Martina Avasoo & Linda Johansson

EVALUATION OF THERMAL PROCESSING TECHNOLOGIES FOR STRAWBERRY JAM

Master’s Thesis in Food Technology

Department of Food Technology, Engineering and Nutrition

Faculty of Engineering, Lund University, Sweden

2011-02-09

Martina Avasoo & Linda Johansson

UTVÄRDERING AV VÄRMEBEHANDLINGSTEKNIKER FÖR JORDGUBBSSYLT

Examensarbete i Livsmedelsteknologi

Institutionen för Livsmedelsteknik

Tekniska Fakulteten, Lunds Universitet, Sverige

2011-02-09

ABSTRACT

This master’s thesis aimed to compare the thermal processing steps of ohmic heating and the MicVac method with the one of conventional strawberry jam production regarding their influence on product quality.

The long cooking times and the dependence of heat transfer from hot surfaces of conventional strawberry jam production lead to undesirable changes in product flavor and color profile. A milder thermal processing step could possibly solve these problems. In ohmic heating, an electrical current passes the food and rapidly heats it internally, without the use of hot surfaces. In the MicVac method, microwaves heat the food rapidly and uniformly, also without hot surfaces. The food becomes automatically vacuum-packed upon cooling due to the specially designed package.

Jam production procedures for ohmic heating and the MicVac method were developed in consideration to equipment and raw material limitations. Strawberry jams of the same recipe were then manufactured in each method and analyzed for vitamin C content, color and microbial growth. Together with a sensory evaluation, this was the basis for comparison.

All of the samples were microbiologically shelf-stable. The ohmic heating jam had the highest and the MicVac jam the lowest vitamin C content after two weeks of accelerated storage at 35°C, even though they had initially similar vitamin C levels. Visible changes in color were observed and showed that the MicVac jam lost its red color faster and the ohmic heating jam retained its color better than the other samples during storage. In the sensory evaluation, the ohmic heating jam received the highest ranking sum, even though it was just above the reference jam. Two consumer groups were identified; one appreciating the traditional cooked flavor of the reference jam; the other one the fresh, intense strawberry flavor of the ohmic heating jam.

The rapid, uniform heating and independence of hot surfaces in ohmic heating enabled the production of a strawberry jam with better preserved color, strawberry flavor and vitamin C content compared to the reference jam. Due to limitations in temperature control and packaging materials, the MicVac method was found to be disadvantageous for the production of strawberry jams.

SAMMANFATTNING

Denna rapport syftade till att jämföra värmestegen vid produktion av jordgubbsylt med ohmic heating (ohmsk värmning) och MicVac-metoden med det vid konventionell tillverkning, med avseende på kvalitetspåverkan.

De långa koktiderna och värmeöverföringen från varma ytor under konventionell produktion av jordgubbssylt ger upphov till oönskade förändringar i produktens smak- och färgprofil, ett problem som eventuellt skulle kunna lösas med ett mildare värmesteg. Ohmic heating är oberoende av varma ytor eftersom en spänning läggs över livsmedlet, som då upphettas snabbt och jämnt. I MicVac-metoden, som också är oberoende av varma ytor, värmer mikrovågor livsmedlet vilket ger en snabb och jämn uppvärmning. Den specialdesignade förpackningen vakuumpackar automatiskt livsmedlet vid nedkylning.

Med hänsyn till begränsningar i utrustning och råmaterial utvecklades processmetoder för sylttillverkning med ohmic heating och MicVac-metoden. Jordgubbssylter av samma recept tillverkades sedan och analyserades med avseende på vitamin C-innehåll, färg samt förekomst av mikroorganismer. Insamlade data från dessa analyser användes, tillsammans med resultaten från en sensorisk analys, som jämförelsematerial.

Alla prover var mikrobiologiskt säkra. Trots att de initialt innehöll ungefär lika mycket vitamin C, hade ohmic heating-sylten högst och MicVac-sylten lägst innehåll efter två veckors forcerad lagring vid 35°C. Observerade förändringar i färg visade att MicVac-syltens färg förändrades fortare och ohmic heating-sylten behöll sin ursprungliga färg bättre än de andra proverna under lagring. I den sensoriska analysen fick ohmic heating-sylten högst poäng i ett rankingtest, tätt följt av referenssylten. Utifrån detta identifierades två konsumentgrupper: en som uppskattade den traditionella, kokta smaken hos referenssylten och en som tilltalades av den färska, intensiva jordgubbssmaken hos ohmic heating-sylten.

På grund av metodens snabba, jämna uppvärmningssteg och oberoende av varma ytor kunde en jordgubbssylt med bättre bevarad färg, jordgubbsmak och med ett högre vitamin-C innehåll produceras med ohmic heating. MicVac-metoden visade sig vara ogynnsam för produktion av jordgubbssylt på grund av begränsningar hos förpackningsmaterial och i processtemperaturreglering.

PREFACE

This report is the result of a 30 credit project in food technology, a master’s thesis part of the Master of Science program in Biotechnology at the Faculty of Engineering, Lund University.

The 20 week long project was carried out during the autumn/winter of 2010/2011 in collaboration with Procordia Food AB, who also financed the project and supplied valuable resources, contacts and knowledge. Most of the work was done at the company’s R&D department in Eslöv. Based on the company’s vision, the authors of this master’s thesis developed a suitable project plan at the beginning of the project. The large scope of the project work, including the finding of suitable equipment and development of jam production procedures, meant that several internal and external parties had to be involved.

Together with this report, an oral presentation of the master’s thesis work was held at the Department of Food Technology, Faculty of Engineering, Lund University, as part of the examination.

Project supervisors were Richard Clerselius, senior product developer at Procordia Food AB, and Ingegerd Sjöholm, teacher at the Department of Food Technology, Lund University.

TABLE OF CONTENTS

1. Introduction ................................................................................................................................................................... 1

1.1. Background ........................................................................................................................................................ 1

1.2. Objective ........................................................................................................................................................... 2

1.3. Sequence of Events ........................................................................................................................................... 2

2. Literature Study ............................................................................................................................................................. 3

2.1. Quality Issues of Strawberries and Strawberry Jams .............................................................................................. 3

2.1.1. Structural Components ............................................................................................................................ 3

2.1.2. Stuctural Changes During Processing ...................................................................................................... 4

2.1.3. Color Compounds..................................................................................................................................... 5

2.1.4. Color Changes During Processing ............................................................................................................ 5

2.1.5. Flavor Compounds ................................................................................................................................... 8

2.1.6. Flavor Changes During Processing ........................................................................................................... 8

2.1.7. Changes in Vitamin C Content ................................................................................................................. 9

2.1.8. Microbial Aspects of Strawberry Jams ..................................................................................................... 9

2.2. Conventional Strawberry Jam Production ............................................................................................................ 11

2.2.1. Preparation of Ingredients ..................................................................................................................... 11

2.2.2. Thermal Processing ................................................................................................................................ 11

2.2.3. Gel Formation ........................................................................................................................................ 12

2.2.4. Filling and Storage ................................................................................................................................. 12

2.3. Ohmic Heating ...................................................................................................................................................... 13

2.3.1. Heating Mechanism ............................................................................................................................... 13

2.3.2. Conductivity ........................................................................................................................................... 14

2.3.3. Process Design ....................................................................................................................................... 15

2.4. The MicVac Method .............................................................................................................................................. 16

2.4.1. Microwave Fundamentals ..................................................................................................................... 16

2.4.2. Heating Mechanisms ............................................................................................................................. 16

2.4.3. Interactions with Foods ......................................................................................................................... 17

2.4.4. Cooking by the MicVac Method ............................................................................................................. 17

3. Introduction to Project Trials ....................................................................................................................................... 19

3.1. Process Development ........................................................................................................................................... 19

3.2. Jam Production ..................................................................................................................................................... 20

4. Material and Methods ................................................................................................................................................. 21

4.1. Materials ............................................................................................................................................................... 21

4.2. Equipment ............................................................................................................................................................. 22

4.3. Analyses ................................................................................................................................................................ 23

4.3.1. Color ...................................................................................................................................................... 23

4.3.2. Flow Properties ...................................................................................................................................... 23

3.3.3. Soluble Solids ......................................................................................................................................... 23

4.3.4. pH .......................................................................................................................................................... 23

4.3.5. Vitamin C ............................................................................................................................................... 23

4.3.6. Microorganisms ..................................................................................................................................... 24

4.3.7. Temperature .......................................................................................................................................... 24

4.3.8. Conductivity ........................................................................................................................................... 24

4.3.9. Sensory Evaluation ................................................................................................................................ 24

4.3.10. Data Analyses ...................................................................................................................................... 25

4.4. Process Development Procedures ........................................................................................................................ 26

4.4.1. Equalization of Density .......................................................................................................................... 26

4.4.2. Equalization of Conductivity .................................................................................................................. 26

4.4.3. Mixing and Pre-Heating of Ingredients .................................................................................................. 26

4.4.4. Determination of Processing Parameters for Ohmic Heating ................................................................ 27

4.4.5. Determination of Processing Parameters for the MicVac Method ........................................................ 27

4.5. Jam Production Procedures .................................................................................................................................. 28

4.5.1. Production of Reference Jam ................................................................................................................. 29

4.5.2. Production of Reference Jam, Short Cooling .......................................................................................... 29

4.5.3. Production of Ohmic Heating Jam ......................................................................................................... 30

4.5.4. Production of MicVac Jam ..................................................................................................................... 30

5. Results and Discussion ................................................................................................................................................. 31

5.1. Process Development ........................................................................................................................................... 31

5.2. Jam Production ..................................................................................................................................................... 35

5.2.1. Thermal Processing Steps ...................................................................................................................... 35

5.2.2. Reprocucibility ....................................................................................................................................... 37

5.2.3. Vitamin C ............................................................................................................................................... 38

5.2.4. Color ...................................................................................................................................................... 40

5.2.5. Sensory Evaluation ................................................................................................................................ 42

6. Project Conclusions ...................................................................................................................................................... 45

7. Acknowledgements ...................................................................................................................................................... 46

8. References ................................................................................................................................................................... 47

Appendix I – Time Plan .................................................................................................................................................... 50

Appendix II - Reaction Kinetic Equations .......................................................................................................................... 51

Appendix III – Photographs .............................................................................................................................................. 52

Appendix IV – Conductivity Data ...................................................................................................................................... 53

Appendix V – Process Development Data ......................................................................................................................... 55

Appendix VI – Temperature Profile Data .......................................................................................................................... 56

Appendix VII – Jam Production Data ................................................................................................................................ 57

1

1. INTRODUCTION

1.1. BACKGROUND

For many decades, Procordia Food AB has produced strawberry jam under strong brands. Even though these products readily live up to the expectations of the Swedish consumers, conventional large scale strawberry jam production has its drawbacks. Long cooking times due to large batch sizes lead to a decrease in product quality through the destruction of important flavors and colors and the production of Maillard compounds. Procordia Food AB has therefore during the past few years sought milder alternatives to the conventional heating step of strawberry jam production.

FIGURE 1. BROWNING IN STRAWBERRY JAM DURING STORAGE

In 2006, the company briefly carried out trial runs with an emerging heating technology called ohmic heating, which uses electric power to heat the food. The results from the trials were promising, but the company concluded that more extensive testing had to be made before they would be able to come to any conclusions.

Due to its short processing times, a process called the MicVac method also interested the company. Microwave heating is here combined with steam boiling through a package equipped with a special valve, resulting in vacuum-packing of the product upon cooling. The method is today used for the production of high-quality ready-to-eat meals.

The quality issues related to the complexity and sensitivity of the strawberry fruit was the basis for selection of raw material and reference jam for this master’s thesis. If an alternative processing method that would better preserve the initial quality of the strawberries could be found, this would potentially help to solve a large problem for the fruit industry.

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1.2. OBJECTIVE

This master’s thesis aimed to compare the thermal processing steps of ohmic heating and the MicVac method with the one of conventional strawberry jam production regarding their influence on product quality. The company was primarily interested in finding ways to preserve more of the original flavors and colors of the strawberries. An important aspect of this was to be able to identify and understand the differences in quality impact of the studied heating technologies.

Since the suitability for processing jam products by the new technologies has not been thoroughly investigated, the project also aimed to develop strawberry jam production procedures tailored for ohmic heating and the MicVac method respectively.

The project work included the selection of suitable equipment as well as suitable methods for analysis that would provide comprehensive information about the impact of the different thermal treatments on storage stability and product quality.

1.3. SEQUENCE OF EVENTS

The project plan was developed at the beginning of the project and was based on the company’s requirements for increased product quality.

Analyses of vitamin C content, color and microbial stability were, together with a sensory evaluation, selected as the basis for evaluation of the studied heating technologies.

A suitable pilot scale ohmic heater was sought out and finally found at C-tech Innovation Ltd, Liverpool, England. A contract was compiled and signed by Procordia Food AB, whereupon the equipment was transported to and installed at the company’s R&D department in Eslöv. In collaboration with MicVac AB (Gothenburg, Sweden), equipment for the MicVac method was also installed at Procordia.

To get a better understanding of and a feeling for the product, the experimental part of the project began with learning how to produce the reference strawberry jam on the stove. After this, pre-trials that aimed to develop processes for jam production by ohmic heating and the MicVac method were carried out. The processes were developed in consideration to the limitations of the available equipment and included the finding of solutions to the technical issues related to the properties of the raw materials.

In the main trials, when tailored production procedures had been developed, strawberry jams of the same recipe were produced in each method. They were then analyzed together with the selected reference jam produced at Procordia’s factory in Tollarp. A sensory evaluation was also carried out in order to further investigate the differences between the produced strawberry jams.

For a detailed description of the project time plan, see appendix I.

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2. LITERATURE STUDY

2.1. QUALITY ISSUES OF STRAWBERRIES AND STRAWBERRY JAMS

Strawberries are the edible fruits of the plants of the Fragaria genus. It is a very popular berry, mainly due to its attractive taste and appearance that varies quite a lot between different cultivars. Strawberries are used all over the world for the production of a range of products such as juices, jams, syrups, desserts and wines. [1]

As the strawberry is a very complex and sensitive fruit, it is a difficult task to try to preserve its initial quality. Therefore, it is important to have knowledge about the mechanisms behind the deteriorative processes that happen during processing and storage of strawberry jams. As an example, strawberry jams, microbiologically considered as very stable products due to their high acid and sugar content, have a much shorter shelf life than they ought to have. This is mainly due to the gradual degradation of color with time, limiting the shelf life to less than twelve months. [2]

2.1.1. STRUCTURAL COMPONENTS

The different tissues of the strawberry fruit are illustrated in figure 2. The outer layer of the strawberry is made up by the epidermal cells, inside which the hypodermal cells and cortical cells are located. The pith, situated in the middle of the strawberry, consists of cells with very thin cell walls. The cells of the pith separate more and more during the growth and ripening of the strawberry, giving this region a porous structure. From the pith out to the surface of the fruit, vascular bundles extend and end up at the small, yellow seeds of the strawberry called the achenes. [3, 4]

FIGURE 2. STRAWBERRY STRUCTURE [5]

Pectins are the main structural components of plant tissue cell walls. They are linear polymers of (1-4)-D-galacturonic acid, where some of the carboxyl groups (-COOH) are esterified with methanol (-COOCH3), as can be seen in figure 3a. Pectins are located in the space between cells, the middle lamella, and help to keep together the cell network [6]. Softening of the strawberry structure during ripening of the fruit is due to the solubilization of the pectin gel network, illustrated in figure 3b [3].

4

FIGURE 3a. PECTIN MOLECULE [7]

FIGURE 3b. SCHEMATIC PECTIN NETWORK STRUCTURE [8]

Both Polygalacturonase (PG) and Pectin Methyl Esterase (PME) belong to a group of enzymes called pectinases, which catalyze pectin breakdown [9]. Both enzymes lead to an increase in soluble pectin during ripening of the strawberry fruit. PME affects methylated galacturonic acid units by removing methanol residues, thus decreasing the amount of methylated groups on the pectin chain [6]. PG hydrolyses unmethylated galacturonic acid units which results in the degradation of the pectin chain into smaller fragments. PG is dependent upon the activity of PME since it only functions when the amount of methylated galacturonic acid units has been lowered by PME to less than 60%. 2.1.2. STUCTURAL CHANGES DURING PROCESSING

Different processing methods such as freezing, thawing and thermal processing will affect the structure of the strawberries [3]. During and after thawing, the strawberries lose liquid due to the rupture of cells by the ice crystals created during the freezing process, a term called drip loss [1]. The size of the drip loss depends on the freezing process. A slow freezing process results in berries with a soggy texture and a higher drip loss due to the creation of larger ice crystals than in a quick freezing process.

It has been proven that during thermal processing and mainly at higher temperatures, the vascular tissue and to some extent also the epidermal cells help to maintain the strawberry structure. Hypodermal and cortical cells tend to collapse due to plasmolysis, when the plasma membrane and cell wall dissociate due to the loss of water through osmosis. [3] Other osmotic effects will also affect the strawberries such as during maceration, where water moves out of the strawberry at the same time as sugar moves into the fruit. This equalization of water and sugar may dewater the strawberries and allow some collapse of the fruit structure. [3]

PME and PG, the enzymes described in section 2.1.1., are not desirable in products containing pectin as a gelling agent. They may, if not inactivated by the thermal treatment, affect the quality of the final product by continuing to break down pectic acids during storage. The thermal stability of PME can be found in table 1 and follows first-order kinetics. No data could be obtained for strawberries, but the systems presented in table 1 may give an idea of the heat sensitivity of the enzyme. [10]

TABLE 1. THERMAL DEGRADATION KINETICS FOR PME IN DIFFERENT FOOD SYSTEMS [10]

Environment Ea (kJ/mol) kT (min-1) Orange juice, pH 3.7 389.3 k65°C = 0.288 Tomato juice 363.8 k68°C = 0.436

5

PG has an optimum pH of 4-6 in strawberries and its activity is very low below 3.5. The PG and PME z-values, the temperature increase to obtain a decrease of the decimal reduction time by 90 %, vary between 6°C and 10°C depending on system and environment. [10]

2.1.3. COLOR COMPOUNDS

Anthocyanins are phenolic compounds and give strawberries their red color. They are water-soluble compounds, all with the basic structure of the flavylium cation, illustrated in figure 4a, but with the addition of a sugar group [11]. Pelargonidine-3-glucoside and cyanidin-3-glucoside are the main anthocyanins in strawberries and can be seen in figure 4b and 4c. Pelargonidine-3-glucoside stands for 72-95 % of the total anthocyanin content [12].

FIGURE 4a. FLAVYLIUM

CATION [13]

FIGURE 4b. PELARGONIDIN-3-GLUCOSIDE [14]

FIGURE 4c. CYANIDIN-3- GLUCOSIDE [15]

2.1.4. COLOR CHANGES DURING PROCESSING

The color stability of strawberry jams varies between different strawberry cultivars. Changes in the color of these products can largely be related to reactions involving phenolic compounds, mainly anthocyanins [11]. These are very unstable compounds and as they are degraded, the product will change in color. The breakdown of anthocyanins results either in bleaching or browning of the system [16, 17]. The stability and degradation of anthocyanins is affected by pH, presence of oxygen, ascorbic acid content, metal ion catalysts, temperature, and enzyme activity.

During processing, anthocyanins are degraded due to their sensitivity to heat [17]. However, a thermal process is often required to ensure color stability during storage following a sufficient inactivation of color-degrading enzymes such as PPO and POD. The degradation of anthocyanins by heat mainly happens through hydrolysis and results in smaller phenolic compounds, anthocyanidins, and sugar [18]. The anthocyanidins can subsequently be broken down into even smaller phenols that may be oxidized by enzymes, a process that will be described later on in this section. The thermal degradation follows first-order kinetics and kinetic data for strawberry-related systems can be seen in table 2 [11].

TABLE 2. THERMAL DEGRADATION KINETICS OF ANTHOCYANINS [11]

Environment Ea (kJ/mol) kT (min-1) Temperature range (°C) Raspberry juice, pH 3.30 92,9 k78°C=0,001561

k108°C=0,01925 78-108

Strawberry juice, pH 3.55

n/a k45°C=0,001083 45 (t1/2 = 640 min)

6

Anthocyanins can also react with other colored compounds and build up complexes, either colored or with discoloring effects [16, 17]. Even though they do not contribute to the red color of strawberries, other phenolic compounds such as quercetin and catechins may also be subjected to enzymatic browning and thus also contribute to the deteriorative browning effects [12]. It is the enzymatic browning reactions that are the main reason for color changes during storage [11].

Polyphenol oxidase, PPO, is an enzyme which reacts with phenolic compounds to induce browning and has been found to have a pH optimum of approximately 5.0 [19, 20]. Strawberry PPO can be found in the membranes of the pith and cortical cells of the strawberry, whereas phenolic substrates are found in the vacuoles inside the cells. When the cells are physically damaged, substrate and enzyme may come together and react to produce brown pigments [21]. PPO catalyses the hydroxylation of monophenolic compounds to colorless o-diphenols. PPO catalyses the oxidation of these compounds to yellowish o-quinones in the presence of oxygen [19]. It is when the o-quinones polymerize with other o-quinones or amino acids that brown pigments are created. The reaction can be seen in figure 5.

FIGURE 5. THE CATALYZATION OF MONOPHENOLS TO DIPHENOLS AND EVENTUALLY

TO O-QUINONE, WHICH MAY REACT TO BROWN COMPOUNDS [22]

Another enzyme that may cause browning in strawberries is the enzyme peroxidase, POD, which catalyzes the oxidation of polyphenols in the presence of hydrogen peroxide, H2O2 [12]. POD can be found in the vascular tissue of the strawberry [20]. Since the amount of H2O2 is often lower than the amount of present oxygen, the polyphenol oxidation mainly happens with PPO. POD is also more thermo-labile than PPO and its contribution to the enzymatic browning of strawberries is therefore usually small [12].

PPO behaves differently in different systems, but has an optimum pH between 5 and 7 in most fruit systems, the strawberry being presented in figure 6 [11]. The pH optimum of POD is around 6.0 and the enzyme has an activity below 50% at pH 3.5 [21].

PPO + O2

PPO + O2

O

R

O

O Amino acids proteins o-quinone (colored) diphenol (colorless)

Reducing Agent

Complex brown polymers

monophenol (colorless)

OH

R

OH

R

R

7

FIGURE 6. STRAWBERRY PPO ACTIVITY AS A FUNCTION OF PH,

WHERE A IS THE ACTIVITY MEASURED AS ABSORBANCE AT 420 NM AT 25°C [19]

Thermal inactivation of PPO often follows a biphasic kinetic model, which means that a labile and a stable fraction of the enzyme exist, where the different fractions have different kinetics, as presented in table 3. To minimize browning reactions in strawberry jams during storage, it is necessary to assure that the stable PPO fraction is totally inactivated during the thermal treatment, since this fraction stands for about 50 % of the total PPO activity [19]. A very small amount of active enzyme is enough for discoloration of the product and the stable fraction is very resistant to inactivation by heat [12]. However, several studies have shown that a short processing at temperatures above 70-90°C is usually enough to inactivate the enzyme completely when in its natural environment [19]. The studies also suggest that when in a buffer solution, strawberry PPO is even more thermo-labile, its activity decreasing to almost zero after 10 minutes at 65°C.

TABLE 3. KINETICS FOR STRAWBERRY PPO IN PHOSPHATE BUFFER, PH 7.0, THERMAL INACTIVATION [19]

ENZYME fraction Ea (kJ/mol) k60°C (min-1) Labile fraction 314.1±4.6 1.198 ±0.077 Stable fraction 321.3±3.5 0.160 ±0.007

The other enzyme responsible for the enzymatic browning of strawberry jams, POD, has been found to be highly thermo-labile in strawberry purée. If PPO and POD have not been completely inactivated following the thermal treatment, a good way of slowing down the enzymatic activity during storage is to cold-store the strawberry jam in an oxygen-free environment. [12]

As can be seen in figure 5, a reducing agent such as ascorbic acid may reduce o-quinones to o-diphenols and through that counteract the discoloration of strawberry jams. At the same time, ascorbic acid can affect the anthocyanins in a negative manner. In the presence of oxygen and iron or copper ions, the oxidation of ascorbic acid produces hydrogen peroxide which can oxidize the anthocyanins to colorless compounds. Therefore, ascorbic acid can be said to both protect and destroy the color of strawberry jams. [23]

Although enzymatic browning of phenolic compounds is the main cause of color degradation in strawberry jams, non-enzymatic caramelization and Maillard reactions do occur. Time and temperature are important parameters affecting these reactions. Since these types of products contain both sugar and amino acids, Maillard browning reactions are quite common. As an indicator of the presence of Maillard compounds, the level of 5-(hydroxymethyl)furfural, HMF, is often measured. HMF is a colorless intermediate in the Maillard reaction and is produced during thermal processing, but to a lesser extent also during storage at room temperature. It reacts easily with other compounds to form brown polymeric pigments. Too little is known about how to control the

A/A

max

(%)

8

different pathways in the Maillard reaction and the only way to minimize the production of Maillard compounds is to keep the thermal processing step as short as possible. Caramelization reactions take place at temperatures above 100°C and contribute to color changes during storage since these reactions also results in the formation of HMF. [24, 25]

2.1.5. FLAVOR COMPOUNDS

More than 360 volatile compounds have been found to make up the very attractive and complex flavor of strawberries, differing between cultivars and with the degree of ripeness [1]. The most important contributors to the strawberry odor are furaneol and mesifurane, illustrated in figure 7a and 7b [26, 27]. Other contributors are esters, giving the berry its flowery and fruity flavors. Alcohols, aldehydes, ketones, lactones and terpenes are also contributing to flavor observations. Non-volatile compounds such as sugars and acids can be directly related to sweet, sour and astringent perceptions [28].

FIGURE 7a. FURANEOL [29]

FIGURE 7b. MESIFURANE [29]

2.1.6. FLAVOR CHANGES DURING PROCESSING

Processes such as freezing and thawing do not affect the furaneol and mesifurane content of strawberries, but do affect esters and thus the fruity flavors of the fruit. [26]

Many flavor compounds are very volatile. Volatile compounds evaporate at all temperatures, with higher temperatures accelerating the evaporation process. Small molecules are more volatile than larger ones and substances with higher vapor pressures vaporize more rapidly than substances with lower vapor pressures. Many volatile compounds such as small alcohols, ketones and esters have relatively low boiling points, demonstrating the importance of short thermal treatments at as low temperatures as possible. [30]

Studies have shown that short thermal treatments preserve flavors better than long thermal treatments, resulting in more fruity and fresh flavors [30]. Flavor changes in a product with time during processing depend on both loss and changed proportion of the volatile compounds. Studies on flavor retention have shown that closed systems, where volatile compounds have a chance to condense and be added back to the mixture result in better tasting jam products [2]. At high temperatures, caramelization of sugars as well as Maillard reactions occur, contributing to cooked, burnt and caramel flavors that are not always desirable for strawberry jams [25, 31].

During storage of the product, oxidation and polymerization reactions as well as interactions with compounds such as pectins may alter the flavor [30]. Taints and off-flavors may be produced if the product is spoiled by microorganisms [28].

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2.1.7. CHANGES IN VITAMIN C CONTENT

Ascorbic acid, vitamin C, is a good indicator of the impact of thermal processing and storage on product quality since it is thermo-labile and sensitive to oxidation. If the ascorbic acid content has been well preserved after processing, this indicates that the other nutrients in the product have been retained as well. The ascorbic acid content of fresh strawberries is approximately 66mg/100g. [32]

The degradation of ascorbic acid can occur either aerobically or anaerobically and is influenced by a variety of parameters such as exposure to oxygen and light, metal ion catalysts, temperature and pH [33, 34]. The degradation of ascorbic acid follows first-order kinetics but can happen through many different pathways since the mechanism is highly dependent on the system and its surrounding environment [33]. Ascorbic acid of fruits and berries is in the form of its L-isomer. Both L-ascorbic acid and its oxidized form, dehydro-L-ascorbic acid, have vitamin activity and are illustrated in figure 8a and 8b.

FIGURE 8a. ASCORBIC ACID [35]

FIGURE 8b. DEHYDRO ASCORBIC ACID [36]

L-ascorbic acid can be oxidized to dehydro-L-ascorbic acid in the presence of mild oxidants [33]. The reverse reaction may also happen in the presence of reducing agents to regenerate ascorbic acid [23]. Metal ions of for example copper and iron as well as metal-containing enzymes can catalyze the oxidation of L-ascorbic acid to its oxidized form. At higher temperatures, the oxidation rate of L-ascorbic acid to dehydro-L-ascorbic acid is increased [33]. The newly formed dehydro-L-ascorbic acid is however rapidly hydrolyzed at elevated temperatures since it is more labile than L-ascorbic acid. At low pH, the oxidation rate of ascorbic acid is decreased [23].

Since ascorbic acid is a sugar acid, it can be broken down in the same manner as sugars at elevated temperatures [23, 33]. During anaerobic conditions, ascorbic acid may be broken down to Maillard compounds such as furfural and 2-furonic acid to eventually form brown pigments. The anaerobic degradation can only be prevented by keeping the temperature low during processing [37]. However, this contribution to ascorbic acid degradation is very small when oxygen is present.

2.1.8. MICROBIAL ASPECTS OF STRAWBERRY JAMS

Pasteurization of high acid products is often maintained to reduce the risk of spoilage of the product when stored at room temperatures. The word pasteurization refers to a mild heat treatment aimed to inactivate the vegetative forms of pathogens and spoilage microorganisms. Since heat-resistant spores are not inactivated, additional forms of preservation such as refrigeration, modified atmosphere packaging, addition of preservatives or a combination of these must be used to ensure product safety during storage. A pasteurization value of 6D90 is a good point to aim at for shelf stable products, and to use kinetic data for fungal spores and enzymes when calculating the pasteurization

10

time. Exceptions do however occur, in the form of foods containing ingredients that provide an antimicrobial effect such as high-acid foods, very sweet foods with a water activity below 65 and/or soluble solids above 70°Brix, salty foods or fermented foods containing alcohol. [38]

Jam products are quite stable products, consisting of large amounts of sugar and relative low pH which makes the environment relatively unpleasant for microorganisms [38]. The spoilage of strawberry jams may come from yeasts, moulds and lactic acid bacteria, although these are quite heat sensitive and therefore usually inactivated during processing. Lactic acid bacteria may spoil products with a pH between 3.7 and 4.5. They are also more heat sensitive than yeasts and moulds. Sorbate or benzoate salts are effective preservatives against yeast and moulds and these substances are often used in jam products to assure a stable product once the jar has been opened by the consumer [39].

Spores from yeasts and moulds, ascospores, are somewhat more heat resistant than the vegetative cells [40]. Ascospores are not often reported as a spoilage problem in high acid products and are often removed during rinsing of the raw material. Normally they are not considered to be a problem in processed foods since they are destroyed below 100°C. However, some varieties of ascospores are more resistant than others such as Byssochlamys nivea [38]. In table 4, heat resistance data of some of the most important spoilers of strawberry products are described.

TABLE 4. HEAT RESISTANCE OF MICROORGANISMS [38]

Microorganism D (min) z (°C) Temperature range (°C) Yeasts: Zygosaccharomyces bailii [11] Saccharomyces cerevisiae

D59°C=16.9 D57°C=9.4 - 23

7.2 7.2

Moulds [6] D65°C=0.5-3.0 5.0 - Ascospores: Byssochlamys nivea Neosartorya fischeri LT025 Talaromyces flavus Eukenicillium javanicum

D93°C=1.7 D93°C=0.5 D90°C=0.9 D90°C=0.8

6.4 6.4 8.2 7.9

� �80-93

The pasteurization step also aims to inactivate some enzymes that may affect product quality. Products that are stored in refrigerators are of minor importance since the activity of enzymes is decreased at lower temperatures. The pH is of great importance in high-acid shelf stable products and may change if some enzymes are not readily destroyed, facilitating the growth of spoilage organisms. [38]

Equations for reaction kinetics can be found in appendix II.

11

2.2. CONVENTIONAL STRAWBERRY JAM PRODUCTION

Jam production is an excellent way of preserving fruits and berries as the season draws to an end. Jams are fruit preserves traditionally made with fruits or berries, sugar and water [41]. They are characterized by their special viscous structure that is the product of interactions between sugars, acid and pectic substances [39]. A traditional strawberry jam typically consists of 65 % soluble solids, 1 % pectin and has a pH of about 3.3. Generally, products with whole fruits or with large pieces of fruit in a concentrated sugar solution are called preserves, whereas products with smaller pieces or fruit pulp are called jams.

2.2.1. PREPARATION OF INGREDIENTS

When selecting the raw material for strawberry jam production, berry cultivar as well as uniformity in ripeness, color and intactness are important factors to consider since this determines the flavor, color and texture properties of the final jam [39]. The strawberries used for jam production are usually individually quick-frozen directly after being harvested, washed and sorted [2]. This minimizes texture quality losses upon thawing and facilitates the mixing of ingredients.

The first step in jam production is the blending of frozen or partly thawed strawberries with some of the crystallized sugar, usually at room temperature. This macerates the fruit and liberates fruit juices to the sugar, but also produces inversion of some of the sugar to glucose and fructose mainly due to the presence of fruit invertase. To speed up these processes, the strawberries are sometimes cut into smaller pieces. The inversion process significantly affects gel formation, improves product brightness and enhances product taste and is therefore an important step of the production process. [39]

2.2.2. THERMAL PROCESSING

After maceration the mixture is heated to evaporate some of the water but mainly to pasteurize the product [38]. A definition of pasteurization and a discussion about the shelf life of strawberry jams can be found in section 2.1.8. The thermal treatment should be kept as short as possible to avoid excessive processing that may negatively influence the quality of the jam. There are mainly two modern heating methods used in jam production; open air systems and closed systems operated under low pressure [2]. The closed system and vacuum approach enables processing at lower temperatures, resulting in less heat damage to important flavor compounds, vitamins and pigments. Furthermore, processing at lower temperatures can help to prevent other quality-affecting processes such as the formation of undesired flavor compounds and hydrolysis of pectic acids [39].

Due to their high sugar content, low pH and relatively low water activity, strawberry jams can be easily pasteurized to obtain a shelf-stable product. The low pH prevents the growth and germination of the heat resistant Clostridium botulinum and its spores as well as most of the acid tolerant microorganisms such as lactic acid bacteria, molds and yeasts, which are relatively sensitive to heat [38]. A short heat treatment at 80-100°C is usually sufficient to inactivate any present microorganisms and undesired enzymes. Even so, addition of preservatives is often done as a complementary way of preventing the growth of deteriorative microorganisms after opening of the jar by the consumer [39].

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2.2.3. GEL FORMATION

Before the pasteurization step, pectin is usually added as a heated mixture with sugar and water [39]. This is done to make sure that a proper pectin distribution is achieved, but also to keep the pectin from heat damage during the addition process. When producing strawberry jams, the addition of pectins is essential for gel formation mainly because strawberries contain rather little pectin themselves, but also because it helps to eliminate variations in gel formation between batches. The function of the pectin is to interact with sugar and berry fibers at a controlled pH to create the desired gel structure characteristic for jam products. Some of the factors affecting pectin gel texture, strength and viscoelastic properties are the degree of esterification (DE), pH, ionic strength, pectin concentration, co-solute concentration and temperature [42].

Commercial pectins are classified according to their degree of esterification, which is the amount of methylated galacturonic acid units on the pectin chain. High methoxyl pectins, with a DE of 55-80%, gel at low pH in the presence of large concentrations of sucrose or similar co-solutes. Therefore, they are often used in the production of high-sugar jellies, jams and preserves. Low methoxyl pectins, with a DE of 20-50%, require calcium to gel but are quite insensitive and gel over a wide range of pH and sugar concentrations. They are therefore commonly used in dairy and low calorie products or whenever the gelling conditions cannot be adequately controlled. Another popular group of pectins are the amidated low methoxyl pectins, where some of the galacturonic acid units contain amide groups (-CONH2). These groups have a positive influence on gelation so that less calcium is required, and enable the production of more elastic and transparent gels. [42]

Acids such as the commonly used citric acid can be added to adjust the pH to a suitable value for gel formation and stability [39]. This should preferably be done carefully and as a final step to prevent pectin degradation by acid hydrolysis. The gel is formed upon cooling and is a three-dimensional polymer network within which the solvent water and sugars are retained. The jam viscosity is proportional to the amount of added pectin and a well-formed gel network should be strong enough to stabilize the fruit pieces in solution [2]. However, jam producers must be aware that too much pectin affects quality attributes such as sweetness, acidity, mouthfeel and strawberry flavor.

2.2.4. FILLING AND STORAGE

After the heating step and the addition of acids and preservatives, the jam is cooled to the desired temperature for filling. At this point the soluble solids, pH and consistency of the jam are controlled and any deviating parameter adjusted for by the addition of water or acid. The jam is then filled into sterilized glass containers either at higher temperatures (hot-fill) or at lower temperatures to ensure an even berry distribution. The jars are subsequently cooled and stored at room temperature in warehouses and stores. Storage at fridge temperatures before opening is not considered to be necessary due to the low pH, high sugar content and the presence of preservatives. It is also a more costly alternative. However, it is recommended to do so after opening of the jar due to the possibility of recontamination. [39]

13

2.3. OHMIC HEATING

The principle of ohmic heating, also known as joule heating, electro-heating, direct electrical resistance heating and electro-conductive heating, is based on Ohm’s law [43]. It is the process by which the passage of an electric current through a conductor, in this case the food, generates heat from within it due to its electrical resistance. The simplest model of an ohmic heater are two plates between which the food flows or is contained, as illustrated in figure 9 [44].

FIGURE 9. THE PRINCIPLE OF OHMIC HEATING. ΔT OCCURS PERPENDICULAR TO THE DIRECTION OF THE ELECTRIC CURRENT, ILLUSTRATED BY THE PARALLEL ARROWS

The technology has gained interest during the past few decades since experiments repeatedly show that it results in processed products with a superior quality to those processed by conventional technologies [45]. The reduced processing times in ohmic heating cause minimal structural, organoleptic and nutritional changes and can be related to the rapid internal heating of the food that is not dependant on any hot surfaces. Studies also suggest that ohmic heating may provide additional non-thermal lethal effects on microorganisms, such as electroporation, which enables the use of an even shorter thermal treatment without interfering with product safety [44].

The many advantages of ohmic heating may very well surpass the negative aspects that the technology can be more costly and difficult to validate than conventional heating methods [46]. The system, batch-wise or continuous, can easily be incorporated into a complete process line with aseptic filling and packaging to provide a complete sterilization process. It is a versatile technology that can be used not only for sterilization or pasteurization but for blanching, evaporation, dehydration, fermentation, extraction, thawing and solidification [43]. Today, the technology is successfully being used for the processing of whole fruits, berries, fruit juices, liquid egg and soups in Japan, the United Kingdom and Northern America [46].

2.3.1. HEATING MECHANISM

In ohmic heating, an electrical current is applied to the food. Owing to the food’s resistance to the applied alternating current, heat is generated within it. The mechanism behind this is that charged molecules in the food such as ions move and collide with their surrounding molecules, which then release energy in the form of heat. [44]

14

Since the heating happens within the food and is not dependent on any hot surfaces, the heating pattern in ohmic heating is much more uniform than in conventional heating technologies [43]. Ohmic heating is therefore suitable for the processing of viscous or multiphase foods which may otherwise be difficult to heat. It is however important to predict or asses the heating pattern when designing an ohmic heating process [46]. This can be done by modeling, which also aims to answer questions such as what the heating pattern will look like, what the lethal effect on microorganisms will be and how it can be ensured, as well as how the food moves within the ohmic heater. Since several food-related factors such as density, specific heat capacity, composition, ability to conduct electricity, viscosity and particle size, distribution and concentration will affect the heating pattern, it is easy to understand that modeling of ohmic heating is a difficult task [43].

2.3.2. CONDUCTIVITY

In order for heating to occur, the food must be in contact with the electrodes of the equipment but must also be able to conduct electricity. The ability of a food to do so is called its electrical conductivity. Since it depends on the food formulation, it must be established for each food system individually. Most foods and even water have some ability to conduct electricity and can therefore be heated successfully using ohmic heating. [44]

The conductivity of a food should ideally be between 0.01 and 10 Siemens per meter (S/m) at 25°C [46]. If the conductivity exceeds this span, the food will not heat due to the low resistance and the electric current will just pass through it. This may be a problem when heating salty liquid foods such as broths or soups. If the conductivity is extremely low, the current will not be able to pass as is the case for pure fats, oils and sugars [43]. The reason for this is that these compounds cause an increase in resistance to ion movement which increases with concentration.

When heating multiphase foods the heating pattern will be extremely uniform if the particles and the liquid have similar or equal conductivities [44]. If the conductivities differ between the two phases, parameters such as specific heat, particle concentration and particle distribution will influence the heating pattern of the food [46]. If the concentration of particles is high and their conductivity is higher than the surrounding liquid, heating will be more rapid since the particles will provide a more uniform resistance to the product. If the concentration of particles is low, particle geometry and fluid motion will have an influence on conductivity due to electrical field condensation around particles having lower conductivities than their surrounding medium. To even out the conductivity of multiphase foods, methods such as salt addition to liquids as well as salt infusion by vacuum impregnation or blanching of solid foods whose conductivities are lower than the surrounding medium can be employed [43].

Conductivity increases linearly with temperature for liquid foods. This is also nearly the case for cellular solid foods, apart from that conductivity increases sharply at around 60°C due to the breakdown of cell wall materials and the subsequent release of ionic compounds to the surrounding medium. [46]

15

2.3.3. PROCESS DESIGN

The main modules of a food ohmic heater are a heating chamber or column in nonconductive materials inside which a pair of electrodes are mounted, an alternating power supply and a control panel for controlling the voltage or current and the temperature of the food [43]. Suitable pumping systems are also essential for continuous processing. The electrodes are the key part and provide rapid heating to sterilization temperatures, without any hot surfaces, which reduces the risk of overheating of the product and fouling at the electrodes [46]. The direct contact between the food and the electrodes also mean that the efficiency in energy conversion is high. In modern ohmic heaters the electrodes are often coated, usually with platinum or titanium, to minimize the risk of corrosion or electrolysis which may occur under certain conditions.

For continuous ohmic heaters, two electrode configurations exist; in the cross-field configuration the food flows perpendicular to the electrodes; in the in-field configuration the food flows parallel to the electric field. The two configuration types are illustrated in figure 10a and 10b. The increase in spacing between the pair of electrodes in the in-field configuration is to compensate for the increase in conductivity and therefore also electric field strength with temperature as the food approaches the outlet of the ohmic heater. [43]

FIGURE 10a. CROSS-FIELD CONFIGURATION [45]

FIGURE 10b. IN-FIELD CONFIGURATION [45]

When designing an ohmic heating process, equipment factors such as electrode configuration, heater geometry, applied voltage, flow rate and flow profile must be taken into consideration [46]. The field strength to be used may be calculated from the flow rate, electrode configuration, conductivity and specific heat of the food as well as from the required temperature rise using equations 1, 2 and 3 below. In the equations, R is the resistance in Ω, P is the effect in J/s, F is the flow rate in kg/s, U is the voltage, L is the electrode spacing in meters, A is the electrode area in m2, σ is the mean of incoming and outgoing conductivities in the ohmic heater in S/m, cp is the specific heat of the food and ΔT is the desired temperature increase.

𝑅 = 𝐿𝜎∗𝐴

(equation 1, [47])

𝑃 = 𝑈2

𝑅 (equation 2, [47])

𝐹 = 𝑃𝑐𝑝∗∆𝑇

(equation 3, [47])

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2.4. THE MICVAC METHOD

The MicVac method is a pasteurization process that combines microwave heating with steam boiling [48]. Compared to conventional microwave heating the MicVac method results in lower energy losses due to its closed-system processing, preventing heat from the evaporated steam to be lost too rapidly to the cavity of the tunnel [49]. Instead, some of the heat has a chance to re-enter the product through convection. The method also has the benefit of the product becoming vacuum-packed upon cooling, resulting in an extended shelf life compared to other microwave processed products.

2.4.1. MICROWAVE FUNDAMENTALS

Microwaves are electromagnetic waves with frequencies ranging from 0.3-3.0 GHz or oscillations per second. Their suitability for the thermal processing of foods has during the past few decades made them attractive for the food industry. Today, microwave heating has many industrial applications such as drying, blanching, pasteurization and tempering. The food is heated by the conversion of electromagnetic energy from the microwaves into heat within the food. [50]

The process differs from conventional thermal processing in a number of ways. Firstly, it is not dependent on any hot surfaces in contact with the food. It also has very rapid dynamics and power can be instantaneously turned on and off. Microwave heating is material selective and will heat foods with diverse thermal properties differently, which allows for dynamic design possibilities. If the process is properly designed, the food will be much more uniformly heated than by conventional heating. [50]

2.4.2. HEATING MECHANISMS

Foods are dielectric materials and heat when exposed to an oscillating electromagnetic field. The microwaves from the electromagnetic field can either be reflected by the food, transmitted after entering into it or be absorbed by the food. As they propagate into the food, they lose some of their energy in the form of heat. This happens by two important mechanisms, ionic and dipolar interaction. [51]

The oscillating electromagnetic field causes any charged particles in the food, such as ions, to move in its direction [51]. This causes collisions between adjacent particles and the generation of heat by the mechanism of ionic interaction. Dipoles such as water interact somewhat differently with microwaves to generate heat. The electromagnetic field forces them to align with it, causing them to collide with neighboring molecules and losing some of their energy as heat [50]. The friction that the oscillating dipoles cause is another source of heat generation by dipolar interaction. In frozen foods, the water molecules are locked in position in the ice crystals and cannot rotate enough to collide with each other [51]. How much they are able to depend on how much unfrozen water the food contains and on its location and salinity. It is therefore not recommended to heat completely frozen foods by microwave heating.

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2.4.3. INTERACTIONS WITH FOODS

The ability of a food to be heated with microwaves is determined by its dielectric properties [50]. They are the effect of the molecular interactions with the electromagnetic field and describe how the energy is deposited and distributed in the food. They depend on the properties of the food such as temperature, viscosity, density, thermal conductivity and heat capacity and directly determine the heating pattern or temperature profile of the food [51]. Most foods have a microwave penetration depth of 0.7-1.0 cm, which is also determined by the dielectric properties and which is an important parameter for process design [50].

The rate of heat generation per unit volume, Q, at a particular location in the food during microwave heating, can be described by equation 4 where f is the frequency, E is the electric field strength, ε0 is a constant called the permittivity of free space and ε’’ is the dielectric loss factor, determined by the dielectric properties of the food and which describes the ability of the food to absorb microwaves. The dielectric loss factor may be determined by extensive experimenting and modeling. [50]

𝑄 = 2 ∗ 𝜋 ∗ 𝑓 ∗ ε0 ∗ ε′′ ∗ E2 (equation 4, [50])

2.4.4. COOKING BY THE MICVAC METHOD

The MicVac method comprises four steps; filling, sealing and valve application, pasteurization, and cooling, as illustrated in figure 11. They will all be thoroughly described in this section. [48]

FIGURE 11. THE MICVAC METHOD PRINCIPLE: FILLING (a), SEALING (b) HEATING (c), COOLING (d) [52]

MicVac AB has designed a special tray suitable for microwave heating. It is made by flexible polypropylene that deforms at the bottom during cooling of the product. The shape is optimized for microwave processing because of its lack of sharp corners and also because of its appropriate height for optimal microwave penetration into the food. The product, often a multi-phase food such as a ready-to-eat meal, is loaded into the tray either manually or automatically. The higher the temperature during filling the shorter the processing time will be, resulting in an increased product quality and an increased production capacity. It is important that the product temperature and weight are rather stable from tray to tray, in order to prevent under- or over-cooking. [48]

The tray is subsequently sealed with a dual-layer, peelable film, designed to withstand the expansion during the pasteurization step and to increase the oxygen barrier, which is otherwise often low for plastic materials. An applicator then punches a hole in the film and places the MicVac valve centered on top of the hole. The valve consists of two PVC labels designed to open only in one direction. As the pressure inside the package reaches a certain point during processing, the valve lets out steam and the geometry of its ends makes it whistle. [48]

a b c d

18

After sealing, the tray is conveyed to the microwave tunnel where the product is heated uniformly due to the special microwave applicators that provide single-mode microwaves. This means that the waves are only reaching the food from one direction, in this case from above, which helps to minimize cold or hot spots in the product. About 60 % of the energy input enters the food and the rest is absorbed by the equipment. The processing time is adapted for each product and can be adjusted by the conveyor belt speed and by the effect of the magnetron, which is the microwave generator. [48]

As microwaves are absorbed and the product heats, water is evaporated from its surface and the vapor pressure above the food increases. The more the product temperature increases, the more water vapor builds up inside the sealed package, causing it to expand. At a certain point, the over-pressure inside the sealed package becomes too high for the valve to withstand and it opens to let out steam. This temporarily decreases the pressure inside the package and the film is somewhat lowered. However, as more microwave energy is absorbed this causes the temperature to rise even more and the film re-expands and the valve begins to whistle continuously as steam exits through it. Air is also pushed out of the package by the evaporating steam and soon there is only food and water vapor remaining inside the package. The over-pressure is maintained and the film remains expanded until the microwaves are turned off and the vapor condenses. [49]

Immediately after the microwave tunnel, a conveyor belt transfers the tray to a cooling unit where it is rapidly cooled to the desired temperature [48]. As the heating process is terminated and as cooling starts, no air is left inside the package and the water vapor quickly condenses, instantly closing the valve [49]. This produces an under-pressure inside the package, causing the tray to deform and to wrap itself around the product that therefore becomes automatically vacuum-packed. The magnitude of the under-pressure is determined by the degree of deformation. The possibility of any air remaining inside the package is small, partly because water evaporates from the surface of the food and gets pushed towards the valve as more steam evaporates, but also because of the design of the valve itself. The oxygen barrier of the tray and film is however lower than for glass, which is not oxygen permeable, possibly allowing oxygen to re-enter the product during long-term storage.

19

3. INTRODUCTION TO PROJECT TRIALS

3.1. PROCESS DEVELOPMENT

Ohmic heating and the MicVac method had not been previously adapted for the production of jam products. Due to their closed-system designs, these methods require proper mixing and distribution of ingredients before the thermal processing step to obtain a stable product. The purpose of these pre-trials was therefore to develop jam production processes for the two methods respectively. This was done with the selected reference jam recipe and its processing procedure as basis, to be able to compare the thermal processing steps of all of the studied methods.

Since an earlier ohmic heating project at Procordia Food AB concluded that it was difficult to obtain an even distribution of berries in the mixing vessel, a pre-trial was carried out to try to increase the density of the air-dense strawberries. A hypothesis was that the porous parts of the strawberry could be replaced with a dense sugar solution by vacuum impregnation.

In order to obtain a uniform heating pattern in ohmic heating, it is vital that the conductivity of the different phases of the food is close to each other. However, this is not the case for strawberries and for solutions containing a lot of sugar [Appendix III and IV]. Since the processing time in ohmic heating is substantially shorter than in conventional jam production, equalization of conductivity will not be adequately achieved during processing. Another pre-trial therefore aimed to equalize the conductivity between the strawberries and their surrounding jam medium. A hypothesis was that an addition of salt to the jam medium would increase its conductivity [Section 2.3.2].

A suitable amount of antifoaming agent for each method was determined in order to be able to process the jam by the MicVac method without obstruction of the valve mechanism but also in order to minimize the risk of an uneven heating pattern in the ohmic heater due to foaming.

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3.2. JAM PRODUCTION

The jam production trials aimed to compare the thermal processing steps of ohmic heating and the MicVac method with the one of the reference jam. To be able to do so in an appropriate manner, one jam was produced in each method. A jam was also produced to see if the shorter cooling step of the pilot-plant strawberry jams would positively influence product quality, thus affecting the comparison of the different jams.

In order to investigate the impact of the thermal processing steps on the quality of the strawberry jams, several parameters had to be analyzed. As supported in the literature, vitamin C content and color are indications of the severity of a thermal treatment, mainly due to their fragile nature [Section 2.1.4. and 2.1.7]. Hypothetically, a shorter thermal treatment should result in a product with better preservation of initially present strawberry compounds. Absence of microorganisms as well as stable pH, color, viscosity and soluble solids values during storage indicate a stable product. The jam production trials therefore aimed to investigate these parameters as well as the vitamin C content of the produced jam samples during storage at ambient temperatures. To provide a rough idea of what reactions would occur during long-term storage at ambient temperatures, the analyses were also carried out under accelerated conditions, when oxidative and enzymatic processes happen more rapidly.

To get a better understanding of the consumer acceptance of the jam samples produced in ohmic heating and the MicVac method compared to the conventional method, a subjective sensory evaluation was performed. This was selected as the most suitable method of evaluation because it is a frequently used method by the company.

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4. MATERIAL AND METHODS

4.1. MATERIALS

The raw materials and the reference recipe were provided by Procordia Food AB and are listed in table 5 and table 6.

TABLE 5. RAW MATERIALS

Raw material Specification Strawberries Senga sengana, individually quick frozen, pH 3.4±0.2, brix 8±1, (figure 17) Pectin Standardized low-methoxyl, amidated pectin Sugar Caster sugar Acid Granulated citric acid Preservative Granulated potassium sorbate Sugar solution 65 % Sodium chloride (NaCl) Antifoaming agent Monoglycerides Packaging materials 400g Flextray (MicVac jam), glass jars (ohmic heating and reference

jams) TABLE 6. REFERENCE JAM RECIPE

Parameter Specification Soluble solids (Brix) 46±1 pH 3.3±0.1 Berry content 52 %

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4.2. EQUIPMENT

The equipment used for the project trials, except for the ohmic heater, was provided by Procordia Food AB and can be seen in table 7. Photographs of the steam vessel and the ohmic heater can be seen in appendix III. TABLE 7. EQUIPMENT

Equipment Specification Vacuum packer MULTIVAC A300 (Multivac Sepp Haggenmüller, Germany)

Pressure drop can be manually adjusted down to 10 mbar Slicer Vegetable Preparation Machine RG-400 (AB Hällde Maskiner, Sweden)

Blade produces 5 mm thick slices Steam vessel 20 liters, jacketed (steam, 4 bar), stirring device

Pressure can be manually lowered to 100 mbar Lobe rotary pump U22 (Waukesha Cherry-Burrell, USA)

Stainless steel, 35 mm ports Ohmic heater C-tech Pilot 10 kW Ohmic Heater, 230 V single phase, 40 A (C-tech

Innovation Ltd, England) Electrode area 225 cm2 (75x300 mm)

Electrode separation 60 mm Output effect can be manually adjusted from 0-100 %

Sealing equipment TraySealer 4 (Færch Plast, Denmark) Manual temperature control, automatic sealing 2-3 sec

Microwave oven Panasonic Inverter NN-GD556 (Panasonic, UK) The inverter power supply minimizes microwave pulsing and therefore provides more uniform heating

Rod mixer Braun (Braun GmbH, Germany)

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4.3. ANALYSES

Measurements of color, flow properties, soluble solids, pH and vitamin C were carried out initially and once per week during four weeks of storage at 22°C. Measurements were also done during accelerated storage at 35°C for two weeks. Yeasts, molds and total count were measured initially as well as during accelerated storage of samples.

4.3.1. COLOR

Color was measured using a Konica Minolta CM-700d Spectrophotometer (Konica Minolta Sensing Inc., Japan). L, a, and b values of the Hunter Lab color space [23] as well as spectral values at 650 nm were registered. The lens was protected using a transparent film that was exchanged after each measurement. Samples were measured in black jars on jam medium to avoid interference from the surrounding light and from intact strawberry pieces. Four replicates per sample were taken.

4.3.2. FLOW PROPERTIES

Flow property measurements were carried out using a Bostwick Consistometer (SCS Scientific Company Inc., USA). The equipment was filled to the brim with jam medium and the flow distance per minute was measured in centimeters. The equipment was cleaned and dried between measurements. This equipment is extensively used by the jam industry. Three replicates per sample were taken.

3.3.3. SOLUBLE SOLIDS

Total soluble solids were measured by the staff at the company’s quality laboratory using a RFM 342 refractometer (Bellingham and Stanley Ltd, UK). Samples were tempered to 20°C and the equipment calibrated before measurements. Three replicates per sample were taken.

4.3.4. pH

pH measurements were carried out by the staff at the company’s quality laboratory using a TIM865 Titration Manager for potentiometric titration (Radiometer Analytical SAS, France) that was calibrated before measurements. Three replicates per sample were taken.

4.3.5. VITAMIN C

Vitamin C content was measured by the staff at the company’s quality laboratory using a TIM865 Titration Manager for potentiometric titration (Radiometer Analytical SAS, France). Samples were homogenized and ascorbic acid extracted from the samples with metaphosphoric and acetic acid to prepare them for the titration process. Three replicates per sample were taken.

24

4.3.6. MICROORGANISMS

Total count, yeast and moulds were measured by the staff at the company’s quality laboratory. Samples investigated for yeasts and molds were incubated on DG18 agar medium (sugar added to 50 % w/w) for one week at 25°C. Samples for total count were incubated on plate count agar (PCA) for 3 days at 30°C. Three replicates per sample were taken.

4.3.7. TEMPERATURE

In the factory, the temperature of the reference jam was measured continuously by the installed thermocouple on the inside of the steam vessel. The filling temperature was displayed on a screen next to the filling equipment and the temperature after the cooling tunnel measured manually.

In the ohmic heating pilot plant, the temperature was measured with thermocouples at several locations: in the steam vessel, at the inlet and outlet of the ohmic heater and in the jars during filling. The temperatures at these locations were manually registered during jam production.

In the MicVac pilot plant, the temperature inside the tray was measured using a DataTrace Micropack III Temperature Data Logger (Mesalabs, USA). The temperature of the vacuum vessel was also registered during jam production.

4.3.8. CONDUCTIVITY

The conductivities of all of the produced jam samples were measured using a CO150 Model 50150 Conductivity Meter (Hach Company, USA). The equipment was calibrated before measurements and the probe cleaned with de-ionized water and dried with a Kleenex towel between measurements. Four replicates per sample were taken.

4.3.9. SENSORY EVALUATION

A subjective sensory evaluation of the final jams produced in the different methods was performed with 36 participants from Procordia Food AB in a room designed for sensory evaluations. The trays containing the samples and questionnaires were prepared in advance and randomized to make sure that the order of the samples would not affect the results. The samples were room tempered and equal amounts of berries and jam medium was portioned in each cup.

Each person received one of each jam sample and were told to evaluate them using a scale from 1-7, where 1 indicated “very bad”, 4 indicated “neither good nor bad” and 7 “very good”, and to support their answers with motivations and comments. Nothing was said about the differences between the samples, which were to be evaluated for strawberry flavor, strawberry chewability and overall impression. The participants were also asked to rank the four samples from 1-4, 1 being the sample they appreciated the most. The obtained data was then statistically analyzed according to section 4.3.10.

25

4.3.10. DATA ANALYSES

The results from color and vitamin C measurements were analyzed for mean and standard deviation.

For the results from the sensory evaluation the replies were normalized for each person individually using equation 5 before performing a Single Factor ANOVA in Excel with a 95 % significance level. A 95 % confidence interval was then calculated using equation 6, where α=0.05, t=1.98 (for (f-1)=120, [53]), n=36 and s is the standard deviation calculated as the square root of the within groups variance (MS) from the ANOVA.

𝑋𝑛𝑜𝑟𝑚𝑎𝑙𝑖𝑧𝑒𝑑 = 𝑋𝑟𝑒𝑝𝑙𝑦𝑖𝑋�𝑟𝑒𝑝𝑙𝑦𝑖

(equation 5, [54])

𝐼𝜇 = 𝑋� ± 𝑡𝛼∗0.5(𝑓 − 1) ∗ 𝑠√𝑛

(equation 6, [53])

The formulated hypotheses that would be either accepted (p<α) or rejected (p>α) were:

𝐻0: 𝜇1=𝜇2 (no difference between samples)

𝐻1: 𝜇1≠𝜇2 (difference between samples)

For evaluation of the ranking test results, the ranking sum of each sample was calculated and is the product of the ranking number and the ranking score. The lower the ranking sum, the higher is the total rating of the sample.

26

4.4. PROCESS DEVELOPMENT PROCEDURES

4.4.1. EQUALIZATION OF DENSITY

Four frozen strawberries were weighed and immersed in a 65 % sugar solution with a density of approximately 1400 kg/m3 [55] in a plastic bag. The bag was placed in the vacuum packer described in table 7 and the pressure lowered to 30 mbar in 5 seconds. The bag was sealed at the end of the packaging process.

The bag was subsequently opened, the strawberries quickly rinsed with cold tap water and dried with a Kleenex. They were then transferred to a glass cylinder filled with cold tap water placed on a scale and their weight and volume registered. The density of untreated frozen strawberries was measured by the same procedure and the increase in density calculated.

4.4.2. EQUALIZATION OF CONDUCTIVITY

The following jam samples were produced on the stove according to the reference jam recipe, presented in table 6, and its lab-scale processing procedure:

• Reference jam (lab-scale) • Jam with added NaCl (0.1 %) • Jam with added NaCl (0.05 %) • Homogenized reference jam (lab-scale)

The homogenized jam was produced in order to obtain the maximum reachable conductivity value for the jam. Homogenization was achieved by mixing the jam for one minute with a rod mixer, described in table 7.

The berries were only gently stirred to keep them intact for conductivity measurements. When all of the ingredients had been mixed, the conductivity of the berries and jam medium was measured between 30°C and 80°C. A reference jam sample was also taken from the factory and its conductivity measured in the same temperature interval.

The suitability for the use of salt as a conductivity increaser was evaluated in a sensory evaluation with 8 participants from Procordia Food AB, who commented on off-flavors, strawberry flavor and aftertaste.

4.4.3. MIXING AND PRE-HEATING OF INGREDIENTS

By talking to the pectin supplier and to product developers at Procordia Food AB, by performing orienting jam production experiments in the steam vessel and by supporting our theories with results from literature studies, a procedure for mixing and pre-heating of the ingredients was developed. The procedure was applied for both ohmic heating and for the MicVac method in the jam production trials.

27

4.4.4. DETERMINATION OF PROCESSING PARAMETERS FOR OHMIC HEATING

Initial process parameters for ohmic heating were approximated based on the equations in section 2.3.3. and on jam conductivity data from appendix IV and V. The average specific heat capacity of the reference jam in the temperature interval, 50°C-90°C, was also approximated based on its water and sugar content by equations 7, 8 and 9.

𝑐𝑝,𝑐𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒𝑠 = 1.5488 + 1.9625 ∗ 10−3 ∗ T − 5.9399 ∗ 10−6 ∗ T2 (equation 7, [56])

𝑐𝑝,𝑤𝑎𝑡𝑒𝑟 = 4.1762 − 9.0864 ∗ 10−5 ∗ T + 5.4731 ∗ 10−6 ∗ T2 (equation 8, [56])

𝑐𝑝,𝑗𝑎𝑚 = 𝑋𝑠𝑜𝑙𝑢𝑏𝑙𝑒 𝑠𝑜𝑙𝑖𝑑𝑠 ∗ 𝑐𝑝,𝑐𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒𝑠 + 𝑋𝑤𝑎𝑡𝑒𝑟 ∗ 𝑐𝑝,𝑤𝑎𝑡𝑒𝑟 (equation 9, [56])

The approximate nature of the calculated data meant that several test runs had to be carried out in order to stabilize the temperature rise in the ohmic heater.

A suitable amount of antifoaming agent for the strawberry jam was recommended by the company.

4.4.5. DETERMINATION OF PROCESSING PARAMETERS FOR THE MICVAC METHOD

The 400g Flextray, as recommended by MicVac AB and which can be seen in appendix VII, was filled with 50°C strawberry jam and mixed with varying amounts of antifoaming agent. The trays were sealed with the MicVac film and processed in the microwave oven for 5 minutes to ensure proper vacuum conditions.

The processing time in the MicVac method was decided based on test runs and on recommendations from MicVac AB after determining a suitable amount of antifoaming agent.

28

4.5. JAM PRODUCTION PROCEDURES

Since only the impact of the heating step of each method was to be compared, the studied temperature interval was chosen to range from 5°C, which was the temperature directly after mixing of the strawberries and sugar, up to the maximum processing temperature and down to 55°C again. The pre-heating step up to 55°C and the cooling step from 55°C down to room temperature were carried out as similarly as possible in the studied methods. The jam samples were produced according to table 8, where the results from the process development trials are presented.

TABLE 8. JAM PRODUCTION METHODOLOGY

Reference Ohmic Heating MicVac Batch size 1400 kg 20 kg 20 kg Frozen strawberries Intact Sliced Sliced Amount of pectin Initial Decreased by 10 % Decreased by 10 % Antifoaming agent As recommended As recommended 10 times as

recommended Pre-heating to 55°C X X X Addition of pectin 85°C 55°C 55°C Lowering of steam vessel pressure X X X From steam vessel to ohmic heater or microwave oven

* Pumping, 5°C temperature drop

Manual filling into trays, sealing and loading into microwave oven

Equipment effect * 74 % 440 W Processing time in ohmic heater and microwave oven

* 2 min (69 kg/h) 4 min

Maximum processing temperature

90°C 90°C 103°C

Filling Filling machine Hot-fill, manual At 50°C, before processing, manual

Cooling To 60°C, in steam vessel, 45 min To 55°C, in cooling tunnel

To 55°C, in freezer, 30 min

To 55°C, in freezer, 30 min

*Not applicable or investigated for the reference jam.

Samples were taken out and analyzed according to table 9.

TABLE 9. ANALYSES TIMETABLE

Parameter Storage temperature (°C) Weeks after production Vitamin C 22

35 0, 1, 2, 4 1, 2

pH 22 35

0, 1, 2, 4 1, 2

Soluble solids 22 35

0, 1, 2, 4 1, 2

Viscosity 22 35

0, 1, 2, 4 1, 2

Color (L,a,b and spectral reflectance)

22 35

0, 1, 2, 4 1, 2

Viable count, yeasts and moulds 22 35

0 1, 2

29

4.5.1. PRODUCTION OF REFERENCE JAM

Samples of the reference jam were collected from the factory and stored at ambient (22°C) and at accelerated conditions (35°C). The process chart for production of the reference jam can be seen in figure 12. After pre-heating the berries and sugar to 85°C at 0.6 bars, the pectin solution and the remaining ingredients were added and heating continued to 90°C. The mixture was held there for 3 minutes and then cooled to 60°C, the pH, soluble solids and viscosity controlled and the jam filled into sterilized glass jars. Samples were subsequently cooled in a cooling tunnel to 55°C.

FIGURE 12. REFERENCE JAM PROCESS CHART

4.5.2. PRODUCTION OF REFERENCE JAM, SHORT COOLING

A 20 kg batch of the reference jam but with a shortened cooling step was produced in the pilot plant steam vessel. The processing procedure up to holding at 90°C was equal to the one when producing the reference jam in the factory. However, it was then hot-filled at 90°C and cooled according to the ohmic heating cooling procedure, described in table 8. The process chart can be seen in figure 13.

FIGURE 13. REFERENCE JAM, SHORT COOLING PROCESS CHART

∞

∞

30

4.5.3. PRODUCTION OF OHMIC HEATING JAM

The ohmic heating jam was produced according to the procedure described in table 8 and figure 14. Since the ohmic heating process of this project had not previously been adapted to strawberry jam production, three final batches were produced to test for reproducibility. Three registrations of process data per batch such as temperatures and applied current were made during the production of the final samples.

FIGURE 14. OHMIC HEATING JAM PROCESS CHART

4.5.4. PRODUCTION OF MICVAC JAM

The MicVac jam was produced according to the procedure described in table 8 and figure 15. Since the MicVac method process of this project had not previously been adapted to strawberry jam production, three final batches were produced to test for reproducibility. 15 trays per batch were processed and initial weight, final weight and evaporated steam registered for each tray.

FIGURE 15. MICVAC JAM PROCESS CHART

∞

∞

31

5. RESULTS AND DISCUSSION

5.1. PROCESS DEVELOPMENT

Strawberry density proved to be a difficult parameter to measure. Even though replicates were taken, the measurements did not result in a significant increase in density. The fact that an increase in density of 6 % was observed was probably due to the combined effect of sugar uptake and strawberry compression. When realizing the difficulties to implement such a method as a pre-treatment step in an industrial production process, the search for a simpler way of reaching an even distribution of strawberries before the thermal processing step became desirable.

The results from the conductivity trial can be seen in figure 16 and clearly show the diversity in conductivity between individual strawberries, but also the difference in conductivity between the berries and their surrounding medium. The homogenized jam was quite similar in conductivity to the reference jam obtained from the factory, indicating that long processing times help to equalize the conductivity between the two phases. This probably happens through the leaching of electrolytes from the strawberries due to shearing and disruption of the structure.

FIGURE 16. CONDUCTIVITIES OF JAM STRAWBERRIES AND MEDIA

y = 3,4216x - 10,697

y = 3,7192x - 108,8

y = 3,0375x - 5,5222

y = 4,6253x - 70,329

y = 5,9664x - 203,28

0

100

200

300

400

500

30,0 40,0 50,0 60,0 70,0 80,0

Cond

uctiv

ity (m

S/m

)

Temperature (°C)

Reference jam

Reference Jam (Lab-scale)

Homogenized jam

Jam 0.1% NaCl

Jam 0.05% NaCl

Strawberries from reference (lab-scale)

32

As seen in figure 16, salt addition increased the conductivity of the medium. The sensory evaluation of these jams indicated that an addition of salt, even as small as 0.05%, resulted in an off-flavor with a metal character that covered the strawberry flavor. As this addition did not increase the conductivity of the jam medium up to the level of the strawberries and because it is questionable to add salt to such a product, it was decided that another way of equalizing the conductivity had to be found.

Based on the information in section 2.1.2. and 2.2.1., it was decided that a good way to equalize the product components such as sugar and water more rapidly would be to use pieces instead of intact strawberries. This would also help to distribute the strawberries evenly and help to reach a more uniform heating pattern in the ohmic heater. It was therefore decided that the strawberries would be sliced when frozen, as seen in figure 17b, before adding them to the steam vessel.

FIGURE 17a. INTACT STRAWBERRIES

FIGURE 17b. SLICED STRAWBERRIES

The setting point of the recipe pectin of approximately 50°C provided a limitation of great importance to the pre-heating step. An addition of the pectin solution to the steam vessel below this temperature would namely result in pre-gelation of the pectin and an uneven formation of the gel network in the finished product. The decision point regarding this was therefore to add the pectin solution at 50°C, which is about the same as the final temperature of the pre-heating step when producing the reference jam in the factory. It was also decided that the amount of pectin should be lowered by 10 % to account for the fact that the product would be less sheared in the pilot plant than in the industrial process.

Mixing of the ingredients as well as lowering of the pressure to facilitate the mixing process during pre-heating was chosen to be carried out as similar to the procedure in the factory as possible. A limitation during the pre-heating step was the difficulty to regulate the flow of steam into the jacketed steam vessel. It was therefore decided that the heating rate up to 50°C did not have to be exactly as long as in the factory, which is 30 minutes. This decision was supported by calculated reaction kinetic values for microbial inactivation and thermal color degradation which showed that the contribution from the pre-heating step was negligible.

33

A temperature rise up to 90°C in the ohmic heater was chosen as this is the maximum temperature when producing the reference jam in the factory. There was however no possibility to control the final processing temperature in the MicVac method since this is, simply put, determined by the boiling point of water. A maximum temperature of approximately 103°C was observed in the trials. This increase of the boiling point of water can be explained by the high sugar content of the jam and by the pressure increase during processing. As ohmic heating and the MicVac method have the capacity to work with larger temperature spans, this would have been done if not for the limitations in the pre-heating step due to the pectin.

A starting flow rate of 69 kg/h for the ohmic heating trials was calculated using equation 1-3 and 7-9. The calculated flow rate was set on the pump by adjusting its effect to 4 Hz and was suitable since the small batch size of the steam vessel limited the flow rate. Furthermore, at this low flow rate it took some time to stabilize the temperature out of the ohmic heater.

𝑐𝑝,𝑗𝑎𝑚,50°𝐶 = 0.46 ∗ 1.632 + (1 − 0.46) ∗ 4.185 = 3.01 𝐽/𝑘𝑔°𝐶

𝑐𝑝,𝑗𝑎𝑚,90°𝐶 = 0.46 ∗ 1.677 + (1 − 0.46) ∗ 4.212 = 3.05 𝐽/𝑘𝑔°𝐶

𝑐𝑝,𝑗𝑎𝑚,𝑎𝑣𝑒𝑟𝑎𝑔𝑒 = 3.03 𝐽/𝑘𝑔°𝐶

𝑅 = 𝐿

𝜎𝑎𝑣𝑒𝑟𝑎𝑔𝑒 ∗ 𝐴=

0.060.2 + 0.11

2 ∗ 0.25 ∗ 0.075= 20.6 𝑜ℎ𝑚

𝑃 = 𝑈2

𝑅=

2202

20.6= 2350 𝑊

𝐹 =𝑃

𝑐𝑝,𝑗𝑎𝑚,𝑎𝑣𝑒𝑟𝑎𝑔𝑒 ∗ ∆𝑇=

23503030 ∗ (90 − 50)

= 0.019𝑘𝑔𝑠

= 69𝑘𝑔ℎ

After performing several trial runs in the pilot plant, an applied voltage of 74 % of the maximum available voltage (230V) was noted as enough to reach a temperature of 90°C out of the ohmic heater. It was decided that since the temperature of the strawberry jam decreased on its way to the ohmic heater inlet, the holding temperature of the steam vessel would be set to 55°C to compensate for this.

To facilitate the production process, hot-fill was chosen as the most suitable filling method. The use of hot-fill as the filling method in ohmic heating can be questioned; in the pilot plant, it resulted in floating berry pieces in the jars which can partly be explained by the low viscosity of the jam at 90°C. A more suitable filling method could be to cool the jam, using a holding tank or a heat exchanger, to the setting temperature of the pectin at around 50°C. The pectin network would then help to evenly distribute the strawberry pieces in the gel upon stirring of the jam. Another solution would be to use a pectin more suitable for hot-fill, with a higher setting temperature. This alternative was not possible in this project since a reference recipe was the basis for evaluation of the different thermal processing techniques.

34

Tests on how to cool the samples showed that a more effective way than to cool them using a water bath was to cool the ohmic heating and MicVac samples to 55°C in a walk-in freezer, which took 30 minutes. The samples would then be stored at room temperature.

Regarding the amount of antifoaming agent in the MicVac recipe, ten times the amount of the ohmic heating recipe was chosen as this amount visibly resulted in a successful cooking process. A processing time in the microwave oven of 4 minutes was chosen, since the continuous whistling of the valve occurred after approximately 3 minutes. The extra minute ascertained that the cooking process was correctly performed and that an adequate vacuum had been achieved.

A summary of the decision points from the process development trials can be seen in table 8 and detailed data from these trials in appendix IV and V.

35

5.2. JAM PRODUCTION

Pilot plants for ohmic heating and the MicVac method were successfully installed during the beginning of the project. Thanks to this, the process development trials resulted in well-functioning jam production procedures for these methods. More comparable results could probably have been reached if intact berries had been used in all of the methods. However, this was not possible due to limitations in the ohmic heating and MicVac jam pilot plants. In the steam vessel, berry distribution was uneven when intact strawberries were used. The low flow rate and the dead angles in the ohmic heater also limited the use of intact strawberries. Another important aspect was that the short thermal treatment after pre-heating would not have been sufficient to even out the added sugar and the water from the berries and would have resulted in an unstable product prone to phase separation. The pre-heating step and use of sliced berries were therefore considered as necessary for successful strawberry jam production procedures for ohmic heating and the MicVac method.

5.2.1. THERMAL PROCESSING STEPS

Measured data from the production of the final samples can be seen in table 10 and table 11.

TABEL 10. PROCESS DATA FOR OHMIC HEATING, FINAL SAMPLES (MEAN±SD, N=9) Output effect (%)

I (A) Temperature, steam vessel (°C)

Temperature, OH in (°C)

Temperature, OH out (°C)

Temperature, filling (°C)

74±1 12±0 57±2 50±1 94±3 88±2

The difficulty to stabilize the temperature in the ohmic heater is illustrated by the variation in jam temperature out of the ohmic heater, presented in table 10. This was found to be due to the low flow rate and the manual control of applied voltage, leading to a slow temperature stabilization process.

TABLE 11. PROCESS DATA FOR MICVAC, FINAL SAMPLES (MEAN±SD, N=45) Initial weight (g) Final weight (g) Loss of steam (%) Continuous whistle after (min:sec) 426.2±2.2 417.3±2.8 2.1±0.3 02:57

As seen in table 11, the low variations in amount of evaporated steam between trays in the MicVac method indicated that the method was reproducible.

The temperature profiles the from production of the different jam samples can be seen in figure 18, where 0 is the time immediately after addition of all of the sugar and strawberries to the steam vessel. The temperature profile for the reference jam was obtained from the factory. Temperature profile data can be found in appendix VI.

36

FIGURE 18. TEMPERATURE PROFILES

The results from the measurements of yeasts, moulds and total count were that nothing grew on the agar plates during incubation of the produced jam samples. Due to the hurdle effect and based on these results, the conclusion can be drawn that all of the methods produced microbiologically shelf-stable products.

In order to compare the impact of the thermal processing steps of the different methods, F90 values were calculated based on the z-value for ascospores. The calculated data can be seen in table 12 and a definition of the F value can be found in appendix II. The number of decimal reductions presented in table 12 were based on enzyme and ascospore kinetics calculated from the data in table 1, 3 and 4 using equation 10 below.

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑐𝑖𝑚𝑎𝑙 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛𝑠 = 𝐹𝑇𝑟𝑒𝑓/𝐷𝑇𝑟𝑒𝑓 (equation 10, [40])

TABLE 12. F90 AND NUMBER OF DECIMAL REDUCTIONS

Antho-cyanins PME PPO

Ascospore (Byssochlamys

nivea)

Ascospore (Neosartorya

Fischeri LT025) Yeasts, moulds

Method

F90 (min) for z=6.4

Number of decimal reductions

Reference 30 0.07 1350 820 6 20 >104 Reference, short cooling

19 0.06 950 450 4 13 >104

Ohmic Heating 21 0.01 730 430 4 14 >104 MicVac 1900 0.10 14000 5200 400 1300 >106

0

10

20

30

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80 90 100 110 120

Tem

pera

ture

(°C)

Time (min)

Reference

Reference, short cooling

Ohmic Heating

MicVac

37

The mildest method regarding thermal anthocyanin degradation was ohmic heating with 0.01 decimal reductions, which can be ascribed to the short thermal processing step of this method. However it was also mildest concerning enzyme inactivation. The number of decimal reductions of yeasts and moulds were numerous, indicating a more than sufficient pasteurization step in all of the studied methods concerning these microorganisms. Although the F90 value is a good way to compare the severity of thermal processes, it may give somewhat biased or angled results when the studied processes have different maximum processing temperatures. The high F90 and number of decimal reductions of the MicVac jam can be explained by the higher processing temperature of the method, a maximum of 103°C compared to approximately 90°C in the other methods. When comparing the F90 values of the reference jam and the jam with the shorter cooling step, it became clear that the cooling step had an impact on the degradation of ascospores and enzymes.

The difficulties with controlling the heating rate during pre-heating up to 55°C resulted in a shortened pre-heating step for the ohmic heating and MicVac jams by almost 50 %. However, the contribution from the pre-heating to the F90 values of all of the methods turned out to be negligible and the methods therefore satisfactorily comparable regarding microbial inactivation. Nothing can be said about how the volatile flavor compounds of the strawberries were affected by the longer pre-heating steps of the reference jams.

If the cooling step down to 55°C in an industrial ohmic heating process would be assumed to be similar to the pilot plant process, the calculated F90 values would be directly applicable to an industrial scale process. Since the pilot-scale MicVac method is comparable to the industrial process, this would also be the case for the MicVac method. The conclusion was therefore that the thermal processing steps of the studied jam production technologies have been appropriately compared.

5.2.2. REPROCUCIBILITY

The pH of all of the samples was 3.4±0.0 and remained stable during storage. The flow property values, presented in table 13, were almost constant throughout the storage period. The variations could be referred to the insensitivity of the method of measurement.

TABLE 13. FLOW PROPERTIES (MEAN±SD, N=3) Temperature (°C)

Time of analysis

Reference Reference, short cooling

Ohmic Heating MicVac

22°C 0 w 4.8±0.6 4.3±0.3 4.2±0.4 3.8±0.7 1 w 3.5±1.3 3.3±0.3 3.5±0.3 3.7±0.7 2 w 4.7±0.3 4.2±0.3 3.9±0.4 3.9±0.5 4 w 4.5±0.0 - - - 35°C 1 w 5.2±0.6 4.7±0.6 3.6±0.7 4.2±0.8 2 w 4.7±0.6 5.3±0.6 4.3±0.8 4.9±0.7

The soluble solids values, presented in table 14, remained within the allowed interval of the reference jam which can be seen in table 6. The slightly higher value of the MicVac jam can be referred to the loss of steam during processing.

38

TABLE 14. SOLUBLE SOLIDS (MEAN±SD, N=3) Temperature (°C)

Time of analysis

Reference Reference, short cooling

Ohmic Heating MicVac

22°C 0 w 45.5±0.1 46.1±0.1 46.2±0.5 46.5±0.4 1 w 45.5±0.1 46.0±0.0 46.5±0.4 48.4±0.6 2 w 45.6±0.0 45.5±0.0 46.7±0.1 48.6±0.3 4 w 45.7±0.0 45.9±0.1 46.4±0.8 48.7±0.3 35°C 1 w 45.7±0.0 45.8±0.0 46.4±1.1 48.0±0.4 2 w 46.1±0.0 45.4±0.0 47.2±0.3 48.8±0.4

The variations in pH, soluble solids and flow properties between the three final ohmic heating and MicVac batches were small and these methods were therefore considered as reproducible.

5.2.3. VITAMIN C

The analyses of vitamin C content of the jam samples resulted in the data presented in figure 19. Detailed vitamin C data can be seen in appendix VII.

FIGURE 19. VITAMIN C CONTENT

The initial vitamin C values indicated that the differences in thermal processing between the analyzed methods influenced the ascorbic acid content. Initially, the vitamin C content of the ohmic heating jam and the MicVac jam was significantly higher than of the two reference jams. The higher values can be related to the shorter thermal processing steps of these methods. After storage at 22°C for 4 weeks, the reference jam contained 61 % of its initial content, the ohmic heating jam 77 % and the MicVac jam 32 % of its initial content. The MicVac jam contained the lowest amount of vitamin C after the storage period even though its initial content was in the same range as of the ohmic heating jam. The quicker degradation could possibly be related to a more pronounced oxidative degradation due to a lower oxygen barrier of the MicVac tray and film. If the vitamin C content had been influenced by the higher processing temperature in spite of the shorter processing time, this should have been seen as a lower initial vitamin C content.

0

5

10

15

20

25

30

Initial 1w,22°C 2w,22°C 4w,22°C 1w,35°C 2w,35°C

Vita

min

C C

onte

nt (m

g/10

0g)

Reference

Reference, short cooling

MicVac

Ohmic Heating

39

After accelerated storage at 35°C for 2 weeks, the sample containing the lowest amount of vitamin C was the MicVac jam, less than 20 % of its initial content. The ohmic heating jam retained its vitamin C content better than the other methods, with 64 % of its initial content left after accelerated storage.

The slightly higher vitamin C content of the reference jam with the short cooling step could indicate that the cooling step influenced product quality. However, no conclusions could be drawn because no differences were observed over time for the samples stored at 22°C. The phenomenon might be explained by the fact that it was difficult to control the berry distribution and shearing of these samples. This resulted in a strawberry content higher than that of the reference jam in many of the jars, as well as more intact berries. This could also have made it difficult to measure the vitamin C content of these samples. If samples had instead been taken out from the factory steam vessel at 90°C, a more even berry distribution would possibly have been reached. This option was unfortunately not thought of until late into the project. Samples would probably also have been more similar to the reference jam due to more vigorous stirring, resulting in less intact berries. These theories could probably explain why the vitamin C content of this jam remained stable during storage at 22°C.

In table 15, calculated vitamin C kinetic data based on the averages of the measured values at 22°C (initial, 1, 2 and 4 weeks) and 35°C (initial, 1 and 2 weeks) are presented. The calculations were based on the equations in appendix II and it was assumed that the activation energy was constant in the present temperature interval.

TABLE 15. CALCULATED VITAMIN C KINETIC KONSTANTS

Method Ea (kJ/mol) kT (min-1) Reference 46.8 k22 = 1.18·10-5

k35 = 2.64·10-5 Reference, short cooling 158 k22 = 0.14·10-5

k35 = 2.05·10-5 Ohmic Heating 50.5 k22 = 1.21·10-5

k35 = 2.89·10-5 MicVac 64.4 k22 = 2.75·10-5

k35 = 8.31·10-5

The calculated kinetic for vitamin C degradation was in the same range for the different jam production methods. However, the activation energy for the reference jam with the short cooling step was higher than for the other samples. This could be explained by the fact that the vitamin C content of this jam remained stable during storage at 22°C, as previously discussed. To be able to draw more precise conclusions, more replicates should have been taken but there was no possibility to do so due to limited resources.

40

5.2.4. COLOR

The color measurements resulted in the L-, a-, b- and reflectance values presented in figures 20a-d. The measured values and standard deviations were too diverse for any conclusions to be drawn.

FIGURE 20a. REFLECTANCE VALUES, 650 nm

FIGURE 20b. a VALUES

FIGURE 20c. b VALUES

FIGURE 20d. L VALUES

0,00

1,00

2,00

3,00

4,00

5,00

6,00

Refle

ctan

ce

0

5

10

15

20

25

30

a-va

lue

02468

1012141618

b-va

lue

02468

1012141618

L-va

lue

41

Changes in color of the different samples were however observed visually but were not detectable by the spectrophotometer. The photographs of one and eight week old samples placed next to each other on two separate plates, presented in figure 21, clearly show that that was in fact the case.

FIGURE 21. REFERENCE, OHMIC HEATING AND MICVAC JAM (LEFT-RIGHT) AFTER 1 (ABOVE) AND 8 (BELOW) WEEKS AT 22°C

Figure 22a-b show that differences in color were also observed during accelerated storage of samples. After one week of accelerated storage, the ohmic heating jam had preserved most of its initial color whilst the MicVac jam had become brown.

FIGURE 22a. OHMIC HEATING JAM, 1W22°C AND

1W35°C (LEFT-RIGHT)

FIGURE 22b. MICVAC JAM, 1W22°C AND 1W35°C

(LEFT-RIGHT)

The measured L, a, and b values were in the range of what was observed in other studies of strawberry products. However, due to the low variability in data over time and the large standard deviations, no conclusions could be drawn regarding color changes, although visible changes in color were in fact observed. Likely explanations as to why this occurred are faulty spectrophotometer settings or disadvantageous preparation of samples such as the interference of present berry pieces, which could have been avoided by homogenizing the samples before measurements.

42

Possibly, another method such as determination of anthocyanin content by chromatography could have been carried out. However, it would have been more time-consuming and costly, and even though it would have provided a good insight in the degradation of anthocyanins, it would not have given any straightforward indications about the consumer acceptance of the analyzed samples.

What was observed in the photographs was that the MicVac jam became brown faster than the other samples during the storage period. This could possibly be explained by a low oxygen barrier of the MicVac tray and film or by the higher processing temperature of this method, resulting in the production of a large amount of Maillard compounds. The ohmic heating jam retained its red color better than the other samples after the end of the storage period. This could be due to its mild processing conditions, preserving the anthocyanins and reducing the production of Maillard components. More photographs should have been taken that could have further confirmed the observed visible changes.

The even distribution of strawberry pieces in the ohmic heating jam seemed to have contributed to its brighter red color: it was less turbid and homogenous than the reference jam and had more distinct strawberry pieces. In the reference jam, the strawberries seemed to have been sheared and disrupted to a greater extent during production, giving it a homogenous appearance with a matte deep red color.

5.2.5. SENSORY EVALUATION

The calculated confidence intervals based on the normalized results from the sensory evaluation can be seen in figure 23-25. Where any participant did not reply, the median number of the other participants’ answers was used.

The only jam that differed significantly from the other samples was the MicVac jam, except for when comparing strawberry chewability where it did not differ significantly from the ohmic heating jam.

FIGURE 23. STRAWBERRY CHEWABILITY

In figure 23, the reference jam with a shorter cooling step differed significantly from the MicVac and the ohmic heating jam. This can be explained by that many of the participants seemed to appreciate more intact berries and by the fact that the berries of this jam had been less sheared during processing.

0,70,80,9

11,11,21,3

Reference Reference, short cooling

MicVac Ohmic Heating

43

An explanation to the low appreciation of berry chewability of the MicVac jam could be due to the more vivid processing, which may have considerably softened the berry structure, resulting in a more homogenous product. Based on the participants’ comments, the ohmic heating jam was perceived to contain more strawberries than it actually did, possibly because of its even distribution of intact yet small berry pieces.

FIGURE 24. STRAWBERRY FLAVOR

The MicVac jam received significantly lower scores than the other jam samples for strawberry flavor. An explanation of this could be that the higher processing temperature resulted in a more severe degradation of flavor compounds and the production of caramelization and Maillard compounds that may have masked the strawberry flavor. Another explanation could be that as steam evaporated during the thermal processing, a lot of volatile flavor compounds accompanied it. This was in fact observed during the production of jam samples as a distinct strawberry odor.

As caramelization reactions can be related to flavor changes during processing at temperatures above 100°C, this could explain why the MicVac jam differed in flavor from the other jam samples. The similar yet not as pronounced flavor profile of the reference jam could be associated with the use of hot surfaces during processing, enabling the formation of undesirable caramelization compounds that could potentially mask the strawberry flavor. On the contrary, ohmic heating is not dependant on any hot surfaces and the ohmic heating jam did not reach temperatures above 100°C. This could explain the absence of these flavor compounds in the ohmic heating jam. Also, the participant’s comments clearly showed that a more distinct and appreciated strawberry flavor, not covered by sugar flavors, were observed for the ohmic heating jam.

FIGURE 25. OVERALL IMPRESSION

0,70,80,9

11,11,21,3

Reference Reference, short cooling

MicVac Ohmic Heating

0,70,80,9

11,11,21,3

Reference Reference, short cooling

MicVac Ohmic Heating

44

Regarding overall impression, the only sample that differed significantly from the others was the MicVac jam. Sliced strawberries were used for the production of the ohmic heating and Micvac jams but not for the reference jams. This may have affected the results from the sensory evaluation regarding overall impression in the sense that people appreciating larger berry pieces or intact berries may have given lower scores to the jams containing sliced berries, even though they may have appreciated these jams regarding strawberry flavor.

In table 16, the results from the ranking test are presented, together with the ranking sum of the different samples.

TABLE 16. RANKING OF SAMPLES (N=36) Number (out of 36) Ranking score (1=highest) Reference Reference,

short cooling Ohmic

Heating MicVac

1 11 6 15 4 2 12 14 8 2 3 8 9 10 9 4 5 7 3 21 1+2 23 20 23 6 3+4 13 16 13 30 Ranking sum = Σ(score x number) 79 89 73 119

In the ranking test, the MicVac jam received the highest ranking sum (119 points), indicating that it was the least appreciated jam. The sample receiving the lowest ranking sum was the ohmic heating jam (73 points) even though it did not differ a lot from the reference jam (79 points).

Based on the participants’ comments, two groups could be identified where one seemed to appreciate the traditional, cooked, jam-like flavors of the reference jam and the other one the fresh, intense strawberry flavors of the ohmic heating jam. This was also confirmed by the ranking test, where these jams received similar ranking sums.

When comparing the produced jams after two months of storage at room temperature, it was observed that the ohmic heating jam had retained most of its original flavors but that the MicVac jam had developed a sharp, acrid odor as well as a cooked flavor. The perceived flavor changes during storage could probably be explained by the formation of Maillard compounds during processing, the reaction rate increasing with temperature. Oxidation and polymerization reactions of these compounds during storage may, together with caramelization compounds formed during the thermal processing, have contributed to the less appreciated flavor profile of the MicVac jam.

45

6. PROJECT CONCLUSIONS

The selected methods for analysis as well as the selected and installed pilot-plant equipment provided useful information which helped to draw conclusions about the impact of the different thermal processing methods regarding the investigated product quality parameters.

The process development resulted in well-functioning jam production procedures for ohmic heating and the MicVac method respectively. A pre-heating step to ensure product homogeneity and stability was identified as necessary, as well as proper mixing of all of the ingredients before the closed-system thermal processing step of these methods.

The assumptions that were made concerning the thermal processing steps enabled the comparison of the different thermal processing technologies on an industrial level regarding color, flavor, vitamin C content and microbial inactivation.

The strawberry jam recipe was easily adaptable to the already existing and well-devised MicVac method. However, due to limitations in process temperature control and packaging materials, the method was found to be disadvantageous for the production of sensitive strawberry jams regarding vitamin C, flavor and color retention.

The rapid internal heating and independence of hot surfaces in ohmic heating enabled the production of a strawberry jam with better preserved color, strawberry flavor and vitamin C content compared to the reference jam. Conclusively, the negative influences on product quality related to thermal processing methods based entirely on convection could probably be avoided by the use of ohmic heating.

46

7. ACKNOWLEDGEMENTS

This master’s thesis could not possibly have been carried out without the support from Procordia Food AB, C-tech Innovation Ltd and MicVac AB.

We would especially like to thank our supervisors Richard Clerselius at Procordia Food AB and Ingegerd Sjöholm at Lund University for guiding and encouraging us during this project.

To Norman Maloney at C-tech Innovation Ltd: we are ever so thankful for your help with installing the ohmic heater and for your technical support throughout the project. It really helped us to reach successful results.

And to Olle Olofsson at MicVac AB: thank you for all of your advice throughout the project and for expressing your sincere interest in our work.

We would also like to show our gratitude to Helén Broström, Kristina Fransson, Magnus Dahlberg and the other employees at the Tollarp factory for answering all our questions and for encouraging us by expressing your interest in our project.

We owe our deepest gratitude to everyone at the quality and chemical laboratory at Procordia Food AB for helping us analyze our samples whenever we asked.

To Blåtand AB (Stöde, Sweden) and SIK - the Swedish Institute for Food and Biotechnology (Gothenburg, Sweden): thank you for sharing your knowledge and experience with us.

Last but not least, we would like to send a million thanks to all of the employees at Procordia’s R&D department for regarding us as co-workers from our very first day there, for always answering our questions and for being such wonderful people!

47

8. REFERENCES

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2. Worobo R.W., Splittstoesser D.F., Microbiology of Fruit Products, in Barrett D.M., Somogyi L.P., Ramaswamy H.S. (editors), Processing Fruits: Science and Technology, 2nd edition, CRC Press, 2005

3. Suutarinen J., Honkapää K., Heiniö R-L., Autio K. and Mokkila M., The Effect of Different Prefreezing Treatments on the Structure of Strawberries Before and After Jam Making, Lebensmittel-Wissenschaft und-Technologie, 2000; 33:188-201

4. Viberg U., Studies of the effects of Industrial Processing of fruit, Lund: The department of Food Engineering, Lund University, 1998, 5-47

5. Jewell G.G, Rantosios A., Scholey J., Factors influencing the breakdown of the fruit in strawberry jam, Journal of Texture Studies, 1973; 4: 363-370

6. Coultate T.P., Food – The chemistry of its components, 5th edition, Cambridge, RSC Publishing, 2009, 60-67

7. Zamora A., Carbohydrates – Chemical Structure, Scientific Psychic, 2005, available at: http://www.scientificpsychic.com/fitness/carbohydrates2.html (accessed 2011-01-09)

8. Bergenståhl B., Polysaccharides and hydrocolloids - pectin network, lecture material, 2009 9. Nogata Y., Ohta H., Voragen A. G. J., Polygalacturonase in strawberry fruit, Phytochemistry, 1993;

34:617-620 10. Duvetter T., Sila D.N., Van Buggenhout S., Jolie R., Van Loey A., Hendrickx M., Pectins in processed fruit

and vegetables: Part I-Stability and catalytic activity of pectinases, Comprehensive reviews in food science and food safety, 2009; 8:75-85

11. Villota R., Hawkes J.G., Reaction kinetics in food systems, in Heldman D.R., Lund D.B. (editors), Handbook of food engineering, Taylor and Francis Group, 2007, 198-243

12. Terefe N.S., Yang Y.H., Knoerzer K., Buckow R., Verseeg C., High pressure and thermal inactivation kinetics of polyphenol oxidase and peroxidase in strawberry puree, Innovative Food Science and Emerging Technologies, 2010; 11:52-60

13. Wikimedia commons, Flavylium ion, Wikimedia, 2011-01-16, available at: http://commons.wikimedia.org/wiki/File:Flavylium-Ion.svg (accessed 2011-01-20)

14. Agrool, pelargonidin-3-glucoside, available at: http://www.agrool.gr/files/pelargonidin%203-glycoside.jpg (accessed 2010-10-01)

15. Agrool, cyanidin-3-glycoside, http://www.agrool.gr/files/cyanidin%203-glycoside.jpg (accessed 2010-10-01)

16. Wesche-Ebeling P. and Montgomery Morris W., Strawberry Polyphenoloxidase: Its Role in Anthocyanin Degradation, Journal of Food Science, 1990; 55(3):731-733

17. Gössinger M., Ullram T., Hermes M., Wendelin S., Berghold S., Halbwirth H., Stich K., Berghofer E., Effects of pre-freezing, puree content and pasteurization regime on colour stability of strawberry nectar made from puree, J Sci Food Agric, 2009; 89:144-149

18. Sadilova E., Carle R., Stintzing F.C., Thermal degradation of anthocyanins and its impact on color and in vitro antioxidant capacity, Mol. Nutr. Food Res., 2007; 51:1461-1471

19. Dalmadi I., Rapeanu G., A. Smout van Loey, C., Hendrickx M., Characterization and Inactivation by Thermal and Pressure Processing of Strawberry (Fragaria Ananassa) Polyphenol Oxidase: A Kinetic Study, Journal of Food Biochemistry, 2005, 30:56–76

20. López-Serrano M., Ros Barceló A., Comparative study of the products of the peroxidase-catalyzed and polyphenoloxidase-catalyzed (+)-catechin oxidation. Their possible implications in strawberry (Fragaria x ananassa) Browning reactions, Journal of Agricultural and Food Chemistry, 2002; 50:1218-1224

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21. Chisari M., Barbagallo R.N., Spagna G., Characterization of polyphenol oxidase and peroxidase and influence on browning of cold stored strawberry fruit, Journal of Agricultural and Food Chemistry, 2007; 55(9):3469-3476

22. Scienceray, The oxidation of an apple, 2009-03-08, available at: http://scienceray.com/chemistry/the-oxidation-of%20an-apple/ (accessed 2010-10-01)

23. Coultate T.P., Food – The chemistry of its components, 5th edition, Cambridge, RSC Publishing, 2009, 233-236, 332-338

24. Aslanovaa D., Bakkalbasia E., Artik N., Effect of Storage on 5-Hydroxymethylfurfural (HMF) Formation and Color Change in Jams, International Journal of Food Properties, 2010; 13: 904-912

25. Coultate T.P., Food – The chemistry of its components, 5th edition, Cambridge, RSC Publishing, 2009, 30-41

26. Deuel C.L and Plotto A., Strawberries and Raspberries, in Barrett D.M., Somogyi L.P., Ramaswamy H.S. (editors), Processing fruits, CRC Press, 2005

27. Coultate T.P., Food – The chemistry of its components, 5th edition, Cambridge, RSC Publishing, 2009, 297-301

28. Cadwallader K.R., Flavor and volatile metabolism in produce, in Ukuku D., Imim S., Lamikanra O. (editors), Produce degradation, CRC Press, 2005, 155-189

29. Siegmund B., Bagdonaite K., Leitner E., Furaneol and mesifuran in strawberries – an analytical challenge, Expression of Multidisciplinary Flavour Science, 2008; 537-540

30. Sun-Pan B., Kuo J-M., Wu C-M., Flavor Compounds in Foods, in Sikorski Z.E. (editor), Chemical and Functional Properties of Food Components, 3rd edition, CRC Press, 2007, 295-328

31. Viberg U., Studies of the effects of Industrial Processing of fruit, Lund: The department of Food Engineering, Lund University, 1998, 47-52

32. Livsmedelsverket, Näringsinnehåll i mat: jordgubbar, Livsmedelsverket, 2010-10-01, available at: http://www.slv.se/sv/grupp1/Mat-och-naring/Vad-innehaller-maten (accessed 2010-11-20)

33. Villota R., Hawkes J.G., Reaction kinetics in food systems, in Heldman D.R, Lund D.B. (editors), Handbook of food engineering, Taylor and Francis Group, 2007, 145-182

34. Livsmedelsverket, Hur påverkas maten när den tillagas?, Livsmedelsverket, 2010-12-27, available at: http://www.slv.se/sv/grupp1/Mat-och-naring/Vad-innehaller-maten/Naringsvinster-och-naringsforluster-vid-matlagning (accessed 2011-01-10)

35. Wikipedia, Ascorbic acid structure, 2007-02-22, available at: http://sv.wikipedia.org/wiki/Fil:Ascorbic_acid_structure.png (accessed 2011-01-14)

36. Wikipedia, Dehydroascorbicacid, 2007-10-01, available at: http://sv.wikipedia.org/wiki/Fil:Dehydroascorbic_acid.png (accessed 2011-01-17)

37. Castro I., Teixeira J.A., Salengke S., Sastry S.K., Ohmic heating of strawberry products: electrical conductivity measurements and ascorbic acid degradation kinetics, Innovative Food Science and Emergeing Technologies, 2004; 5:27-36

38. Marques da Silva F.V., Gibbs P.A., Principles of Thermal Processing: Pasteurization, in Simpson R. (editor), Engineering Aspects of Thermal Food Processing, CRC Press, 2010

39. Figuerola F.E., Berry jams and jellies, in Y. Zhao (editor), Berry Fruit: Value-Added Products for Health Promotion, CRC Press, 2007, 367-386

40. Adams M.R., Moss M.O., Food Microbiology, 3rd edition, UK, RSC Publishing, 2008, 63-78 41. Livsmedelsverket, Livsmedelsverkets Föreskrifter om Sylt, Gelé och Marmelad, Livsmedelsverket, 2003,

LIVSFS 2003:17 42. Lopes da Silva J.A., Rao M.A., Pectins: structure, functionality and uses, in Stephen A.M., Phillips G.O.,

Williams P.A. (editors), Food Polysaccharides and Their Applications, CRC Press, 2006, 354-397 43. Li F-D., Zhang L., Ohmic Heating in Food Processing, in Farid M.M. (editor), Mathematical Modeling of

Food Processing, CRC Press, 2010, 659-672 44. Coronel P.M., Sastry S., Jun S., Salengke S., Simunovic J., Ohmic and Microwave Heating, in Simpson R.

(editor), Engineering Aspects of Thermal Food Processing, CRC Press, 2010, 73-81

49

45. Vicente A.A., Castro I., Teixeira J.A., Ohmic Heating for Food Processing, in Sun D-W. (editor), Thermal Food Processing - New Technologies and Quality Issues, CRC Press, 2006, 425-464

46. Lima M., Food Preservation Aspects of Ohmic Heating, in Rahman M.S., Handbook of Food Preservation, 2nd edition, CRC Press, 2008, 741-745

47. Ekholm, Fraenkel, Hörbeck, Ivarsson, Schale, Former och Tabeller i Fysik, Matematik och Kemi för gymnasieskolan, Göteborg, Konvergenta HB, 2002, 37-39

48. MicVac, Microwave In-Pack Pasteurization by MicVac, Göteborg, MicVac, 2009, 25-33 49. Norström H., Produkttemperatur och förpackningstryck under tillagning av livsmedel med mikrovågor

enligt MicVac-metoden, Examensarbete i Livsmedelsteknik, Göteborg, Chalmers, 2002 50. Coronel P.M., Sastry S., Jun S., Salengke S., Simunovic J., Ohmic and Microwave Heating, in Simpson R.

(editor), Engineering Aspects of Thermal Food Processing, CRC Press, 2010 81-89 51. Buffler C.R., Microwave Cooking and Processing – Engineering Fundamentals for the Food Scientist,

New York, Van Nostrand Reinhold, 1993, 1-13, 38-60, 73-77 52. Wikipedia, MicVac-metoden, 2010-06-04, available at:

http://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/The_Micvac_method.jpg/300px-The_Micvac_method.jpg (accessed 2010-11-10)

53. Olbjer L., Experimentell och industriell statistik, 5th edition, Luns Universitet, Lund, 2000 54. Bergenståhl B., professor, department of food technology, Lund University 2011-01-05 55. Sugartech, Density of sugar factory products, Sugartech, 2010-11-05, available at:

http://www.sugartech.co.za/density/index.php (accessed 2010-12-01) 56. Singh R.P, Heating and cooling processes for food, in Heldman D.R, Lund D.B. (editors), Handbook of

food engineering, Taylor and Francis Group, 2007, 402

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APPENDIX I – TIME PLAN

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

ACTI

VITY

WEE

K Li

tera

ture

sear

ch

Proj

ect p

lan

Mat

eria

ls, m

etho

ds, a

naly

ses

Visit

to T

olla

rp

Visit

to S

IK

Visit

to M

icVa

c

Orie

ntin

g tr

ials

Ord

er ra

w m

ater

ials

Pre-

tria

ls (P

art I

)

Ohm

ic H

eatin

g eq

uipm

ent

Mic

Vac

Tria

ls

Ohm

ic H

eatin

g Tr

ials

Refe

renc

e ja

m p

rodu

ctio

n

Stor

age

and

anal

ysis

of O

H sa

mpl

es

Stor

age

and

anal

ysis

of M

V sa

mpl

es

Stor

age

and

anal

ysis

of R

ef sa

mpl

es

Sens

ory

eval

uatio

n

Stat

istic

al a

naly

sis

Repo

rt w

ritin

g

Calc

ulat

ions

(sca

le-u

p et

c)

Pres

enta

tion

51

APPENDIX II - REACTION KINETIC EQUATIONS

The reaction kinetics for microorganisms can be described by first-order reaction kinetics, described in the equation below where is the concentration, t is the time and k is the reaction constant. First order reaction kinetics is often also valid for the degradation of many chemical substances. [33]

−𝑑𝐶 𝑑𝑡

= 𝑘 ∗ 𝐶

The Arrhenius equation describes the temperature influence on first-order reaction processes. In the equation below, k0 is a system-dependent constant, Ea is the activation energy, R the gas constant and T the temperature (°K). [33]

𝑘 = 𝑘0 ∗ 𝑒�−𝐸𝑎𝑅∙𝑇 �

The Arrhenius equation is often used to describe the degradation kinetics of chemical substances while the thermal death time (D- and z-values) is often used for microorganisms. In the following equations, N0 is the initial and N is the present number of microorganisms, t is the time, D is the decimal reduction time, z is the temperature increase to decrease the D value by one log unit and T1 and T2 the temperature interval. [33, 40]

log(𝑁0) − log(𝑁) = 𝑡/𝐷

𝑧 = 𝑇1−𝑇2log (𝐷𝑇2)−log (𝐷𝑇1)

The F-value is the time at instantaneous heating at a reference temperature that the temperature profile of the pasteurization process corresponds to. The F-value is useful when comparing the lethal effects of different thermal treatments and can be calculated by the following equations. [40]

𝐿𝑅 = 1/10(𝑇𝑟𝑒𝑓−𝑇)/𝑧

𝐹𝑇𝑟𝑒𝑓 = �𝐿𝑅 𝑑𝑡

The values below have been calculated based on the data in table 1-4.

CALCULATED D90 AND Z VALUES

Component D90 (min) z (°C) Anthocyanins 540 27.5 PPO 0.042 11.9 PME 0.025 10.0 Ascospore (Byssochlamys nivea) 5.0 6.4 Ascospore (Neosartorya Fischeri LT025) 1.47 6.4 Yeasts 8.36*10-4 7.2 Moulds 3.0*10-4 5.0

52

APPENDIX III – PHOTOGRAPHS

STEAM VESSEL

C-TECH PILOT 10 KW OHMIC HEATER

MICVAC JAM IN THE 400g FLEXTRAY

OHMIC HEATING JAM

53

APPENDIX IV – CONDUCTIVITY DATA PROCESS DEVELOPMENT CONDUCTIVITY DATA

Temperature (°C)

Conventional jam, Tollarp

Conventional jam, medium

Conventional jam, berries

Conventional jam,

homogenized

NaCl 0,1% w/w

NaCl 0,05%

w/w 37 276,9 39 242,3 43 305 80,6 44 135,9 43 349,9 103,6 44 104,8 44 42,5 102,7 45 59 45 61 65,2 46 65,7 46 73,4 46 66 46 327,3 104,8 75,1 47 298,4 47 202,5 48 104,7 48 85,5 48 92,7 49 143,1 49 371,2 50 146,6 93,4 50 103,7 50 152,9 152,6 52 272 52 154,5 55 146,2 116,1 55 176,9 159,6 56 180,2 157,8 56 262,1 56 327,6 158,6 117,5 57 192,3 57 427,4 58 115,5 58 195,4 59 195,2 164,7 60 165,1 61 199,3 160,7 62 203,3 123,9 63 122,1 218,2 160 63 126,5 157,9 64 224,9 166,9 64 221,3 64 221,4 65 224,7 65 214,8 228,5 66 377,8 66 286,4 194,9 66 149,5 295,7 67 221,3 195,9 67 153,3 376 68 153,3 440,9 68 413 202,3

54

68 207,7 69 148,2 274,2 69 477,7 69 445,4 208,9 70 200,5 211 70 210 268,3 212,1 71 154,4 263,2 71 440,4 266,2 72 232,1 209,7 72 156,1 246 73 158,4 73 249,5 74 451,8 75 243,5 247,8 75 244,1 75 322 75 246,5 331,6 249 76 312,4 77 266,4 2331 256,8 77 230,2 78 161,2 379,7 234,9 78 378,1 82 269,5

55

APPENDIX V – PROCESS DEVELOPMENT DATA STRAWBERRY CONDUCTIVITY (MEAN±SD, N=3)

Temperature (°C) Conductivity (mS/m) Thawed strawberries 17.5 220±10 Sugar solution (65 w/w) 25.0 0.27±0.01 REFERENCE JAM CONDUCTIVITY (MEAN±SD, N=3)

Temperature (°C) Conductivity (mS/m) Reference jam, medium 23 69±3.0 Reference jam, berries 22 62±6.4 DETERMINATION OF AMOUNT OF ANTIFOAMING AGENT IN MICVAC

Antifoaming agent x original amount

Initial weight (g) Final weight (g) Loss of steam (%) Whistles continuously after (min)

10x 417 398 4,6 02:55 8x 421 404 4,0 03:10 7x 424 406 4,2 03:00 6x 423 404 4,5 03:00 DETERMINATION OF PROCESSING TIME IN MICVAC

Processing time (min)

Initial weight (g) Final weight (g) Loss of steam (%) Whistles continuously after (min)

5,0 423 404 4,5 03:00 4,5 423 414 2,1 03:20 4,0 424 417 1,7 03:05

56

APPENDIX VI – TEMPERATURE PROFILE DATA REFERENCE JAM

Temperature (°C) Time (min) 5 0 55 30 85 50 85 52 90 57 90 60 60 107 57 110 57 111 55 115 REFERENCE JAM (SHORT COOLING)

Temperature (°C) Time (min) 5 0 55 30 85 50 85 52 90 57 90 60 88 65 55 95 OHMIC HEATING

Temperature (°C) Time (min) 5 0 45 5 55 9 57 11 50 16 94 18 88 22 55 52 MICVAC

Temperature (°C) Time (min) 5 0 45 5 55 9 57 11 50 12 103 15 103 16 55 46

57

APPENDIX VII – JAM PRODUCTION DATA VITAMIN C (MEAN±SD, N=3)

Temperature (°C)

Time of analysis

Reference Reference, short cooling

Ohmic Heating MicVac

22°C 0 w 14.7±0.6 18.3±0.6 23.9±1.5 24.7±1.3 1 w 13.0±2.0 18.0±0.0 20.0±0.0 17.3±2.3 2 w 13.7±1.2 18.0±0.0 18.7±0.6 16.7±3.1 4 w 9.0±0.0 18.3±0.6 18.3±1.5 8.0±1.0 35°C 1 w 11.0±1.0 15.0±0.0 16.7±1.2 10.7±2.3 2 w 9.0±0.0 12.0±0.0 15.3±2.5 <5.0 COLOR (MEAN±SD, N=12)

Jam Temperature (°C)

Time of analysis

Reflectance (650 nm)

L* a* b*

Reference 22°C 0 w 2.7±0.1 3.7±0.3 16.6±0.9 6.3±0.5 1 w 3.3±0.4 4.7±1.4 19.0±2.2 7.6±1.4 2 w 3.3±0.5 4.0±0.7 18.5±0.7 6.8±1.1 4 w 3.7±0.7 8.0±3.1 17.8±4.4 8.8±1.7 35°C 1 w 2.9±0.6 5.0±2.8 15.6±2.5 6.7±1.5 2 w 2.5±0.2 3.0±0.4 13.8±1.3 5.4±0.7 Reference, 22°C 0 w 3.9±1.5 6.4±3.3 19.6±5.2 9.0±3.2 short cooling 1 w 4.3±1.1 9.2±5.0 18.3±5.8 9.0±4.3 2 w 2.0±0.3 2.7±0.7 12.1±2.2 4.6±1.3 4 w 4.0±1.1 10.6±4.1 16.8±5.6 9.5±4.2 35°C 1 w 2.9±1.1 7.3±5.2 12.3±3.0 6.2±2.1 2 w 2.7±0.8 5.5±3.0 14.3±4.1 7.0±1.7 Ohmic 22°C 0 w 3.6±1.5 5.7±2.6 20.4±3.9 9.7±4.4 Heating 1 w 3.9±1.5 7.0±3.5 20.5±3.9 10.7±4.5 2 w 3.5±0.7 4.7±1.2 19.2±3.2 8.0±2.0 4 w 4.2±0.9 9.9±4.6 19.3±3.9 10.7±2.8 35°C 1 w 3.2±1.5 11.7±5.2 17.4±6.6 9.7±5.2 2 w 3.6±0.5 6.6±2.9 18.8±4.8 10.7±4.5 MicVac 22°C 0 w 3.9±0.9 6.4±1.8 22.0±3.3 10.6±2.7 1 w 3.9±0.6 7.8±3.1 20.2±3.3 10.1±2.2 2 w 3.5±0.7 5.4±1.4 20.8±3.1 9.3±2.5 4 w 4.6±0.6 10.6±2.8 19.4±4.7 12.3±2.2 35°C 1 w 3.2±0.6 6.4±1.8 18.1±1.8 10.3±1.9 2 w 2.4±0.5 4.4±1.2 14.6±2.6 7.6±2.1 REPLIES BEFORE NORMALIZATION FROM THE SENSORY EVALUATION (MEAN±SD, N=36)

Attribute Reference Reference, short cooling

Ohmic Heating MicVac

Strawberry flavor 5.3±1.1 5.4±1.0 5.8±1.1 4.7±1.4 Strawberry chewability/texture 5.2±1.1 5.6±1.1 4.7±1.5 4.2±1.2 Overall impression 5.4±0.9 5.2±1.2 5.4±1.1 4.5±1.2