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Part 7 Food Processing

Food Biochemistry and Food Processing (Simpson/Food Biochemistry and Food Processing) || Thermal Processing Principles

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Part 7Food Processing

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38Thermal Processing Principles

Yetenayet Bekele Tola and Hosahalli S. Ramaswamy

IntroductionThermal Processing Basics

IntroductionReaction Rate

Zero-Order ReactionFirst-Order ReactionSecond-Order Reaction

Common Food MicroorganismsThermal Resistance of Food MicroorganismsKinetics of Microbial Death

Decimal Reduction Time and Thermal Death TimeTemperature Dependency of Kinetic ParametersConcepts of Process Lethality

Thermal Process Determination MethodsGeneral Method

Original General MethodImproved General Method

Formula MethodsCharacterization of Heat Penetration DataHeat Penetration ParametersThe Retort CUT, f h and jh ValuesStumbo’s Method

Quality OptimizationHigh-Temperature Short-Time and Ultra-High

Temperature ProcessingAgitation ProcessingAseptic ProcessingThin Profile and Retort Pouch ProcessingNovel Thermal Food Processing Technologies

Microwave and Radio Frequency HeatingOhmic Heating

Novel Nonthermal Processing TechnologiesHigh-Pressure ProcessingPulsed Electric Field

Retort Types for Commercial ApplicationBatch Retorts

Steam Heating RetortWater Heating Retort (Immersion and Spray Modes)

Steam-Air Heating RetortContinuous Retorts

References

Abstract: Food intended for human consumption must be producedin safe and stable forms to ensure availability, distribution, as wellas normal growth and development. Thus, foods are processed invarious forms to achieve these desired effects. Thermal process-ing entails the application of heat energy for food transformationinto the desired safe and stable forms. This chapter describes thebasic principles of thermal processing and surveys conventional ver-sus novel thermal processing methods currently available for foodtransformation.

INTRODUCTIONFood processing is used to transform and/or preserve raw ingre-dients from the farm into various food forms for consumptionby human being. Food processing often takes clean, harvestedproduce or edible portions and uses them to produce attractiveand marketable food products. Fresh agricultural products (plantand animal origin), due to their high moisture content, are highlyperishable and can be kept only for a short period of time. Es-pecially during peak harvesting seasons, due to large volumeof these products, unless and otherwise pertinent measures aretaken, the problem leads to spoilage and high economic losses.Furthermore, food products should be available throughout theyear in market to fulfill the demand of consumers in all seasons.To do this, we need to minimize postharvest losses and preserveagricultural products in a safe way with no or minimum qualityloss. The major emphasis of food processing is shelf life exten-sion by preventing undesirable changes in the wholesomeness,nutritive value, and sensory qualities (Ramaswamy and Marcotte2006).

Food Biochemistry and Food Processing, Second Edition. Edited by Benjamin K. Simpson, Leo M.L. Nollet, Fidel Toldra, Soottawat Benjakul, Gopinadhan Paliyath and Y.H. Hui.C© 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

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726 Part 7: Food Processing

Various food processing operations make the food productsmore attractive, satisfying, safer, and easier to eat. Commonfood-processing techniques include pasteurization, sterilization,cooking, drying, cooling and freezing, fermentation, addition ofpreservatives and reduction of water activity, and use of two ormore of the above techniques to inhibit or stop chemical, bio-chemical, and microbiological activities. Most foods available inthe market are subjected to some form of thermal processing. Al-though the canning process started from Nicholas Appert’s timein 1809, food processing via the canning process still providesa universal and economic method for preserving and process-ing foods. Thermal processing of canned foods has been one ofthe most widely used methods for food preservation during thetwentieth century and has contributed significantly to the nutri-tional well-being of much of the world’s population (Teixeiraand Tucker 1997).

The two very common industrial conventional thermal foodprocessing technologies in production of canned products arepasteurization and sterilization. Pasteurization is the process ofheating liquids and/or solid foods for the purpose of destroy-ing principal pathogens capable of growing under aerobic con-ditions and reducing the level of spoilage vegetative bacteria,protozoa, and fungi from high acidic foods (pH < 4.5). WhenpH > 4.5, foods produced by this process should be stored atlow temperature to avoid further growth spore forming micro-organisms. However, commercial sterilization refers to an in-tensive heat treatment process that effectively kills or eliminatesall pathogens and vegetative microorganisms as well as bulkof spore-forming bacteria from the low acid foods (pH ≥ 4.5).Products produced through this method can be shelf stable forup to two years. In order to reduce the process severity, thethermophilic spore formers are not targeted to be completelyeliminated, instead the canned product is advised to be stored attemperatures below 30◦C to prevent the growth of thermophiles.Because of the high safety implications and severe process-ing conditions, the conventional thermal sterilization has beenaccepted to result in considerable product quality degradation.However, many improvements have been implemented to im-prove thermal processing operations and techniques, and novelprocessing approaches have been introduced in recent years toimprove the quality of thermally processed foods without com-promising safety of these products. Therefore, the main focusof this chapter is to describe the basic principles of thermalprocessing operation in terms of production of safe and betterquality products.

THERMAL PROCESSING BASICSIntroduction

The major objective of thermal processing is production of safeand stable products that consumers are willing and able to buy.To achieve this goal, it is necessary to understand the scientificbasis on which the process is established. Silva et al. (1992) in-dicated that commercial thermal processing is a function of sev-eral factors, such as product thermo-physical properties (prod-uct heating rate), surface heat transfer coefficient, initial food

temperature, retort temperature, heating medium (hot water orsteam), heating medium come-up time (CUT), temperature re-sistance of food microorganisms and quality factors, and targetdegree of lethality or safety level we need to achieve.

The success of thermal processing does not depend on theelimination of the entire microbial population, because thiswould result in low product quality due to the long heatingrequired. Instead, all pathogenic and most spoilage-causing mi-croorganisms in a hermetically sealed container are destroyed,bulk of the spore formers is killed, and an environment is createdinside the container that does not support the growth of remain-ing spore formers. There are different mechanisms that enableone to control the germination and growth of such type of sporesin canned foods. These are based on the microbial growth andinactivation with respect to oxygen requirement, pH preference,and temperature sensitivity.

In general, canned foods have a 200-year history and are likelyto remain popular in the foreseeable future owing to their conve-nience, long shelf life, and low cost of production. The technol-ogy is receiving increasing attention from thermal-processingspecialists to improve both the economy and quality of somecanned foods (Durance 1997). However, the sterilization pro-cess not only extends the shelf life of the food, but also affectsits nutritional and sensorial qualities. Process optimization istherefore necessary in order to promote better quality retentionwithout sacrificing safety.

Reaction Rate

During thermal processing of foods, several types of chemi-cal reactions occur. Some reactions result in a quality loss andsuch type of reactions must be minimized, whereas others resultin the development and formation of a desirable flavor, taste,or color, and these ones must be optimized to obtain the bestproduct quality (Toledo 2007). In order to maintain quality offood products through optimization of processing conditions,predictive mathematical models are very important. To realizethis goal, information is needed on the rates of destruction ofmicrobes as well as quality parameters and their dependence onvariables such as temperature, pH, light, oxygen, and moisturecontent, which can be expressed by mathematical models. Abetter understanding of kinetics of food products can providebetter opportunities for developing food processes to maximizequality parameters and ensuring safety.

Each reaction undergoes on its own rate and the rate of reac-tion is described by the reaction kinetics. Kinetics is the studyof the rate at which compounds react. Reaction kinetics (ratetheory) deals to a large extent with the factors that influence thereaction velocity. The rate depends on several factors, includingthe contact between the reacting components, their concentra-tion, temperature, and pressure at which the reaction takes place.The “collision theory” implies that the molecules need to collidewith each other in order for the reaction to take place. If thereare a higher number of collisions in a system, there is a greaterchance for the reaction to occur. The reaction will go faster, andthe rate of the reaction will be higher. In collision theory, twomain things are to be considered: the activation energy, which

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is the minimum energy required to initiate the collision, and thestatistical probability for collisions between certain moleculesthat possess an adequate energy level for the reaction to occur ata given temperature. To measure a reaction rate, it is necessary tomonitor the concentration of one of the reactants or products as afunction of time. Therefore, the rate of reaction can be expressedas a rate of change in concentration to change in time.

On the basis of this concept, the rate law is an expressionrelating the rate of a reaction to the concentrations of the chem-ical species present, which may include reactants, products, andcatalysts. Many food reactions follow a simple rate law, whichtakes the form

r = k[A]a[B]b[C]c (1)

that is, the rate (r) is proportional to the concentrations of thereactants (A, B, C) each raised to some power. The constantof proportionality, k, is called the rate constant. The power aparticular concentration is raised is the order of the reactionwith respect to that reactant. Note that the orders do not have tobe integers. The sum of the powers in Equation 1 is called theoverall reaction order.

Zero-Order Reaction

A zero-order reaction is independent of the concentration of thereactants. A higher concentration of reactants will not speed upthe zero-order reaction. This means that the rate of the reaction isequal to the rate constant, k, of that reaction. Zero-order reactionis described as

Rate = r = −dA

dt= k[A]0 (2)

After separating variables and integrating both sides of Equa-tion 2∫ A

A0

dA = −∫ t

t0

kdt (3)

This provides the integrated form of the rate law

[A] = [AO ] − kt (4)

where k is the rate constant of the reaction, A is concentration attime t. In a zero-order reaction, when concentration data [A] isplotted versus time (t), the result is a straight line.

First-Order Reaction

A first-order reaction is one where the rate depends on the con-centration of the species to the first power. Most of the reactionsinvolved in the processing of foods are of first-order reactions.For a general unimolecular reaction, the decrease in the concen-tration A over time t can be written as

Rate = r = −dA

dt= k[A]1 (5)

Rearranging the equation

− dA

A= kdt (6)

Integrate both sides of the equation∫ A

A0

dA

A= −

∫ t0

t0

kdt (7)

and the linear for of Equation 7 is:

ln[A] = ln[A0] − kt (8)

where [A] is the concentration at time t, [A0] is the concentrationat time t = 0, and k is reaction rate constant (s−1).

Plotting ln [A] with respect to time (t) for a first-order reactiongives a straight line with the slope of the line equal to −k, wherethe rate constant is calculated.

Second-Order Reaction

A second-order reaction is one where the rate depends on theconcentration of the species to the second power. The reactionrate expression for a unimolecular second-order reaction is

dA

dt= −k[A]2 (9)

Separation of variables and integration of both sides of equa-tions will give us

dA

[A]2= −kdt (10)

∫ A

A0

A

[A]2= −

∫ t

t0

kt (11)

1

[A]= 1

[A0]+ kt (12)

Second-order unimolecular reaction is characterized by a hy-perbolic relationship between concentration of the reactant orproduct and time. A linear plot will be obtained if 1/A is plottedagainst time.

Second-order bimolecular reactions may also follow the fol-lowing rate equation:

A + B → P

Rate = dA

dt= −k[A][B] (13)

where A and B are the reactants. After separating variables, thedifferential equation may be integrated by holding B constant togive

dA

[A]= −k[B]dt (14)

∫ A

A0

dA

A= −

∫ t0

t0

k′dt (15)

1

[A]= 1

[A0]− k′t (16)

k′ is a pseudo-first-order rate constant: k′ = kB.A second-order bimolecular reaction will yield a similar

plot of the concentration of the reactant against time as afirst-order unimolecular reaction, but the reaction rate constantwill vary with different concentrations of the second reactant(Table 38.1).

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Table 38.1. Summary of Zero-, First-, and Second-Order Reaction Rates(M-molar Concentration)

Rate Law/Order Differential Form Integral FormLinear Plot toDetermine k

Units of ReactionConstant (k)

ZerodA

dt= −k [A] = [Ao] − kt [A] vs. t

M

s

FirstdA

dt= −k[A]1 ln[A] = ln[Ao] − kt ln[A] vs. t

1

s

SeconddA

dt= −k[A]2 1

[A]= 1

[Ao]+ kt

1

[A]vs. t

1

M.s

Common Food Microorganisms

Foods by their very nature involve complex biological moleculesthat strongly support the survival and growth of food microor-ganisms. One of the major causes for food deterioration is thegrowth and activity of microorganisms. Microorganisms cancontaminate the food before and after processing from varioussources. The major categories of microorganisms involved infood spoilage and deterioration are fungi and bacteria. Each mi-croorganism has its own optimum temperature, pH, and oxygenlevel to grow. Broadly, food microorganisms can be categorizedas pathogenic and spoilage ones. Food pathogenic microorgan-isms are those groups that can cause illness in human being oranimals due to food poisoning or infection. Food poisoning andfood infection are different, although the symptoms are similar.True food poisoning or food intoxication is caused by eatingfood that contains a toxin or poison due to bacterial or fungalgrowth in food. The bacteria or fungi that produced and ex-creted the toxic waste products into the food may be destroyedduring processing, but the toxin they produced can cause theillness or digestive upset to occur. Staphylococcus aureus andClostridium botulinum are two common species of bacteria thatcause food poisoning. Food infection is the second type of food-borne illness. It is caused by eating food that contains certaintypes of live bacteria, which are present in the food. Once thefood is consumed, the bacterial cells themselves continue togrow and illness can result. Salmonellae, C. perifringens, Vibiro

parahaemolyticus, Yersinia enterocolitica, and Listeria mono-cytogenesis are common infectious bacteria of food.

The other category includes food spoilage microorganisms.Food spoilage can be defined as the process or change leading toa product becoming undesirable or unacceptable for consump-tion. This can be chemical, physical, or microbial spoilage orthe combination of these factors. Food spoilage is more of aneconomic concern than safety issue.

Food microorganisms can be grouped based up on their oxy-gen and temperature requirement for survival and growth (Table38.2). In foods that are packaged under vacuum, low oxygenlevels are intentionally achieved. Therefore, the prevailing con-ditions do not support the growth of microorganisms that requireoxygen (obligate aerobes). Likewise, the thermophilic microor-ganisms require temperatures much higher than 30◦C for theirgrowth.

Acidity level of a food is another intrinsic food factor that de-termines the survival and growth of food microorganisms. Fromthermal processing standpoint, foods are divided into three pHgroups: (i) high-acid foods (pH < 3.7), (ii) acid or medium-acidfoods (3.7 < pH < 4.5), and (iii) low-acid foods (pH > 4.5).The most important distinction in the pH classification, withreference to thermal processing, is the dividing line betweenacid and low-acid foods. Most laboratories concerned with ther-mal processing have devoted attention to C. botulinum, whichis a highly heat-resistant, rod-shaped, spore-forming, anaerobic

Table 38.2. Classification of Microorganisms and Bacteria Based on Their Oxygen Demand and OptimumTemperature for Growth

Classification Groups of Microorganisms Examples of Microorganisms

Oxygen-based classificationObligatory aerobes (require oxygen) Most molds, Micrococcus, Serratia marcescens, Mycobacterium

tuberculosisObligatory anaerobes (require absence of oxygen) C. botulinum, C. sporogenes, C. thermosaccharolyticumFacultative anaerobes (tolerate some oxygen) Bacillus coagulans, Staphylococcus aureus

Optimum temperature for growthThermophilic (55–35◦C) B. coagulans; S. thermophilus; C. thermosaccharolyticum, C. nigrificansMesophilic (40–10◦C) C. pasteurianum, B. mascerans, C. sporogenes, B. licheniformisPsychrophilic (35–<5◦C) Pseudomonas, Micrococcus, C. botulinum Type E, S. aureus

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pathogen that produces the deadly botulism toxin. It has beengenerally recognized that C. botulinum does not grow and pro-duce toxin below a pH of 4.5. Hence, the dividing pH betweenthe low-acid and acid groups is set at 4.5. There may be othermicroorganisms that are more heat-resistant than C. botulinum.

Thermal Resistance of Food Microorganisms

As indicated in the above sections, the success of thermal pro-cessing depends on the destruction of all pathogenic and mostspoilage-causing microorganisms in a hermetically sealed con-tainer and creating an environment inside the package that isnot conducive to the growth of spoilage-type microorganismsand their spores that resist the thermal process. The microorgan-isms vary in terms of heat resistance and require different degreeof thermal processing. In order to establish a thermal process,first, the target heat-resistant microorganisms of concern needto be identified. C. botulinum is the principle target microorgan-ism of concern in low-acid canned foods. The second step isevaluating the thermal resistance of target organism. The ther-mal resistance data should be well described by an appropriateand reliable mathematical model. Data on the temperature de-pendence of the microbial destruction rate are also needed tointegrate the destruction effect through the temperature profileunder processing conditions.

Kinetics of Microbial Death

An understanding of the mechanism and kinetics of thermaldeath of food microorganisms would be helpful in the practicaluse of heat in processing of foods. Thermal destruction of bacte-ria is generally accepted to be exponential with time, and processcalculations used in thermal processing are generally based onthis assumption (Stumbo 1973). However, there are also nonex-ponential thermal death patterns indicated in different literatures(Stumbo 1973, Toledo 2007). The theory that thermal death ofbacteria follows the first-order kinetics has been well recognized(Stumbo 1973). A first-order reaction is one in which the rate isproportional to the number of microorganisms present.

Decimal Reduction Time and Thermal Death Time

When a suspension of microorganisms is heated at constanttemperature, the decrease in number of viable organisms followslogarithmic order of death. In other words, the logarithm of thesurviving number of microorganisms following a heat treatmentat a particular temperature plotted against heating time will givea straight-line curve (Fig. 38.1). These curves are commonlycalled survivor curves. The microbial destruction rate is definedas a decimal reduction time or D-value (minutes), which is theheating time in minutes at a given temperature required to resultin one decimal or log unit reduction in the surviving microbialpopulation. In other words, D-value represents a heating timethat results in 90% destruction of the microbial population fromthe initial number. The rate of microbial destruction with timeat constant temperature expressed as

dN

dt= −k[N ]1 (17)

0.1

1

10

100

1000

10000

403020100

Surv

ivor

s

Time at a constant temperature (min)

D

Log100- log 10= 1

Figure 38.1. Survivor curve that depicts the number of survivormicroorganisms versus time at a constant temperature.

where dN/dt is the change in microbial population with time t, kis the reaction rate constant, and 1 is to indicate the order of re-action (first order). After separation of variables and integration,the above equation can be written as

ln N = ln N0 − kt (18)

where N0 is initial population of microorganisms.D is based on common logarithms, in contrast with k, which

is based on natural logarithms. But D and k are related as

ln

(N

N0

)= ln (10) log

(N

N0

)= −kt (19)

From this

log

(N

N0

)= − k

ln(10)t (20)

Thus, the D value is the negative reciprocal of the slope of aplot of log (N) against t, and D and k are related as

− 1

D= − k

ln(10); D = ln(10)

kor D = 2.303

k(21)

Therefore, the D value from survivor curve can be expressedas

D = (t2 − t1)

(log N0 − log N )(22)

where N0 and N represent the survivors following heating for t1

and t2 (minutes), respectively.Commonly in thermal processing applications, survivor

curves are plotted on specially designed semi-log papers for easyhandling and interpretation data. The survivor number of mi-croorganisms is plotted directly on the logarithmic y-axis againsttime on the linear x-axis. From this graph, the D value can be

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Table 38.3. Probability of Residual Fraction Survivorsafter Multiple D Values

D Value (No = 103)

Probability of ResidualFraction Survivors After

90% of Destruction

1D 102

2D 101

3D 100

4D 10−1

5D 10−2

nD 10n−3

calculated from the time interval on x-axis between which thestraight-line portion of the curve on the y-axis shows a reductionby logarithmic unit. For instance, as indicated in Figure 38.1the number of survivor microorganisms is reduced from 100(two logarithmic unit) to 10 (one logarithmic unit) in approx-imate time interval of 10 minutes (24–14 minutes), which isthe D value at a given constant temperature. However, in to-day’s computer era, the D value can be obtained from a log(N) versus t computer graph on a spreadsheet as the negativereciprocal slope. The logarithmic nature of the survivor curveindicates that complete destruction of microbial population isnot theoretically possible; a decimal fraction of the popula-tion should remain even after an infinite number of D values(Table 38.3).

Thermal death time (TDT) is another concept that is com-monly used in thermal processing of food. The concept some-what contradicts the logarithmic destruction approach. As com-pared to decimal reduction concept, TDT is the minimum heat-ing time required to cause complete destruction of a microbialpopulation. In sequential study with microbial destruction asa function of time (at a given temperature), TDT then repre-sents a time between the shortest destruction and the longestsurvival times. The difference between the two is sequentiallyreduced or geometrically averaged to get an estimate of TDT.The “death” in this instance generally indicates the failure of agiven microbial population, after the heat treatment, to show apositive growth in the subculture media. Comparing the TDTapproach with the decimal reduction approach, one can easilyrecognize that TDT value depends on the initial microbial load(whereas D value does not). Further, if TDT is always measuredwith reference to a standard initial load or load reduction, itwould simply represent a certain multiple of D value. For exam-ple, if TDT represented the time to reduce the population fromN0 = 102 to N = 10−1, then TDT is a measure of 3D values(Figure 38.2). On the other hand, if it is based on 104–10−6, itwould represent a 10D value. Therefore, TDT is a multiple of D(TDT = nD, were n is the number of decimal reductions).

Temperature Dependency of Kinetic Parameters

The D value (and TDT) is strongly dependent on the temperatureapplied during heating of bacterial suspension. Higher temper-

0.1

1

10

100

1086420

Time (min)

z

2D

3DD

1D

D

Sur

vivo

rs

Figure 38.2. Multiple D and TDT.

atures will give shorter D values and vice versa. Temperaturedependency of kinematic parameters is expressed in terms of D-z or TDT-z models. By plotting the various D-values (or TDT)against temperature, again on a logarithmic scale, the z-value canbe obtained as temperature range for the D-value (or TDT) curveto pass through one log cycle (Fig. 38.3). In this concept, thetemperature sensitivity indicator is defined as the z value. Phys-ically, z represents the temperature change need to increase ordecrease the decimal reduction time (or TDT) by a factor of 10.

0.1

1

10

100

130120110100

Temperature (°C)

z

log

D-v

alue

log

(100

) –

log

(10)

= 1

Figure 38.3. D value versus temperature curve and z value.

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Mathematically, using reference temperature (T r) and referenceD value (D0), z value (◦C) is expressed as

z = Tr − T

log D − log D0(23)

or in between two temperature ranges of T1 and T2,

z = T2 − T1

log D1 − log D2(24)

where D1 and D2 are D values at T1 and T2, respectively.A larger z value indicates that the rate of destruction of mi-

croorganism is less temperature sensitive. A small z value in-dicates higher temperature sensitivity and a small change intemperature will result in a significant change on microbial pop-ulation. Most commonly, quality parameters like nutrients andcolor have large z values as compared to microorganisms thathave small z values. Again, microbial spores will have a higherz as compared to vegetative bacteria.

The D value at any given temperature can be obtained from amodified form of the above equation using the reference D value(D0 at a reference temperature, T r, usually 121.1◦C (250◦F) forthermal sterilization and 82.2◦C (180◦F) for pasteurization).

D = D010( Tr−Tz ) (25)

Concepts of Process Lethality

Lethality (F0 value) is a measure of the heat treatment or steril-ization processes. To compare the relative sterilizing capacitiesof heat processes, a unit of lethality needs to be established. Forconvenience, this is defined as an equivalent heating time of 1minute at a reference temperature, which is usually taken to be121.1◦C (250◦F) for the sterilization processes. The criteria forthe adequacy of a process must be based on two microbiolog-ical considerations: (i) destruction of the microbial populationof public health significance and (ii) reduction in the number ofspoilage-causing bacteria.

In the canning industry, the F0 value is often used for the low-acid canned foods and refers to the lethality value (Fz

Trefvalue)

with a z value of 10◦C (18◦F) and a reference temperature (T ref)of 121.1◦C (250◦F). The z value of 10◦C is used as the thermalcharacteristic for the pathogenic microorganism, C. botulinumspores.

Experimental work and experience have shown that a processthat achieves 12 log reductions of C. botulinum can be consideredcommercially sterile (Tucker 2008). If such a process is correctlyapplied to a product, then the health risk to the consumer willbe insignificant. Applying a 12D process reduces the probabilityof spore survival by a factor of 1012. If cans contain one initialspore, then for trillion cans produced and given a 12D process,only one can would contain a surviving spore.

The D121.1◦C (D250◦F or D0) value can be used for establishingsuch a process. The D0 for C. botulinum is taken as 0.23 minute.Thus, 12D value would represent 12 × 0.23 or 2.76 minutes.The minimum F0 value should be 2.76 minutes to accomplishthe 12D process. In general, if N0 is the initial spore concen-tration and N the target spore concentration need to achieve,then for exposure to a theoretical constant process temperature

of 121.1◦C at the coldest or slowest heating point, the targetprocess would be:

F0 = D0 log

(N0

N

)(26)

However, several low-acid foods are processed beyond theminimum F0 value of 2.76 minutes in order to deal with spoilage-causing bacteria of much greater heat resistance. For these or-ganisms, acceptable levels of spoilage probability are usuallydictated by economic considerations. Most food companies ac-cept a spoilage probability of 10−5 (5D = TDT) from mesophilicspore formers (organisms that can grow and spoil food at roomtemperature). The organism most frequently used to characterizethis classification of food spoilage is a strain of C. sporogenes,known as PA 3679, with a maximum D121.1◦C value of 1.00minute. Therefore, the 5D minimum value would be 5 min-utes and hence a F0 of 5 minutes is more commonly used forthese foods. Where thermophilic spoilage is a problem, moresevere processes may be necessary because of the high heat re-sistance of thermophilic spores. Fortunately, most thermophilesdo not grow readily at room temperature and require incubationat unusually high storage temperatures (>38◦C) to cause foodspoilage.

Sterilization time required at temperature (T) for the samevalue target microorganism to deliver equal degree of sterility ismathematically expressed as

FzT = F0 ∗ 10

(Tref −T

z

)(27)

The same general relationships as were discussed under ster-ilization apply to pasteurization. A combination of temperatureand time that is sufficient to inactivate the particular speciesof bacteria must be used. Fortunately, most of the pathogenicorganisms, which can be transmitted from food to the personwho eats it, are not very resistant to heat. For thermal processcalculation of high-acid canned foods with an extended refrig-erated shelf life, for shelf-stable low pH < 3.9 products (such asfruit juices) and acidified foods under normal storage conditions,food industries commonly use the pasteurization value (Po withz value of 10oC and Tref of 82.2oC) instead of the sterilizationvalue (Fo value), since foods generally are processed below oraround 100◦C.

THERMAL PROCESS DETERMINATIONMETHODSThermal process evaluation is a process of determining the de-sired processing time at a given heating condition to achievedesired process lethality or vice versa. Determination of processlethality (F0) can be done in two ways: using calculation method(Equation 27) and microbiological data (Equation 26). Differentcalculation methods have been used to determine the requiredprocessing time to achieve the desired lethality. The reliabil-ity of the method depends on how accurately it integrates thelethality effect of transit temperature response of the food un-dergoing thermal process, with respect to test microorganism ofpublic health concern. This implies that accurate determination

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of microbial kinetic data and reliable measurement of tempera-ture profile of the product at the worst case or slowest heatingpoint within the container are important inputs to determine orevaluate the reliability of a given thermal process.

The desired degree of lethality in terms of minimum equiva-lent time at a given reference temperature is generally preestab-lished for a given product considering kinetic data of target mi-croorganism, and processes are designed to deliver a minimumof this preset value at the thermal center. There are essentiallytwo widely accepted categories of methods for using these datato perform thermal process calculations. The first of categoryis the General Method of process calculation based on Bigelowet al. (1920) method of calculation, and the second is the For-mula Method of process calculation based on concepts of Ball(1923) or Stumbo (1973).

General Method

The General Method developed by Bigelow et al. (1920) is agraphical procedure to calculate thermal processes. It is a simplemethod and can be applied to any product and process situation.The method is used to determine sterility equivalents at knownproduct temperatures measured directly by the use of thermo-couples positioned within the slowest heating test container atthe slowest heating spot in the container. It is the most accuratemethod for evaluating under a given set of heat penetration datagathering. It is universally applicable to essentially any type ofthermal processing situation and used to get the baseline targetvalue upon which all other experimental and formula calculationmethod performances are compared.

However, the time–temperature data is at the coldest spotand the method is only used to calculate process times for thegiven heat profile data. Because of this main limitation of themethod, it lacks predictive power for different processes. There

are two General Method approaches: Original General Methodand Improved General Method.

Original General Method

The Original General Method uses the time–temperature dataat slowest heating point of the product and converts this datato destruction rate graph to determine sterilization value of theprocess (area under the curve in the graphical procedure). Theprocedure is to plot the inverse of TDT against time to producea destruction rate curve. The area beneath the curve correspondsto the accumulated sterilization value delivered by the processduring heating and cooling (Fig. 38.4). Conversion of availabletime–temperature data to destruction rate is done using Equation27. From gathered time–temperature data, TDT is calculated andthen the reciprocal of TDT (TDT−1) is computed for correspond-ing temperature (Table 38.4 columns 3 and 4). The reciprocalof TDT is known as the destruction rate achieved at that par-ticular time–temperature combination. Using destruction rateversus time (1/TDT vs. Time), the graph is drawn and the areaunder the curve is determined by counting the squares, or usingplanimeter. A unit of sterilization area on the curve is equal tothe product of destruction rate and time of unity, which is theminimum processing requirement to achieve desired degree ofcommercial sterilization value (SV) with respect to target mi-croorganism. The sterilization value of the overall process canbe calculated by considering the area under the curve divided bythe unit sterilization area.

Graphical method of process calculation is summarized inTable 38.4 for C. botulinum with D0 value of 0.21 minute toachieve TDT value of 12 D0 (F0) with z value of 18◦F.

In order to get a target sterilization value, the process (steamoff) time is appropriately increased or decreased, as the casemay be by shifting cooling curve to right or left direction on

Unit SV = Unit area = 0.05x20 Total SV= Total area under the curve

Lines drawn to the left parallel to the cooling line to reach desired sterilization value

0.45

0.4

0.3

0.2

0.1

0

0 20

Time (min)

Let

halit

y (1

/TD

T)

40 60

0.35

0.25

0.15

0.05

Figure 38.4. Graphical approach of process calculation. Broken lines are parallel lines drawn to the left after carrying out over-processing inorder to get desired SV.

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Table 38.4. Process Calculation by General Graphical and Numerical Integration Method

Time(min)

Temperature(◦F) F = TDT = Fo

∗10(T r−T/z) 1/TDT 1/TDT ∗�t

0 62 7.01E+10 1.4E−11 04 71 2.22E+10 4.5E−11 1.8E−108 99 6.17E+08 1.6E−09 6.5E−09

12 134 7.01E+06 1.4E−07 5.7E−0716 171 6.17E+04 1.6E−05 6.5E−0520 202 1.17E+03 8.5E−04 3.4E−0324 231 2.86E+01 3.5E−02 1.4E−0128 241 7.97E+00 1.3E−01 5.0E−0132 247 3.70E+00 2.7E−01 1.1E+0036 248 3.25E+00 3.1E−01 1.2E+0040 250 2.52 4.0E−01 1.6E+0044 250 2.52 4.0E−01 1.6E+0048 231 2.86E+01 3.5E−02 1.4E−0152 181 1.72E+04 5.8E−05 2.3E−0456 137 4.78E+06 2.1E−07 8.4E−0760 97 7.97E+08 1.3E−09 5.0E−09

SV = �(1/TDT)�t = 6.27

a trial-and-error basis until the desired lethality achieved. Inother words, this whole process is repeated until the desired SVis obtained, and the corresponding processing time is known.From food safety point of view and commercial processing con-ditions, the SV should be more than unity to ensure completedestruction of pathogens and reduction of spoilage microorgan-isms to the level that do not cause spoilage in postprocessingduration.

However, this graphical method is a tedious and cumbersomeapproach and requires a graphical trial-and-error works. Be-cause of this reason, several approaches have been attempted todevelop a simple approach in order to get similar informationfrom gathered time–temperature data. One of the methods is thenumerical integration method (Tabular Method) proposed byPatashnik (1953) (Table 38.4 column 5). In this method, the cor-responding SV of each temperature is numerically calculatedfor each time interval, and the total SV is obtained for equal(Equation 28) and unequal (Equation 29) interval of time.

Sterilization value =∑(

1/TDT

)�t (28)

Sterilization value =∑(

1/TDT�t

)(29)

In Table 38.4, we obtained a SV of 6.14, which can be in-terpreted that the given heat treatment is 6.14 times more thanwhat is required to destroy a given target microorganism. Thisis definitely an overprocessing and will have an impact on thequality of the product. Therefore, SV of 6.14 should be reducedto the value close to unity in order to achieve the desired degreeof lethality of target microorganism.

Improved General Method

Like the original graphical approach, this method also requiresheat penetration data and the conversion of product temperatureto lethal rate. For the development of Improved General method,the two main contributions of Ball (1928) played a significantrole. The first important point was the construction of a hypothet-ical reference TDT curve passing through 1 minute at referencetemperature of 121.1◦C (250◦F) having a given z value. In ad-dition to this, Ball (1928) introduced the term lethal rate (L) fora given temperature according to the following Equation 30 onthe basis of Equation 27.

FzT

F0= 10

(Tref −T

z

)(30)

For commercial sterilization process, the numerator on leftside of Equation 30 indicates TDT at temperature T , which isequivalent to thermal treatment of 1 minute at reference temper-ature of 121.1◦C (250◦F) (Fz=10

Tref=121.1◦C). Therefore, based uponthis concept, Equation 30 can be written as

FzT

1= 10

(Tref −T

z

),

where FzT = TDTT and after rearranging, we get

1

TDTT= L = 10

(TT −ref

z

)(31)

The left-hand side term refers lethal rate (L) and L is calcu-lated as a function of temperature versus time from heat pene-tration data at a specific temperature. The lethal effects at thedifferent time–temperature combinations in a thermal processare integrated so as to account for the total accumulated lethal-ity, since each temperature is considered to have a sterilizing

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Table 38.5. Example of Improved General Method for ProcessEvaluation at Reference Temperature of 250◦F and z Value of 18◦F

Time(min)

Temperature(◦F)

Lethality Rate (min)L = 10(T −Tr/z)

Accumulatedlethality (min)

L ∗ �t

0 62 3.59E−11 0.0E+004 71 1.14E−10 4.5E−108 99 4.08E−09 1.6E−08

12 134 3.59E−07 1.4E−0616 171 4.08E−05 1.6E−0420 202 2.15E−03 8.6E−0324 231 8.80E−02 3.5E−0128 241 3.16E−01 1.3E+0032 247 6.81E−01 2.7E+0036 248 7.74E−01 3.1E+0040 250 1.00E+00 4.0E+0044 250 1.00E+00 4.0E+0048 231 8.80E−02 3.5E−0152 181 1.47E−04 5.9E−0456 137 5.27E−07 2.1E−0660 97 3.16E−09 1.3E−08

Accumulated lethality = 15.80

value during the entire heating and cooling phases (Equation 32;Table 38.5).

Accumulated lethality = F0 =t∫

0

10( T −Trz )dt =

t∫0

L =t∑0

L�t (32)

Note that general methods do not have any specific re-quirement about the shape of the time–temperature curve.They are therefore accurate and they should be the meth-ods of choice for handling “complex” temperature data (Ball1928). In 1980s, computerized mathematical models for pre-dicting time–temperature data using improved general methodwere developed by different researchers (Teixeira and Manson1982, Datta et al. 1986, Sastry 1986, Chandarana and Gavin1989, Chang and Toledo 1989, Lee et al. 1990) for F0 valuecalculations.

Even though the General Method is the base for comparingthe accuracy of other formula methods, the process is criticallydependent on product, container, and process delivery systemconditions as studied at the time of the test. Extrapolation out-side the given process condition yields undefined, invalid lethal-ity results. Additional testing must be performed to verify thesterilizing value delivered under the altered conditions.

Formula Methods

The formula method of process calculation simplifies the te-dious steps associated in calculating process time using gen-

eral methods. However, it requires characterization of the non-linear heat penetration data to compute the key heat penetrationparameters.

Characterization of Heat Penetration Data

Heat penetration data are time–temperature combination datathat are collected after exposing a given food for a given periodof time under a given heating condition. For products in metalcans, which are mainly heated by conduction, the slowest heatingpoint is the center of the container, and for products whose modeof heating predominated by convection, the slowest heating pointis 1/10th above the bottom of the can along the central verticalaxis. Accurate determination of heating profile of a given systemis very important for accurate establishment of process schedule.The profile should be collected under conditions that simulatethe actual process situations. Typical heat penetration profilesfor conduction and convention heating products is shown inFigure 38.5.

Because of the limitations of the General Method, a morerobust Formula Method was developed and proposed by Ball(1923). It makes use of the fact that the difference between retortand cold spot temperature decays exponentially over processtime after an initial lag period. Therefore, a semi-logarithmic plotof this temperature difference over time appears as a straight linethat can be described mathematically by a simple formula andrelated to lethality requirements by a set of tables that must beused in conjunction with the formula. The originally developedmethod was later modified by Ball and Olson (1957), which hasbeen still widely used in the canning industry.

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Temperature ofretort

Convective heatingof liquid

Conductive heatingof solid particle

00

20

40

60

80

100

120

10

Time (min)

Tem

pera

ture

(°C

)

20 30

Figure 38.5. Heat penetration curves foods in container in relationto retort temperature (♦), convective heating (�), and conductiveheating (�).

Equation 33 derived from the heat penetration curve is usedto estimate the process time, B (minutes).

B = fh logjhIh

g(33)

In order to understand the contents of this equation and de-termine the process time (B), it is necessary to recognize heatpenetration parameters, retort CUT, other several inputs, and as-sumptions of Ball (1923) as indicated in the following section.

Heat Penetration Parameters

The following data are normally obtained from the heat pene-tration data and heating conditions for calculation purposes aswell:

f h—Heating rate index. It is the time required for the straight-line portion of the heating curve to pass through one log cycle.It is also the negative reciprocal slope of the heating rate curve.

jh—Heating rate lag factor. This is a factor that, when mul-tiplied by Ih, locates the intersection of the extension of thestraight-line portion of the semi-log heating curve and the ver-tical line representing the effective beginning of the process(0.58 CUT).

jch = Tr − Tpih

Tr − Tih(34)

Iih—Difference between the retort temperature and food tem-perature at the start of the heating process (T r−T ih).

T r—Retort temperature.Tpih—Pseudo-initial temperature during heating. It is the tem-

perature indicated by the intersection of the extension of theheating curve (original Tpih) and the vertical line representingthe effective beginning of the process (0.58 CUT) (New Tpih).

T ih—Initial food temperature when heating is started.CUT—Come-up time. In batch processing operations, the retort

requires some time for reaching the operating condition. Thetime from steam on to when the retort reached T r is called theCUT.

0.58 CUT—Effective beginning of the process. Implies 58% ofCUT from the start, which do not significantly contributes forprocess lethality. Ball recognized that the last 42% of the CUTonly has a significant contribution on sterilization process.

g—Difference between the retort temperature (T r) and the max-imum temperature (T) reached by the food at the slowestheating point (T r−T).

T ic—Food temperature when cooling is started.Tcw—Cooling water temperature.Ic—Difference between the cooling water temperature and food

temperature at the start of the cooling process (T ic−Tw).gc—The value of g at the end of heating or beginning of cooling

(T r−T ic).f c—Cooling rate index. The time required for the straight-line

portion of the cooling curve to pass through one log cycle. Itis also the negative reciprocal slope of the cooling rate curve.

Tpic—Pseudo-initial temperature during cooling. It is the tem-perature indicated by the intersection of the extension of thecooling curve and the vertical line representing the start ofcooling.

jcc—Cooling rate lag factor. This is a factor that, when multipliedby Ic, locates the intersection of the extension of the straight-line portion of the semi-log cooling curve and the vertical linerepresenting the start of the cooling process.

jcc = Tcw − Tpic

Tcw − Tic(35)

Formula methods make use of heat penetration data ob-tained from a semi-logarithmic plot of the temperature difference(T r−T) on log scale against time (on linear abscissa).When thetemperature difference is greater than one, three-cycle log paperis recommended (Fig. 38.6), and when the difference is belowone, four-cycle log paper is required.

Gathered time–temperature data may be plotted in two differ-ent ways. In the first case, log (T r−T) versus time is plotted onthe paper (spread sheet), and the slope is obtained from the lin-ear portion of the negative reciprocal slope of the graph. The lagfactor is calculated from the intercept of the equation of the line.In the second case, the semi-log paper is rotated 180◦ (Jacksonplot) and the top line of the paper is marked 1◦ less than retorttemperature (T r−1◦C), the next log cycle is marked 10◦ less(T r−10◦C), and then the third cycle 100◦C less (T r−100◦C).Using this interval, the product temperatures are directly num-bered without conversion on semi-log graphing paper to producethe linear plot needed to determine heating rate index (f h) andlag factor (jh) (Fig. 38.7).

The Retort CUT, f h and jh Values

Once heating medium (hot water or steam) is injected into theretort, the retort is not reached immediately to the specifiedoperating temperature of 121.1◦C. The time from introduction of

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0 12 24 36 48 60 72 84 96

Tem

pera

ture

(°C

)

Time (min)

21

111

120

-79

-879

Tr-1

Tr-10

Tr-100

Tr-1000

T Tr-T

One log cycle on 3 log cycle paper

Figure 38.6. Diagram showing how the axis of semi-logarithmic graphing paper (rotated 180◦C) is marked for plotting heat penetration datafor retort temperature of 121◦C.

96847260483624120

Tem

pera

ture

(°C

)

Time (min)

21

111

120

- 79

-879

fh

Tih

Tpih

TR-1

TR-10

TR- 100

TR-1000

T Tr-T

58% of CUT

Figure 38.7. Heat penetration curve on semi-log paper duringheating phase.

heating medium to when processing temperature (T r) is reachedis the retort CUT. Among his several assumptions, Ball (1923)assumed that 58% of the retort CUT has no significant heatingor lethality value; therefore, heating starts from a pseudo-initialtime (tpih), which is 058% of CUT (0.58 CUT). According toFigure 38.7, the total CUT is assumed as 12 minutes. Therefore,the corrected zero time or pseudo-initial time (tpih) is 7 minutes(0.58∗12). From hypothetical data of Figure 38.7, the heatingparameters of jh and f h are

jh = Tr − Tpih

Tr − Tih= 121 − 61

121 − 71= 1.2 and,

fh = 74.4 − 14.4 = 60 minutes

The heating rate index (f h−value) is obtained as the time forthe curve to traverse one logarithmic cycle (Fig. 38.7).

For cooling part, to plot the cooling curve on semi-log paper(in this case you do not need to rotate 180◦), the bottom line ismarked 1◦C, the second log cycle 10◦C, and the third log cycle100◦C above the cooling water temperature (Figure 38.8). Then,temperatures are plotted directly on the y-axis of semi-log paperand time of cooling on x-axis. Cooling parameters, f c and jcc,(Equation 35) are obtained likewise during heating, except thereare no come-down time considerations during cooling.

In addition to this, the cooling curve can be obtained by plot-ting a cooling curve graph from log difference of temperatureof a product to that of cooling water temperature (log(T−Tcw))

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

Tem

pera

ture

(oC

)

Time (min)

fc

Tpic

Tic

Tcw+1

Tcw+10

Tcw+100

Tcw+1000 1000

100

10

1

Figure 38.8. Heat penetration curve on semi-log paper duringcooling phase.

versus time. The beginning of cooling time (time zero) is con-sidered the end of heating time. The cooling rate index (f c) iscalculated from the negative reciprocal slope of cooling curveand the cooling lag factor (jcc) from the intercept of the equation.

Ball calculated the process time based on straight-line equa-tion of heating curve and integrating the effect of process timeover the kinetic data of microbial destruction. He developed ta-bles (Table 38.6) and graphs to relate process time and process

Table 38.6. f h/U Versus Log g Values for the BallMethod

f h/U Log g f h/U Log g

0.4 −1.79 5 0.7420.5 −1.29 6 0.8050.6 −0.949 8 0.8940.7 −0.736 10 0.9550.8 −0.544 20 1.1120.9 −0.392 30 1.1871 −0.273 40 1.2351.2 −0.09 50 1.271.3 −0.019 60 1.2961.4 0.042 70 1.3181.5 0.097 80 1.3361.6 0.146 90 1.3521.7 0.183 100 1.3651.8 0.229 120 1.3881.9 0.265 140 1.4062 0.298 160 1.4223 0.525 180 1.4354 0.655 200 1.447

lethality. These were based on relating two parameters: g (tem-perature difference between heating medium and product at theend of heating) as a measure of process time and f h/U (ratioof heating rate to sterilization value) as the measure of processlethality.

The lag factor for cooling cycle is important, because a sig-nificant contribution of sterilization takes place during the earlypart of the cooling period. Ball (1923) assigned a value of 1.41for jcc and also assumed f h = f c so that the needed informationcan be obtained just from the heating curve. The values for f c

tend to be larger f h because of the slower heat transfer with watercompared to steam. Considering these factors, Ball related di-mensionless ratio f h/U versus g and expressed their relationshipin the form of tables and figures. With the above assumptions,and experimentally obtained heating parameters (f h, jch, Ich),process calculations can easily be accomplished. Table 38.7 isan illustration of how process time (B) can be calculated. It canbe seen it is necessary to get the input from Table 38.7 for find-ing the value of log g from f h/U. As discussed before, the retortdoes not reach the desired retort temperature immediately afterthe steam is turned on, rather needs a finite heating time to cometo operating temperature (CUT). On the basis of the assumed42% effectiveness for the CUT, the operator’s process time (Pt)is obtained from the Ball process time (B) (Equation 36).

B = Pt + 0.42CUT (36)

The operator’s process time Pt is the time interval from thetime the retort reaches the desired process temperature to thetime the steam is turned off. The CUT correction concept is onlyapplied in Formula Methods, because in General Methods, theeffect of the length of the CUT will be automatically includedin the calculated lethality value because all the temperaturesused in the calculation will reflect the effect of heat flowing intothe product during the CUT. The stepwise procedure for calcu-lating process lethality using Ball formula method is shown inTable 38.8.

Stumbo’s Method

Stumbo’s f h/U versus log g tables (Stumbo 1973) includes thecontribution from both the heating and cooling lags. These tableswere developed after considering some restrictive assumptionsmade by Ball (1923).

Generally, the Formula Method assumptions were

1. Ball assumed a constant hyperbolic lag factor of 1.41(jcc = 1.41) for the cooling conditions, which is not thesame in many cases. Early part of the cooling curve con-tributes significantly to the process lethality and hence jcc

is very important. Stumbo and Longley (1966) publishedtables that considered variations in cooling lag factor in-stead of assuming it to be constant (Table 38.9).

2. This is one of the assumptions made by Ball (1923) as-suming the heating rate is equal to cooling rate duringthermal processing time at the slowest heating part. Com-monly under practical condition f h<f cc, which is more

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Table 38.7. Procedure in Calculation of Process Time Using Ball Method for ConductiveHeating Food

Process Time Calculation

1 jh 1.412 fh 60 min3 Retort temperature (Tr) 245◦F4 z (◦F) value 18◦F5 Lethality(Fo) 5 min6 CUT 8 min7 Initial product temperature (Ti) 170◦F8 Ih = Tr−Ti 75◦F =245−1709 jh∗Ih 105.75 = 1.41∗7510 Log (jh∗Ih) 2.024

11 Fi = 10(

250−Trz

)1.9 =10( 250−245

18 )

12 fh /U = fh /(Fo∗Fi) fh /U = 60/(1.9∗5)=6.32

13 Log g From Ball Table 38. 6 by interpolation = 0.82114 Log(jh.Ih) − log g 1.203 = 2.024−0.82115 B = fh [log(jh∗Ih)−log g)] 72.18 min =60∗1.20316 Pt = B − 42% CUT 68.82 min = 72.18 − 0.42∗8

conservative assumption from safety point of view butfrom quality point of view it has an over processing ef-fect. In practice, most of the heating is done with steam oragitated water, while cooling is normally done in slowlycirculating water. Therefore, generally f h < f c and f h = f c

gives a more conservative processing conditions.3. The temperature difference between the retort temper-

ature and cooling water temperature (T r−Tcw) is thedriving force. This is expressed in terms of Ic, which is(Ic = T ic−Tcw). Ball assumed that the sum of Ic + g =T r−Tcw = 180◦F (for steam) or 130◦F (for full

immersion water overpressure). Stumbo (1973) discov-ered that, a 10◦F change in T r−Tcw can result in 1% dif-ference in calculated lethality (F0) and can be corrected ifrequired.

4. CUT correction for jh value. Ball noticed that only about42% of the CUT can be considered as effective contribu-tion to the process time.

5. No further product heating after cooling starts. However,this is not always valid for the critical point of conduc-tion in heated canned foods. In such cases, considerableoverprocessing can occur.

Table 38.8. Calculation of Process Lethality Using Ball Method From Hypothetical Given Data

Process Time Calculation

1 jh 2.052 fh 34.9 min3 CUT 8 min4 Pt 45 min

Process time B = Pt + 42% CUT5 (B = Pt + 42%CUT) B =45+0.42∗8 = 48.36 min6 Retort Temperature (Tr) 260◦F7 Initial temperature(Ti) 100◦F8 Ich = Tr − Ti 160 ◦F =260−1009 jh∗Ich 328 ◦F = 2.05 ∗16010 log (jh∗Ih) 2.5211 z value 18◦F12 Fi = 10( 250−T r

z ) 0.27813 B/fh 1.39 = 48.36/34.914 log g = log(jh∗Ih)−B/fh log g= 1.13 = 2.52−1.3915 fh /U From Ball Table 38.6 by interpolation = 22.416 Fo = f h

(f h/U∗Fi )Fo = 34.9/(22.4 ∗0.278) = 5.60 min

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Table 38.9. fh/U Versus g Relationships When z = 18◦F for Stumbo Method (1973)

Values of g When j of Cooling Curve is

f h/U 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

0.20 4.09–05 4.42–05 4.76–05 5.09–05 5.43–05 5.76–05 6.10–05 6.44–05 6.77–050.30 2.01–03 2.14–03 2.27–03 2.40–03 2.53–03 2.66–03 2.79–03 2.93–03 3.06–030.40 1.33–02 1.43–02 1.52–02 1.62–02 1.71–02 1.80–02 1.90–02 1.99–02 2.09–020.50 4.11–02 4.42–02 4.74–02 5.06–02 5.38–02 5.70–02 6.02–02 6.34–02 6.65–020.60 8.70–02 9.43–02 1.02–01 1.09–01 1.16–01 1.23–01 1.31–01 1.38–01 1.45–010.70 0.150 0.163 0.176 0.189 0.202 0.215 0.228 0.241 0.2550.80 0.226 0.246 0.267 0.287 0.308 0.328 0.349 0.369 0.3900.90 0.313 0.342 0.371 0.400 0.429 0.458 0.487 0.516 0.5451.00 0.408 0.447 0.485 0.523 0.561 0.600 0.638 0.676 0.7152.00 1.53 1.66 1.80 1.93 2.07 2.21 2.34 2.48 2.613.00 2.63 2.84 3.05 3.26 3.47 3.68 3.89 4.10 4.314.00 3.61 3.87 4.14 4.41 4.68 4.94 5.21 5.48 5.755.00 4.44 4.76 5.08 5.40 5.71 6.03 6.35 6.67 6.996.00 5.15 5.52 5.88 6.25 6.61 6.98 7.34 7.71 8.077.00 5.77 6.18 6.59 7.00 7.41 7.82 8.23 8.64 9.058.00 6.29 6.75 7.20 7.66 8.11 8.56 9.02 9.47 9.939.00 6.76 7.26 7.75 8.25 8.74 9.24 9.74 10.23 10.73

10.00 7.17 7.71 8.24 8.78 9.32 9.86 10.39 10.93 11.4715.00 8.73 9.44 10.16 10.88 11.59 12.31 13.02 13.74 14.4520.00 9.83 10.69 11.55 12.40 13.26 14.11 14.97 15.82 16.6825.00 10.7 11.7 12.7 13.6 14.6 15.6 16.5 17.5 18.430.00 11.5 12.5 13.6 14.6 15.7 16.8 17.8 18.9 19.935.00 12.1 13.3 14.4 15.5 16.7 17.8 18.9 20.0 21.240.00 12.8 13.9 15.1 16.3 17.5 18.7 19.9 21.1 22.345.00 13.3 14.6 15.8 17.0 18.3 19.5 20.8 22.0 23.250.00 13.8 15.1 16.4 17.7 19.0 20.3 21.6 22.8 24.160.00 14.8 16.1 17.5 18.9 20.2 21.6 22.9 24.3 25.770.00 15.6 17.0 18.4 19.9 21.3 22.7 24.1 25.6 27.080.00 16.3 17.8 19.3 20.8 22.2 23.7 25.2 26.7 28.190.00 17.0 18.5 20.1 21.6 23.1 24.6 26.1 27.6 29.2

100.00 17.6 19.2 20.8 22.3 23.9 25.4 27.0 28.5 30.1150.00 20.1 21.8 23.5 25.2 26.8 28.5 30.2 31.9 33.6200.00 21.7 23.5 25.3 27.1 28.9 30.7 32.5 34.3 36.2250.00 22.9 24.8 26.7 28.6 30.5 32.4 34.3 36.2 38.1300.00 23.8 25.8 27.8 29.8 31.8 33.7 35.7 37.7 39.7350.00 24.5 26.6 28.6 30.7 32.8 34.9 37.0 39.0 41.1400.00 25.1 27.2 29.4 31.5 33.7 35.9 38.0 40.2 42.3450.00 25.6 27.8 30.0 32.3 34.5 36.7 38.9 41.2 43.4500.00 26.0 28.3 30.6 32.9 35.2 37.5 39.8 42.1 44.4600.00 26.8 29.2 31.6 34.0 36.4 38.8 41.2 43.6 46.0700.00 27.5 30.0 32.5 35.0 37.5 39.9 42.4 44.9 47.4800.00 28.1 30.7 33.3 35.8 38.4 40.9 43.5 46.0 48.6900.00 28.7 31.3 34.0 36.6 39.2 41.8 44.4 47.0 49.7999.99 29.3 31.9 34.6 37.3 39.9 42.6 45.3 47.9 50.6

6. Also, the Stumbo method accommodates different valuesof z and can be used for not only cold-point values but alsofor integrated lethality computations. The use of this yieldmore accurate process times (Smith and Tung 1982).

Typical calculations of process time and process lethality us-ing Stumbo method are illustrated in Tables 38.10 and 38.11.

QUALITY OPTIMIZATIONThe effect of the thermal sterilization process on quality andnutrient retention of food has been of major concern for foodprocessors since the invention of canning. During sterilizationof foods, heat has two simultaneous effects: sterilization andcooking. Therefore, it is necessary to optimize between these

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Table 38.10. Calculation of Process Time (B) UsingStumbo Method

1 jh 0.962 fh 12.6 min3 Process lethality (Fo) 5 min4 Retort temperature (Tr) 245◦F5 Initial temperature (Ti) 175◦F6 Ih = Tr−Ti 70◦F = 245−1757 jh∗Ih 67.28 Log(jch

∗Ih) 1.759 Z value 18◦F

10 Fi = 10(

250−Trz

)1.9

11 fh/U = fh/(Fo∗Fi) 1.33

12 jcc

From Stumbo’s table for z = 18◦F(18◦F) (jcc = 1.6), obtain gvalue by interpolation

fh/U g value1.00 0.6381.33 ?2.00 2.34

Interpolated g value is 1.2 forfh/U value of 1.33

1.6

13 log g 0.079214 B = fh [log (jch Ih)−log g)] 21.68 min

Table 38.11. Calculation of Process Lethality (F0) UsingStumbo Method

1 jh 1.92 fh 36.2 min3 Operator process time (Pt) 42 min4 CUT 10 min5 Retort temperature (Tr) 255◦F6 Initial temperature (Ti) 160◦F7 Ih = Tr−Ti 95◦F8 jch

∗Ih 180.59 Log(jch

∗Ih) 2.2610 z value 18◦F

11 Fi = 10(

250−Trz

)0.53

12 B = Pt + 42% CUT 46.2 min12 B/fh 1.2813 log g = log(jch

∗Ih)−B/fh 0.9814 g 9.5515 jcc

From Stumbo’s table for z = 18◦F(jcc = 0.8). Obtain fh/U valueby interpolation

fh/U g value10.00 8.24? 9.5515.00 10.16

Interpolated fh/U value is13.41 for g value of 9.55

0.8

16 F0 = fh/[(fh/U)∗Fi] 5.1 min

two effects to get a better quality safe product. This goal can beachieved by optimizing thermal process conditions. In order toachieve proper process optimization, accurate determination ofkinetic parameters of microorganisms, spoilage enzymes, andquality factors are crucial. The need to optimize processing con-ditions arises when the kinetic behavior of the different com-ponents is considered because the rate of a chemical reactiongenerally doubles for a 10◦C rise in temperature, whereas ratesof bacterial destruction increase tenfold under similar condi-tions (Holdsworth 1985). Gathered kinetic parameters are usedto develop reliable predictive models that will ensure to producebetter quality products without compromising food safety.

There are two key issues that should be explored to pro-duce better-quality safe products. The first choice may focuson reducing thermal intensity levels, through reducing of theheating, and cooling times associated with the process throughimproving heat transfer property of processing condition andmodification of geometry of packaging material. The other op-tion is to look for novel thermal or nonthermal food processingtechnologies that will ensure better quality products with rel-atively same degree of safety. New preservation technologiesare utilized because of their expected potential to inactivate mi-croorganisms and spoilage enzymes with a very little damageon product quality.

High-Temperature Short-Time and Ultra-HighTemperature Processing

As the names implies, high-temperature short-time (HTST) andultra high temperature (UHT) processes use higher tempera-tures and shorter processing times than conventional thermalprocesses to improve quality of foods and beverages. Becausethe products are exposed to high temperatures for short times,there is reduced degradation of quality factors of the productswhile causing a greater effect on destruction of food microor-ganisms.

The principle of HTST or UHT process is easily understood bycomparing the heat resistance of nutrient components, microbialspores, and vegetative bacteria (Figure 38.9). The D value atreference temperature for vegetative bacteria are in fraction ofseconds (z = 5◦C); microbial spores, 0.2 minute (z = 10◦C); andnutrient components, 100–150 minutes (z = 25−30◦C). Thisvariation gives an opportunity to optimize processing conditionfor better quality production of foods and beverages.

From destruction of vegetative bacteria point of view, area (A,B, and C) beneath the dotted line of the graph are not acceptable(Figure 38.9). In these sections, the vegetative bacteria are notyet destroyed. The same is true for section D, E, and F, wherethe product is cooked but spore formers still survive the heattreatment. All sections of G, H, and I above the bold line givessterile product. However, the question that comes here is that,to what extent the product is cooked with respect to the qualityfactors. The top and bottom broken D lines represent 90% and10% destruction of a nutrient component, respectively. Particu-larly, section I of the graph gives a safe product with less than10% destruction of nutrient. This is the region corresponding toHTST or UHT treatment as indicated by the arrows.

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90% destruction of nutrient

40% destruction of nutrient

10% destruction of nutrient

Logtime

Temperature

C

B

AD

E

F

G

H

I

Figure 38.9. A diagram of resistance of nutrient (- - -), microbial spores (–), vegetative bacteria (•••), and HTST heating principle.

For instance, in UHT sterilization process, milk is exposed for2–5 seconds at high temperatures of 140–145◦C to kill bacteriaspores. UHT treatment limits browning of the milk, developmentof cooked flavor, and denaturation of proteins. In HTST pasteur-ization, milk is subjected to a temperature of 71.1◦C for 15–20seconds. HTST or UHT approaches are not always beneficialfor conduction heating food products that heat relatively slowly,exhibiting large temperature gradients between surface and cen-ter of the container. However, they are best for liquid foods andliquids containing small particles that can heat rapidly while sub-jected to in-container thermal processing or in-heat exchangers,as an aseptic processing.

Agitation Processing

Some retorts agitate the cans during processing in order to in-crease the rate of heat penetration in cans. As compared to staticretorts, the process time may be reduced by 80% because the con-tents are heated up faster and more evenly. Agitation processingis mainly groped into axial and end-over-end (EOE) type (Fig.38.10). EOE involves the containers being loaded vertically anda crate rotating around a central horizontal axis. During axialrotation, cans are rotated individually in the horizontal plane.

As the containers are agitated inside a retort, the contents ofthe containers are mixed uniformly; this eliminates cold spotsand increase heat penetration rate. Mixing largely is due to themovement of the headspace bubble during agitation, and to beeffective, there must be sufficient headspace in cans. Both smalland large headspace may result in lower heat transfer, leading tounderprocessing because of limitation of mixing of food com-ponents in cans. In addition to headspace, fill of the container,solid to liquid ratio, consistency of the product, and the speed ofagitation are the crucial factors to be standardized in agitatingprocessing. Unless and otherwise these conditions are properlyoptimized for a given product and processing condition, the re-quired fast heat penetration may not be achieved.

Aseptic Processing

The word “asepsis” implies the process of removing pathogenicmicroorganisms or protecting against infection by such organ-isms. It can be defined as a state of control attained by usingan aseptic work area and performing activities in a mannerthat precludes microbiological contamination of the exposedsterile product. In food processing, aseptic processing involvesa section that precludes microbiological contamination of the

Cans in vertical position Cans in horizontal position

Figure 38.10. End-over-end and axial agitation orientation of cans in retort cage.

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final sealed product. During aseptic processing, the product isexposed to desired treatment temperature to eliminate food mi-croorganisms and finally packed in sterile container in an asepticenvironment. This processing method is more efficient for liquidproducts and liquid products containing small particles. Asepticprocessing sterilizes food and beverages in a way that puts theleast amount of thermal stress on a product, so nutrients and nat-ural flavors, colors, and textures are maintained while sterility isensured.

The aseptic process begins with sterilization of both the pro-cessing system and the filler. Generally, this is done with hotwater or saturated steam. Food is then pumped into the asepticprocessing system for sterilization. The flow of food is con-trolled via a timing pump, so that no food flows too fast or tooslow.

This process consists of heating, holding, cooling, and packingsteps. First, the product is heated and held for some time to attaindesired degree of sterilization. Once the food leaves the holdtube, it is sterile and subject to contamination if microorganismsare permitted to enter the system. The best way to keep theproduct sterile is to keep it flowing and pressurized. Heating stepis followed by cooling of the product. The next step is moving theproduct into an aseptic surge tank to hold the product just beforepackaging. Meanwhile, the packaging material is sterilized fromthe other side of the aseptic environment. Packaging material issterilized by liquid hydrogen peroxide at a high temperature.After the food and package are sterilized, the sterile package isfilled with the food, closed, and sealed in a sterile chamber.

The direct and indirect heating processes used in aseptic pack-aging can affect the final taste of the food. Indirect heating, thefood comes into contact with either a metal plate or tube, whichcan give food, such as milk, a burnt flavor. There are threetypes of indirect heating processes: plate heat exchangers, tubu-lar heat exchangers, and scraped-surface heat exchangers. Theformer two are mainly used for liquid foods and the latter one isfor more viscous and particulate foods to prevent fouling due tohigh temperature effect. All use a physical separation betweenthe product and the heating medium and transfer heat througheither a plate or tube to the product.

Direct heating uses steam injection or steam infusion andminimizes the burnt flavor of the product by letting the foodcome into direct contact with the heat source. With steam in-jection, product and steam are pumped through the same cham-ber; while with steam infusion, the product is pumped through asteam-filled infusion chamber. In all these heat exchangers, sincethe product is directly exposed to heating medium or subjectedto thin profile mode of heating, it allows faster heat penetra-tion within short period of time, which destroys microorganismswith limited effect on quality of the product. That is why asepticprocessing is considered as HTST processing in terms of opti-mization of quality with desired degree of safety. However, forparticulate foods, the rate of heat penetration in the center of theslowest heating point of the product should be determined to en-sure desired degree of pasteurization/sterilization. In particulatefoods, first the heat is exchanged between the heating mediumand liquid food, and then transferred to the particle. In this case,it is difficult to measure the temperature of the moving particle.

The time–temperature profile in the particle can be estimatedusing mathematical models. Based upon the data of the modelused, the particle should be held at appropriate temperature forsufficient time to achieve the required sterilization value at thecenter.

Thin Profile and Retort Pouch Processing

Quality optimization is mainly achieved through enhancing theheat transfer rate from the heating medium to the product. Anyresistance to rapid heat penetration slows down the heat pene-tration rate and exposed the product for longer heating time.The nature of packaging material, thickness, and its overallthermal conductivity determine the heat transfer rate. In con-ventional thermal processing, metal cans and glass containersare the main type of containers used. However, the thicknessof these containers limits fast transfer of heat between heatingmedium and product. Modification of geometric configurationof the packaging materials from material thickness and geom-etry point of view is additional opportunity to optimize qualitythrough improving heat transfer rate. Foods in rigid polymertrays or flexible pouches heat more rapidly, owing to the thinnermaterial and smaller cross-section of the container.

Much of the retort pouch development was conducted by theU.S. Army Natick Research and Development Center for usein the Meal Ready-to-Eat (MRE), having relatively light weightas compared to conventional metal containers. However, thethin thickness of the pouch and its flat shape allows fast heatpenetration and contributes much on improvement of quality offoods as compared to conventional packaging materials. The re-tort pouch is a flexible, heat sealable container that is thermallyprocessed like a can and used to produce shelf stable, commer-cially sterile food products. It is constructed of a 3-ply laminatecomposed of an outer layer of polyester film, a middle layer ofaluminum foil, and an inner layer of polypropylene. The layersare bonded together with a special adhesive. The tri-laminatematerial provides seal integrity, toughness, puncture resistance,printability, and superior barrier properties for long shelf life. Italso withstands the rigors of thermal processing up to 135◦C.

Retort pouches are filled with wet foods sealed and then heat-treated in steam/hot water retort kettles to achieve commercialsterilization (for shelf-stable foods) or pasteurization (for re-frigerated foods). Because processing time typically is faster inthe pouch than in metal, glass, or rigid plastic containers, theproduct tends to end up with better quality and safety. Pouchesalso offer lower shipping and storage costs (pouch material islighter than cans, and pouches take up less storage space thancans); easier, safer handling (no can openers or thawing timerequired); and reduced product waste and reduced volumes ofdisposed packaging waste material as compared to cans.

Some of the disadvantages include a lack of physical dura-bility and slow production rates due to slow filling and sealingrate as compared to cans or glass jars. Furthermore, it needs anoverpressure processing to protect the integrity of the packagesduring processing. Pouches are more easily punctured; they canoverwrap for safe distribution.

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Novel Thermal Food Processing Technologies

Microwave and Radio Frequency Heating

Microwave (MW; 300–300,000 MHz) and Radio Frequency(RF) waves (0.003–300 MHz) are a part of the electromag-netic spectrum. MW and RF energy generates heat in dielectricmaterials such as foods through dipole rotation and/or ionic po-larization. MW ovens are now common household appliances.Popular industrial applications of MW heating in food process-ing operations include tempering meat or fish blocks and pre-cooking bacon or meat patties, while RF heating is commonlyused in finishing drying of freshly baked products. Such applica-tions shorten processing times, reduce floor space, and improveproduct qualities compared to conventional methods.

Extensive research has been carried out over the past 50 yearson MW and RF energy in pasteurization, sterilization, drying,rapid extraction, enhanced reaction kinetics, selective heating,disinfestations, etc., but with limited applications. Technologi-cal challenges remain and further research is needed for thoseapplications. MW/RF sterilization applications demand morethorough and systematic studies as compared to other applica-tions. These studies will have far reaching impacts to the foodindustry and research communities.

Several commercial 2450 MHz MW sterilization systems pro-duce shelf-stable packaged foods in Europe (e.g., Tops Foods,Olen, Belgium) and Japan (Otsuka Chemical Co., Osaka), butthese systems are designed with multi-mode MW cavities. Gen-erally, MW/RF heating is a promising alternative to conventionalmethods of heat processing as it is regarded as a volumetric formof heating in which heat is generated within the product, whichreduces cooking times and could potentially lead to a more uni-form heating. Reduction in processing time and uniform heatingresults in getting high-quality product in terms of its nutrientcontent, desired flavor, texture, color, and taste.

Ohmic Heating

Ohmic heating (OH) is based on the passage of alternating elec-trical current through a food product that serves as an electricalresistance. Because of the current passing through the food sam-ple and its resistance to the flowing current, relatively rapidheating occurs. OH has good energy efficiency since almost allof the electrical power supplied is transformed into heat. Manyfactors affect the heating rate of foods undergoing OH: electri-cal conductivities of fluid and particles, the product formulation,specific heat, particle size, shape, and concentration as well asparticle orientation in the electric field.

As compared to conventional heating technologies, internalheat generation by OH eliminates the problems associated withthe heat conduction in food materials and then prevents theproblems associated with overcooking. Aseptic processing hasbeen used commercially for long time for liquid foods, but forproducts containing particulates, the use of conventional heat-transfer techniques leads to overprocessing of the liquid phase toensure that the center of a particulate is sterilized. This can resultin destruction of flavors and nutrients, and mechanical damage to

the particulate. However, OH-treated product is clearly superiorin quality than those processed by conventional technologies.This is mainly due to its ability to heat materials rapidly anduniformly, leading to a less-aggressive thermal treatment. Hence,OH can be considered as a HTST aseptic process (Castro et al.2004).

Novel Nonthermal Processing Technologies

High-Pressure Processing

High-pressure Processing (HPP) is an innovative technologicalconcept that has a great potential for extending the shelf lifeof foods with minimal or no heat treatment. It is a processaimed at controlling deteriorative changes such as microbialand enzymatic activity without subjecting the product to drasticthermal processing and mass (drying) transfer techniques suchthat the original quality is retained.

The application of hydrostatic pressure to food results in theinstantaneous and uniform transmission of the pressure through-out the product independent of the product volume. The treat-ment is unique in that the effects neither follow a concentrationgradient nor change as a function of time. A significant advan-tage is the possibility of operation at low or ambient temperaturesso that the food is essentially raw. Gelation, gelatinization, andtexture modification can be achieved without the application ofheat. Apparently, HPP is a physical treatment and is not expectedto cause extensive chemical changes in food system. Once the de-sired pressure is reached, the pressure can be maintained withoutthe need for further energy input. Liquid foods can be pumpedto treatment pressures, held, and then decompressed asepticallyfor filling as with other aseptic processes.

HPP is a novel technique for processing of foods and hasattracted considerable attention in recent years. It has been com-mercialized for a variety of acid and acidified food products. Forlow-acid foods, it has been used only as a temporary measureof extending the shelf life under refrigerated storage conditions.It has also been used for several other purposes, including con-trol of some pathogens and viruses, for inducting functionalchanges, as well as improving nutritional and sensory quality offoods. HPP has been used mainly for refrigerated and high-acidfoods. Pasteurization by HPP can be carried out at pressuresin the range of 400–600 MPa at relatively moderate tempera-tures (20–50◦C). Under these conditions, HPP can be effectivein inactivating most vegetative pathogens and spoilage microor-ganisms, but the quality factors remain unaffected.

HPP for producing shelf-stable low-acid foods is still a topicof considerable controversy. It has the potential to produce betterquality foods than possible from the use of processing noveltiessuch as MW, RF, or OH techniques in combination with asepticprocessing. This is because HPP allows the product temperatureto increase very rapidly (due to adiabatic heating) from around90–100◦C to the sterilization zone (120–130◦C; in about 2 min-utes) and bring it back almost instantaneously by depressuriza-tion. The process can be formulated either as a pressure-assistedthermal processing. A better understanding of the inactivationkinetics of pathogens or their surrogates under pressure-assisted

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thermal process conditions is the key for the success of the HPPand for regulatory approval.

The critical factors in the HPP include pressure, time toachieve treatment pressure, time at pressure, depressurizationtime, treatment temperature (including adiabatic heating), prod-uct initial temperature, vessel temperature distribution at pres-sure, product pH, product composition, product water activity,packaging material integrity, and concurrent processing aids.Although HP processing related research work has increasedtremendously in the last decade, there is serious lack of infor-mation in this area to permit establishing a reliable process.

Pulsed Electric Field

Pulsed electric field (PEF) processing is a nonthermal methodof food preservation that uses short bursts of electricity for mi-crobial inactivation and causes minimal or no detrimental effecton food-quality attributes. PEF can be used for processing liquidand semi-liquid food products. PEF processing involves treatingfoods placed between electrodes by high-voltage pulses in theorder of 20–80 kV (usually for a couple of microseconds). Theapplied high-voltage results in an electric field that causes micro-bial inactivation. A series of short, high-voltage pulses breaks thecell membranes of vegetative microorganisms in liquid mediaby expanding existing pores (electroporation) or creating newones. The membranes of PEF-treated cells become permeable tosmall molecules; permeation causes swelling and eventual rup-ture of the cell membrane. The treatment is applied for less thanone second, so there is little heating of the food, and it main-tains its “fresh” appearance, shows little change in nutritionalcomposition, and has a satisfactory shelf life (Castro et al. 1993,Kozempel et al. 1998). Since it preserves foods without usingheat, foods treated this way retain their fresh aroma, taste, andappearance.

RETORT TYPES FOR COMMERCIALAPPLICATIONRetorts generally are either batch or continuous types. Theirconfiguration may be either vertical or horizontal. Horizontalretorts are commonly preferred due to their ease of loading andunloading facilities as compared to vertical retorts.

Batch Retorts

Steam Heating Retort

In this type of retort (static or rotary), saturated steam is used asa heating medium. Latent heat is transferred from steam to thecans when saturated steam condensed at the surface of the cans.The saturated steam process is the oldest method of in-containersterilization. Since air is considered an insulating medium, satu-rating the retort vessel with steam is a requirement of the process.If air enters in the retort, it forms insulation layer of the surfaceof the can and prevents the condensation of steam and causesunderprocessing of the product. It is inherent in the process thatall air be evacuated from the retort by flooding the vessel with

steam and allowing the air to escape through vent valves. Thereis no overpressure during the sterilization phases of this process,since air is not permitted to enter the vessel at any time duringany sterilization step. However, there may be air-overpressureapplied during the cooling steps to prevent container deforma-tion. This is because during cooling, the steam rapidly condensesin the retort and the food cools more slowly and pressure in thecontainer remains high. When the food temperature becomesbelow 100◦C, overpressuring is stopped and the food is allowedto continuously cool up to 40◦C.

Water Heating Retort (Immersion and Spray Modes)

The water immersion process is the most widely acceptedmethod of sterilizing product using an overpressure process.The water immersion process is similar to a saturated steam pro-cess in that the product is totally isolated from any influence ofcooling air. The product is totally submerged in preheated waterat preset temperature. But it is different from saturated steamin that air can be introduced into the vessel during sterilization.The water is preheated in different chamber with steam to de-sired temperature and pumped to retort chamber. Overpressureis provided by introducing air (or steam) on top of the water. Insome instances, air is added to the steam (which then heats theair). The heated air agitates the water as it flows to the surfaceand serves to pressurize the process load. Because it is an over-pressure process, the machine can handle most, if not all, of thefragile containers. For cooling, the water present in the retortpasses through a heat exchanger, where it is gradually cooled byfresh water circulating in the service side.

The water spray process is also an overpressure process, likewater immersion, except that the product is exposed to the influ-ence of the overpressure air. In this retort, low amount of wateris stored in the bottom of the retort and circulated by a pumpwith high flow rate and sprayed on containers. It is similar tothe saturated steam process in that steam is the driving forcefor reaching the center of the load. But the water spray processis different from saturated steam in that air can be introducedinto the vessel during sterilization. Overpressure is provided byintroducing air (or steam) into the retort. To overcome the insu-lating effects of the air, the spray nozzles vaporize the steam andmix the steam with the air. The condensates are automaticallyevacuated by a drainer and can be returned to the boiler.

Steam-Air Heating Retort

The steam-air process is an overpressure process, like water im-mersion, except that the product is exposed to the influence ofthe overpressure air. Steam is directly injected inside the re-tort chamber and distributed and mixed uniformly with air withthe help of a fan to prevent cold spots in the retort. It is a“ventless” process, resulting in significant energy savings. Fur-thermore, a precise control of the steam/air process results ina shortened cycle time for maximum production and consistentquality, and reduces the process time and optimal heat transfercharacteristics, which allow foods to retain more of their naturalqualities. The opening of the steam inlet valve is automatically

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monitored, depending on the programmed temperature param-eters. The pressure is also independently controlled from thetemperature by injecting or venting compressed air.

Continuous Retorts

In continuous retort, filled and sealed containers are contin-uously moved from the atmospheric condition into steam-pressurized environment for desired degree of processing. Inthis process, cans enter a continuous retort and are instanta-neously at the processing temperature; therefore, the processtime is exactly the residence time of the cans within the retort.

REFERENCES

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