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ENGINEERING PRINCIPLESAPPLIED IN FOODS

SUMMARY - RESUMEN

Juan deDios Alvarado

INGENIERO EN ALIMENTOS, M. en C.FACULTAD DE CIENCIA E INGENIERA EN ALIMENTOS

UNIVERSIDAD TCNICA DE AMBATO - ECUADOR

SECRETARIA GENERAL DE LA ORGANIZACIN DE LOS ESTADOS AMERICANOS

PROGRAMA REGIONAL DE DESARROLLO CIENTIFICO Y TECNOLGICO

PROYECTO MULTINACIONAL DE BIOTECNOLOGIA Y TECNOLOGA DE ALIMENTOS


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Engineering PrinciplesApplied in Foods

SUMMARY

Most of the scientific information including technical articles and books have been published inEnglish. Iberoamerican contribution, specially Food Engineering texts, is limited and this book has

been written in order to fill in part this necessity.

Content is going to serve as a guide or as a text to lead food engineering and food technologyundergraduates students. Information about several kinds of foods are provided and speciallyphysical properties could be considered. In addition, certain topics like heat process calculationsand heat transfer coeficients determination could help in postgraduation studies.

Nine chapters and thirty eigth topics are presented. It is important to point out experimental datapresentation in whole topics because they were obtained acuting skills to use a small quantity ofavailable equipments and reactives. Scientific principles application is unlimited if the diversity of

foods are consider.

Chapters order is the result of fifteen years of experience as a teacher of the University. Bookincludes basic principles first, food physical properties later and process which involve formerknowledge finally. In addition, a nomenclature for each chapter and indexes of subjects andproducts are presented to make it usefull.

Whole topics are structurated to be use like independient experimental units. They include:Introduction and Bibliography, which consider cognitive aspects of teaching. Materials andMethods and Experimental Results, which explain procedure aspects. Objectives and Questions,in order to cover formative aspects the main purpose of educational activity.

Chapters and topics consider are:

Chapter 1. Material and Energy Balances.

Use of these balances is fundamental for development of several engineerings including FoodEngineering.

Considering a didactic view, this chapter indicates basic concepts. Determinations and calculations

are made to demonstrate balances compliment. The chapter emphasizes the importance andfrequent use of balances in many practical experiences during processing and food study.

Topic 1.1. is about application of material balance during jelly preparation. It presents thefundamental equation and limits to identify total mass entering and leaving the system.Refractometric data are used to quantify soluble solids changes during coction, this values arecompared with the established by solids balance in order to prove their concordance.Jelly, marmelade and jam preparation is common in food industry and when refractometers are

not available these balances could be used by technitians to control process and its conditions forpreparing these products or similar ones. Obteined results demonstrate the worth of solids balance

during concentration for water evaporation.

Topic 1.2. provides a definition of specific heat and an energy balance example to determine it.An equation for specific heat calculation in fruits and fresh vegetables is presented. This topic


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includes the most usefull equations and models for calculation of this thermal property. Hwang

and Hayakawa method to evaluate experimentally specific heat, without direct contact between

product and environment, are based in energy balances. This method and its fundamentals are

described giving details.Cow milk experimental data obteined with calorimeters made from commercial thermos bottles

are provided. They are similar to those ones reported in technical publications and calculated by

equations and models included.Experimental method worth is emphasized, furthermore its use is easy and it could be applied to

several liquid food like juices. This topic shows clearly one of the multiple applications of energy

balances.

Possibly, the most common chart used by engineers is the psychrometric chart, this graphic

presents relationships obtained by material and energy balances established in air and vapour

mixtures. Psychrometric chart calculations and their elaboration are presented in Topic 1.3.

Fundamental equations for physical properties calculations were compiled. A flow diagram and

a computer program, which were made using Fortran language, are included. The main

characteristic of the computer program is its versatility. Program includes the possibility to

introduce changes in calculation due to variability of pressure caused by the different altitud above

the level of the ocean.

Chart plotted for Ambato, a city located at 2540 meters above the level of the ocean, are provided.

The information is important for montainous locations because computer program could be

modified easily according to altitude of particular places.

Chapter 2. Basic Principles of Transport Phenomena.Nowdays, a better understanding of mechanisms and fundamental phenomena related with food

processing is required. This requirement is increasing quickly. A deep knowledge about these

mechanisms is necessary to improve or design more reliable and safer processes. To understand

a particular mechanism is important to know about thermodynamic, kinetic, transport and other

fundamental principles related to the mechanism. Three cases are considered in this Chapter, two

of them are about heat transfer and the other is about material transfer.

It is pretended that students learn about principles and equations that explain this mechanisms.

Methods to determine involved parameters are indicated. But the most relevant point is to inculcate

in students the importance of a real knowledge of this phenomenons for food engineering

development.

Topic 2.1. discusses about Fourier law. This law quantifies thermal energy that flows

perpendiculary to isothermic surfaces in motionless and isotropic materials. Fundamental equations

about heat transfer by conduction are included. Mounting of a system to determine thermal

conductivity of solids materials are described.Fourier law and method were proved using thermal conductivity experimental data measured in

coconut. A computer program in basic language was developed in order to use experimental data

applied in spheres, as an example of the multiple possibilities the computers offer.

Topic 2.2. considers first law of Fick with the purpose to study one of the basic mechanisms of

material transfer.Several equations, which were published in order to determine diffusion coefficient or material

diffusivity, are compiled in this topic. Toledo equation, which could be applied only in food whose

water activity values are high, is included.Diffusivity experimental data from water, fruit juices and wine were determined using tubes made

of glass and the minimal quantity of equipments. These values are compared with data calculated

using as the compiled equations as an included computer program in Basic language.

Comprehension of a phenomenon, that is apararently easy as vapour diffusion from liquid to air,

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is pointed out because it allows to explore important aspects like molecular mass effect and

molecular collisions.

Topic 2.3. discusses conjunctly tire mechanism of heat transfer by convection and radiation. It i

indicated that Newton law is important to determine convective heat transfer coefficient. Some othe most useful equations to quantify heat flow due to free convection in cylinders in contact wit

air, were chooses and transcribed. Some of these are dimensionless, others have dimensio

numbers and they are specific for materials in contact with air.

Experiments using horizontal and vertical ice cylinders were done An analogy between heat an

mass transfer was applied to determined heat transfer coefficients of free convection to air. Thes

values were compared with those ones calculated by chosen equations. Relationship betweedifferent mechanisms of heat and mass transfer is important if they are considered simultaneousl

Chapter 3. Kinetic of Reactions that Occur in Foods.

The most important characteristic of foods is their continued tranformation i n the whole timChemical and biochemical reactions happen all around and they cause positive and negativ

changes. This Chapter treated about the quantification of these changes.

Fundamental models of chemical kinetic are studied. Reactions and common changes that occ

in foods are characterized by kinetic equations. The main purpose is inculcate into students th

worth of kinetics study to understand food complexity and to applied this principles in practica

situations, for example shelf life determination.

Topic 3.1. is about oil stability. A general model to determine kinetic reactions of any order wderived.

Peroxid index changes were determined, they are a measurement of fatty acid oxidation. Dat

were obtained from samples of linseed, lupin and pumpkin seeds. The kinetic model was used t

determined the reaction order, kinetic equations and its reaction rates.

A possible application of these values to select antioxidant substances are mentioned. Fatty ac

composition of seventeen oils and fats are included.

Browning is a complex process and occurs as a result of chemical and biochemical reaction

Topic 3.2. are related to banana browning.

With the purpose to calculate the order of the kinetic reaction, based on shelf life concept, a

equation was established.

Easy measurements as spectrophotometric ones were made in samples of four banana varieties

Some samples were under natural conditions and others were under thermal treatment to destro

enzymes. After that, all samples were mantained at 20 and 30C. These values were used t

determine the reaction order. In all of the cases, the reaction order was near to 1, that correspon

to a first order kinetic. A little higher values were registered in samples put down therma

treatment.

Shelf life concept are indicated. A Q10 value, which quantifies sensibility of the changes duetemperature, was calculated. As a result of the thermal treatment, treated samples had Q10 valuehigher than the others, specially in "Seda" and "Cavendish" commercial varieties.

Food nutritional aspects and nutritive value must be consider always during food process. Topi

3.3. discusses ascorbic acid or vitamin C degradation because it is a typical thermolabil compound

Several researches made about this are mentioned. Ascorbic acid content was determined by

chemical method in fresh juices of lime, lemon, tangerine and grapefruit. Samples were maintaineunder aerobic conditions and four temperatures during different times.

It was proved that degradation follows a first order kinetic, equations and its practical application

are included. Arrhenius equation, which are used to calculate activation energy, describe

temperature effect on this reaction from 20 to 92C. The highest value of activation energ

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correponds to grapefruit juice, 58.09 [kj/mol], and the lowest one was obtained in Sierra lemonjuice, 35.02 [kJ/mol].Thermal process reduce vitamin C content in citrus fruit juices. Results are used in order to

evaluate this reduction.

Chapter 4. Mechanical and Optical Properties.

This is the longest chapter because physical properties are indispensable for engineering

calculations. Laboratories are generally equiped to do basic measurements and they could be

modify in order to determine physical properties of foods. As a result of the big amount ofproducts several laboratory experiences, including methods and techncas to measure mechanicalproperties. are presented.

In the seven developed topics it was pretended students remember basic concepts of Physics,specially those ones related to Mechanics. Some methods and techncas to measure basic properties

are indicated. Main didactic purpose is to inculcate into students that if they did not have really

physical data, every calculation o simulation could be a simple numerical exercise.

Topic 4.1. is about an application Arquimedes Principle. Basic principles about impulse and

flotation are indicated . Published information about the relationship between specific gravity and

solids content of potatoes, are reported.Weigth determinations in samples of four potatoe varieties was made in air and immersed them

in water to determine specific gravity. Average data lead to established linear equations which

describe relationship between dry material and specific gravity.In addition, a general equation established using information of all varieties is compared with other

published equations.

It is indicated that this method could be used to predict the yield during french fries production

or as a control index of materials.

Cereals and legume seeds are basic ingredients of daily diet around the world. People eat them

as entire grains or as flour. Topic 4.2. provides different methods used to determine density, its

variations and rest angle. In grinding samples, it is necesary to use a microscopy to caracterized

particles and the results are reported as diameter averages.Real density, apparent density and voids are reported for grains of l upino, six varieties of corn,

wheat and rice; flours of corn, wheat, rice, barley, peas, faba bean, and for starches of potato and

cassava.

These determinations could be used as control parameters.

Topic 4.3. is about density changes in liquid food due to different causes. It contents many

experimental data and empirical equations established with them.

Many technical articles related to milk, oils, fruit and vegetable juices are mentioned.The relationships between density and temperature, density and main solid components as fat,

carbohidrates and protein; are studied in raw and pasteurised cow milk.

Polinomic equations to determine juice density in twenty six different fruits in a range of 10 to

80 C are included. These fruits are: apple, avocado, banana, banana-little, blackberry, cherry-

black, grape, grapefruit, guava, lemon, lime, mango, muskmelon, naranjilla, nectarine, orange,papaw, passion-flower, passion-fruit, pear, peach, pineapple, plum, prickilypear, starfruit,

strawberry, tangerine, tomato, treetomato, watermelon.

A general equation obtained by multiple correlation analysis, which describes density changes with

temperature a range of 10 to 40C and with soluble solids content within 5 to 25Brix, ispresented. Values of volumetric thermal expansion coefficients are included for all of these

products.

In addition, density values for juices of ajipepper, peas, sugar-cane, onion, cauliflower, bean, faba

bean, potatoe, gherkin, green pepper, radish, beet, cassava, carrot, pumpkin; which were

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determined in a range of 10 to 70C, are provided.

Inverse linear relationship between density and temperature was determined in oils of avocado,

cotton, lupino, sunflower, linseed, corn, peanut, olive, castor-oil, soybean and sesame; and fatsof cocoa, coconut, almond and palm pulp. Calculation of volumetric thermal expansion coefficients

was made easily and data are included. The terms of linear equations and data of density as afunction of temperature are presented. A multiple correlation equation was obtained, it establish

density changes with average molecular weight, iodine index and temperature.

Topic 4.4. is related to surface tension of fruit juices and provides its principal theorical aspects.

Surface tension data of twenty two different fruit juices were determined in a range of 10 to 50C.

Equations obtained by linear correlation analysis between surface tension and temperature, areincluded. Soluble solids content as Brix and moisture content values are presented.

By a multiple correlation an equation was established between juice surface tension, Brix andtemperature.

Elastic or Young's module, a typical property of solid materials, is refered in Topic 4.5. This

topic provides a revision of principal concepts and it describes a measurement method using

narrow cylinders.

Elastic modulus data determined in noodles, which had different moisture content, are presented.

A linear correlation describes the inverse relationship between moisture content and Young'

modulus.

Determined values are similar to published ones.

Anisotropy of noodles are proved.

In Topic 4.6., it was intented to use tension-deformation diagrams to obtain information about

meat mechanical resistance.

The principles of strength of food materials and tension-deformation diagrams used are included.

Cylindrical muscule portions of beef and pork meat were lengthened by consecutive and alternativeaddition of weigths.

Making use of tension-reformation diagrams, next mecanic properties were determined:proporcionality limit, elastic limit, fluency limit, rupture limite, resilience limite, tenacity modulus

striction and yield point elongation. Notorial differences prove that beef is more ressistant tostrength than pork meat.

The optical property named refractive index, whose determination is easy, quick and require small

quantity of sample, is measured by modern refractometers with precision. This property lets

explore food structural aspects.Topic 4.7. includes refraction index values which were determined in fourteen oils and fats in arange of 10 to 80 C for oils and of 30 to 80C for fats. Linear equations established betwee

refractive index and temperature for each product are included. Iodine index is a measurement of

fatty acids insaturation, this property was related with refractive index using a linear correlationwith all data at 40C. Another linear relationship between refractive index and average molecular

weigth is provided. Average molecular weigth is considered as an indicative of fatty acids length

In addition, a multiple correlation equation between the refractive index as function of iodine index

and average molecular weigth, is presented.

Fatty acids composition for the fourteen oils and fats are presented.

Chapter 5. Rheological Properties.

Food technicians established that rheology is a powerful tool to solve food process problems which

include tranportation. Most of the food products are non-Newtonian and rheology is indispensable

to understand their behaviour.

Chapter fifth provides a review of basic principles, for example, Newton viscosity law and non

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Newtonian models, in both cases temperature and composition effect are analyzed. Procedures to

measure viscosity using capillary viscosimeters and to measure rheological parameters using

rotational viscosimeters or rheometers are detailled. In the formative part, two points areconsidered, the first one is to emphatize the need of using specific data and equations forcalculations related to flow of Newtonian and non-Newtonian fluids; and the second one is to

indicate the posibilities for a better understanding of the complex structure of foods by rheology.

Newtonian fluids are those whose behaviour could be described by Newton viscosity law. Only

a small quantity of foods are Newtonian fluids for example: milk, oils, clarified and filtered juices.

Topic 5.1. is about viscosity of these products. It starts with a transcription of the derivation of

Poiseuille equation. Specific equations; which describe temperature and composition effect overviscosity of cow milk, fruit juices and oils; are included.

Cow milk viscosity data measured by Ostwald viscometer at different temperatures and lactose

concentrations are presented. Activation energy average calculated by Arrhenius equation was 18.1

[kJ/mol] for cow milk flow. Rotational viscometer were used to determine viscosity values, at

three different temperatures and four concentrations of soluble solids, in filtered fruit juices of

lime, lemon, tangerine, orange and grapefruit. For each juice, a multiple correlation equation, that

describes temperature and soluble solids content effect over viscosity, was found. Activation

energy values for these citric fruits are in a range of 19.5 to 22.2. [kJ/mol].

Viscosity values were determined in nine samples of oils extracted from avocado, sesame, cotton,

lupino, castor-oil, linseed, peanut, passion-fruit seed, soybean. Determinations were made using

a rotational viscosimeter in a range of 20 to 70C. Inverse effect of temperature over viscosity

is described by linear equations. All viscosity values are similar except for castor-oil whose

behaviour is anomalous. Activation energy values are in a range of 22.2 to 29.0 [kJ/mol], butcastor-oil activation energy value is 45.0 [kJ/mol].

Most of processed foods are non-Newtonian fluids. Milk is a special case, it is a Newtonian fluid

but its products are non-Newtonan fluids and sometimes they become solids for example chesse.

Topic 5.2. provides basic information about rheological treatment for non-Newtonian fluids, time

effect is analysed. This topic also provides specific references for cream, yoghurt, butter and a

sweetened milk.Pseudoplastic behaviour of whipped cream was proved using a rotational vicometer with concentric

cylinders geometry with narrow gap. Flow behavior index and consistency coefficient variations

depend on process conditions and time of whipping.

Rheological parameters were determined in three samples of yoghurt consider them aspseudoplastic fluids. It is indicated the importance to consider raw materials and process influence

over rheological parameters. These parameters could be used as control indexes for final productsor during fermentation.

According to the behaviour of butter samples, they could be defined as Bingham fluids. Yield

stress could be used as a control index for unctuosity.Thixotrophy was proved in sweetened milk. This product becomes weaker and loses consistence

with the time of shear stress.

Liquated pulps of apple, avocado, blackberry, cherry-black, guanabana, guava, mango, naranjilla,

papaw, passion-flower, passion-fruit, pear, peach, plum, starfruit, strawberry, tomato andtreetomato were considered to determine fluid behavior index and consitency coefficient by the

Power fluid law equation, for pseudoplastic fluids.Six samples were considered and measurements were made in duplicate. Rotation speed was

increased and decreased using a simple cylinder, concentric cylinder geometry with wide gap, of

a rotational viscometer. Thixotrophy was analysed and temperature effect on consistency

coefficient was established by Arrhenius model.

Rheological parameters of liquated banana pulps of four varieties: banana, banana-little,

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" maqueo" and "limeho", were determined using a rotational rheometer. Two model were

considered, the power law model for pseudoplastic behaviour and the Herschel-Bulkley model (H-

B) for mixed type behaviour.

Data are included in Topic 5.4.. Value differences between varieties are discussed. Advantages

of using a determinate model are pointed out. Activation energy values for flow are provided. It

is pointed out the difficulty to obtain exact values and the heterogeneity of samples presents

problems.

Chapter 6. Thermal Properties.

The values of thermal properties have multiple applications. In Food Engineering these propertiesare used in heat transfer calculations and to established critical control points during processes.

These properties are also employed in Food Techonology as control indexes and to compare the

efficiency of equipment and industrial plants. In addition, they are used to control raw materials,

materials during process and final products.

A review of essencial aspects and concepts are provided. Simple methods to determine therma

properties are described. In a formative way, it is pointed out the convenience of specific technical

data availability as a fundamental basis to study or applyed heat transfer process.

In the topic 6.1. are included the development of an equation which related specific heat and sugar

content of syrups. Several published equations for specific heat calculation in foods with high

contents of water and sugar are provided.

Specific heat data were determined in samples at four different contents of soluble solids. The

syropes were obtained by addition of different quantities of sugar to juice of apple, avocado,babaco, banana, banana-little, blackberry, cherry-black, grape, grapefruit, guava, lemon, lime

mango, muskmelon, naranjilla, nectarine, orange, papaw, passion-flower, passion-fruit, pear,

peach, pineapple, plum, prickilypear, strawberry, tangerine, tomato, treetomato, watermelon. A

linear correlation was established between 'Brix and specific heat and it could be used by

processers of this kind of preserved food.

Several publications are mentioned in Topic 6.2., they are about fruit thermal diffusivity and

equations for it calculation. A simple method to determine thermal diffusivity, in a practical

manner, shown by Charm is provided.

Moisture content and thermal diffusivity data are presented. This topic includes heating factors

which were measured using finite and infinite cylinders in apple, avocado, banana (four varieties),

blackberry, cherry-black, cucumber, grape, grapefruit, lime, naranjilla, nectarine, melon, orange,

papaw, passion-flower, pear, peach, pineapple, plum, prickilypear, starfruit, strawberry,

tangerine, tomato, treetomato and watermelon. When a specific product is analysed, the

compliment of a linear relationship between thermal diffusivity and moisture content occurs.

However, it was established that this relation is not satisfactory if several products are considered,for example different fruits.

Topic 6.3. is about thermal conductivity of beef, chicken, lamb and pork meat. Some published

equations to determine this property are included. Using a method which considers the change of

temperature with time in controled conditions, transient heat transfer, heating rate values were

determined and thermal conductivity were calculated.

Results were compared with published data and also with those calculated by reported equations

for meat products. It was found they are similar. Thermal condutivity value were aproximately0.5 [W/m. K] in all samples.

In foods, whose geometry is similar to spheric form, Fourier law for unsteady-state heat transfer

could be applied in order to determine thermal conductivity . Data measured in potato and orange

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samples, with and without peel, are provided in topic 6.4.

A review of Fourier equation is included. Published equations for themal conductivity calculation

in fruits and vegetables are compiled.

Some examples, which include heat transfer coefficient calculation in surface of samples and heat

conductivity determination, are provided.

Results are similar to published ones. It was observed that the method is imprecise, however

products with peel showed higher values than their similar without peel.

Latent heat of vaporization is a thermal property largely used in food process when contained

water changes from liquid to vapour. Topic 6.5. includes a calorimetric method to determine this

propety in food with a high moisture content.Thermodynamics principles are shown and Trouton rule is mentioned. Experimental values

determined in water, milk and carrot juice are presented. Calorimetric method worth is confirmed.

Latent heat of vaporization for foods is only a little higher that the value determined in pure water.

To evaporate water from food with a low moisture content, is required more quantity of energy

than the required to evaporate water from foods with a high moisture content. Data obtanined in

samples of wheat products, by measuring vapour pressure and applying Othmer law, are shown

in Topic 6.6.A review of thermodynamics principles which explain fugacity and activity concepts and their

relationship with the Othmer law. This Topic also includes data obtained in milling samples ofbiscuits, noodles and bread by measuring of vapour pressure at three temperatures. Water activity

values are calculated and presented. Employing mathematical technics, values of latent heat of

vaporization were determined, they are higher than pure water values until 23%.

Chapter 7. Heat Transfer.

This is one of the most important chapters, because in any moment of food processes exist changesof temperature. Process success or failure depends on correct control of these changes.

It is included a review of heat transfer principles, specially equations related to heat transfer that

occurs in the most popular equipmets employed in food industry. Methods to determine heat

transfer coefficients in equipments are described as well as their employment in controlperformance. It is important that new professionals apply these knowledge in layout andconstruction of simple equipments of heat transfer. This knowledge could be also applied in the

development of storage systems in order to decresed food wastage.

Topic 7.1. is about a particular kind of heat exchangers, which consist of a bundle of tubes inside

a suitable shell named reversed-current exchanger. Exchange surface is incresed due to the

particular design.Shell exchangers as a part of a system used in food industry for heating or cooling are shown.

This topic includes principal heat transfer equations to determine its flow and heat transfer

cofficients inside and outside of surface pipes. Corrections made for these equipments and

resistance due to fouling are analyzed.The Characteristics of laboratory equipment used in experimentation, main accesories and itsproperties were included.It is presented an example of heat transfer coefficients calculation using experimental data in order

to determine the overall heat transfer coefficient. Four conditions of vapour pressure in the shell

and water flow inside pipes were considered.

A computer program to calculate heat transfer coefficients and resistance due to fouling areprovided. It employs different equations and selects the best one according to experimental data.

This information is related to equipment efficiency and its performance, for that reason it could

be used to determine cleaning or maintenance periods.

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Topic 7.2. is about plate heat exchangers, one of the most popular exchangers in food pro

industries. Main technical characteristics and general conditions to use them properly are inclu

Exchanger sections are described, and they are regeneration, heating, cooling, pu

homogenization, regulation tank, holding, tmal cooling and control system. Published relation

to determined heat transfer in these exchangers and to calculate resistance due to encrustati

are transcribed.

A plate heat exchanger of laboratory was employed to shown tire operation of these equipm

Data obtained in a water-water system, with flow of cold and hot liquid in concurrent or cou

current flow, are presented. Three different flow rates were considered to determine dimensio

numbers like: Reynolds, Prandtl, Peclet and Nusselt. The last one is used to calculate heat tran

coefficients employing experimental values or equations. Heat transfer determination anoverall heat transfer coefficients calculation could be used to quantify milk fouling influence

heat transfer.

Topic 7.3. is about steam jacketed kettle. Main resistances offered to heat transfer by int

surface, are analyzed. Main equations to calculate heat transfer coefficients for condensed st

and boiling water, were compiled. Information about thermal conductivity of stainless steel pl

is included.

Trials are done employing two jacketed kettlers, one is old and spheric, the second is new

cylinderc. Steam at different pressures as the heating agent and boiling water were consid

calculate overall heat transfer coefficient. Comparison of calculated values with experimental

measuring condensed steam mass and evaporated water mass, was used to calculate resistance

to encrustations. A computer program in BASIC language to simplify resistance calculation

to encrustacions, is included.

Based on condensed steam in contact with inner surface, heat tranfer coefficient were determi

these data were compared with heat transfer coefficient calculated using the equations prop

by Brown, Charm, Perry and Green. This comparison leads to propose the last equation u

contol these equipments and to stablished cleaning and maintenance periods accordinencrustations. Heat transfer coefficient data determined in boiling water contained in the ke

are compared with those calculated by Rohsenow equation.

It deserves particular attention that heat transfer coefficients could be used as control and la

tools, accordig to the methodology described in this Topic.

The importance of heat transfer in unsteady-state during food processing is emphatized in T

7.4.

This topic included, published equations for spherical and similar forms, which consider t

cases: negligible internal resistance to heat transfer, negligible surface resistance to heat tran

and finite surface and internal resistance to heat transfer.

Heating and cooling trials in potatoes at air and maintained in cartoon boxes without protec

lead to confirm published equations and charts used to calculate temperature changes with t

when external heat transfer conditions are steady.

Differences of convective heat transfer coefficients data, that occur on the potato surface contac

air, are analyzed. Experimental data were used to determine this film coefficient by trial and e

procedure.

Sterilization process of canning foods was accomplished in retorts. In this situation, heat tra

occurs in unsteady-state and through the finite cylinders, with initial nonuniform temperatu

is a quite complex situation of heat transfer and it could be analysed by the numerical metho

finite differences, whose principles and equations are presented in Topic 7.5. An exapresented by Charm was used to make a computer program in BASIC languaje, which calcu

temperature changes in different points in the can during process. The program includes the

properties calculation as function of temperature employing Choi and Okos model. It calcu

mass average temperature inside the can for different times. Employing this information, tim

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cooling could be determined.

Data obtained using a small can and a product whose proximate composition was known, are

presented.

Chapter 8. Thermal Process.

This is a very important Chapter because it contents the principles which lead Food Engineeringbecome an independient profession.

In a cognositive way, a review of principles development at the begining of the century until

current computer programs, is included. Nowdays, thermal process conditions are calculated based

on the microbial growth, enzyme activity and food nutrition value. Current computer programscould calculate process conditons exactly and easily. A procedimental part pay more atention to

calculation than experimental trials to obtained kinetical data. In the formative part, it is important

to emphasize that if thermal process conditions were incorrect, consummers would get sick or die.

For that reason, a responsable professional, who undesrtant process importance, is required.

Two topics are discussed: the first one is about juice and milk pasteurization and the other is

related to commercial sterilization of canned products.

In Topic 8.1., an easy form to applied the General Method to determine the time of pasteurizarion

for cow milk and orange juice including its mathematic principles, are development and presented.

A compilation of D and z published values, specially those ones for cow milk pasteurization or

sterilization, are included. Equations to calculate thermal death time could be obtained analyzing

data of thermal death time curve. A method suggested by Toledo to determine food pasteurization

time in liquids that flow inside tube heaters exchangers, is described. Tube heaters exchangers are

commonly employed in aseptic processing.

Temperature data registered in heating and cooling trials with cow milk and orange juice, are

included. Considering letal rate ofSalmonella spp. and Costridium botulinum Tipe E, a simple

form to applied the General Method using experimental data and temperature secuence, are

provided. A similar procedure was used for orange juice but considering Candid tropicalis and

Rodotorula rubra letal rate.

Pasteurization times calculated in both cases correspond to specific characteristics of each product.

Boil milk during five or more minutes is a common practice. Obtained results employing

household instruments, proved that a correct pasteurization process require less time than five or

more minutes. Less time involves a less reduction of nutrition value.

In Topic 8.2., the Mathematical Method developed by Ball was employed to calculate thermal

process times required for commercial sterilization of canned prawns.

Basic equations that were used to develop Ball method are presented, the terms involved in

determination of the process time are defined.

It presents examples of the method application considering pre-heating, sterilization and cooling

periods, and when heating speed changes. To make calculation easier, two nomographs; which are

used to determine thermal process times in foods with and without "broken" heating curves; are

included. The nomographic method description and graphics employing are enclosed.

Experimental data obtained in canned prawns during sterilization in retort, lead to show Ball

method application and nomograph utilization considering thermal resistance of Clostridium

botulinum and Bacillus stearothermophilus . Results were employed to analyse microorganisms

destruction and on the other hand, nutrition value reduction.

Chapter 9. Dehydration.

Dehydration is probably the most applied food preservation method. It is apparently simple, but

it involves several mechanisms of heat and mass transfer which act simultaneous. Considering this,

many equipments was developed and constructed. In few cases their description is included.

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This chapter provided fundamental concepts of water activity and equations which describe drying

kinetic at different conditions. Experimental methods to study this process as an industrial one or

in a general manner are presented. Simple drying methods were pointed out because they could

be applied in rural regions. Obtained results were used for equipment layout, one of the most

important activities for Food Engineers.

Water activity rather than moisture content influences biological reactions. Its study is usefull to

understand and to control food dehydration process. Water activity data and a simple determination

in products with high moisture content, are discussed in Topic 9.1.

Basic equations established by the relationship between water activity values and no-electrolitic

constituens of binary solutions are reported, as well as specific equations for liquid products.Freezing point determinations in sweet whey obtained from chesse process, with glucose, fructose,

lactose and saccharose addition; were used to shown equations used and Norrish equation worth.

Correlations, between soluble solid content (Brix) and water activity, lead to establish differences

between the effect of monosaccharide and disaccharide addition. Monosaccharides are more

efficient to reduce water activity than disaccharides.

Presented equations allow to calcule water activity for high moisture content products employing

a simple refractometric measure as Brix.

Topic 9.2. is about osmotic dehydration of bananas, and dehidration of grapes to make raisins.

Equations for water activity calculation in systems of multicompounds with high moisture content,

are provided. Equations to describe water and solute flow during osmotic process are also

included.

Results of tryals at 35 and 45C with bananas, are used to compare process, by measuring ofproduct weight and water content reduction. Water activity values were calculated at different

times during osmotic dehydration to establish permeability constants, which are used to evalueprocess efficient.

Data obtanined in a variety of grapes during osmotic preconcentration to obtain raisins, considering

three pre-treatments to accelerate the process, were included. Moisture content and solids, water

loss, weight loss, solid increased and permeability constant values determined at threetemepratures.

Coefficients of heat and mass transfer, which were determined after osmotic preconcentration in

drying trials employing air, were presented. Four sorption isotherms for raisins at 20C with

different sugar contents are included.

Solar drying is the most employed method to preserve foods in rural regions. Advantages of solar

radiations at the Equator zone are discussed and equations for drying solar energy calculation arepresented in Topic 9.3. Different kinds of solar dryers and equations for solar drying analysis in

cereal grains, are mentioned.

Tryals with corn employing simple equipments of local construction, lead to compare direct and

indirect solar drying. Efficience values shown indirect solar drying (8.1 %) were quite better than

direct one (7.1 %).

Drying data obtained in quinua, with and without saponines, are included. Bitter taste is caused

by saponines, for that reason, consumers try to remove them washing the product; but drying

process get more difficult, according to effective diffusion coefficient and drying constant data.

It is emphatized that if quality of rural zones products wanted to be improved, solar drying

principles would be study and applied properly.

An application of drying at low temperatures in potatoes is presented in Topic 9.4. Atmospheric

freeze-drying principles and its equations were discussed.

Data measured in 3 [mm] slices mantained at -2C, using sodium hidroxide as hygroscopic agent

and air movement inside of the chamber, were used to determine overall heat transfer value and

Heldman an Hohner number. According to this number it was established that the process is

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controlled by mass transfer from the product surface to air.

To pointed out the benefits of atmospheric freeze-drying, technology of potato slices preparation

is described. It was provided a description of the tryals at cooling and freezing temperatures, three

chemical sustances were employed as hygroscopic agents. Biological values and rehydration tests,

lead to establish high quality of the product protein and very small physical damaged, specially

when it is obtained by atmospheric freeze-drying. Potato slices dehydrated employing cooling

temperatures showed considerable physical changes and microorganism growth, however this

method with a proper control, could be consider as an alternative for food preservation, specially

in mountain regions whose temperature is nearly to the water freezing point.

Topic 9.5. is about Fick's second law application to control drying process which are verycommon in vegetable products industries.First part, includes basic equations which established a relationship between heat transfer and

water transfer when drying rate is constant. A detailed description of Fick's second law derivation

is included. Equations obtained, which describe moisture changes with time considering diffusivity

as the main mechanism of mass transfer in the falling rate period, are enclosed. Equations to

establish temperature effect on the diffusion coefficient and multiple equations which also consider

relative humidity and air velocity influence, are compiled. Equations published by several authors

to analyse drying process, when there are more than one falling rate period, are also provided.

Data obtained in slices of beet employing a tunnel dryer, lead to define two drying periods,

constant and falling rate periods. Graphics plotted using the presented equations, confirm that

water diffusion inside the product, controlled drying process. Diffusion coefficients were

determined and compared with those calculated by specific multiple correlation equations fordiffusivity. Values of activation energy for drying were calculated us ing Arrhenius model.

Mathematical models were used to calculate succesfully drying times when specific and real data

were available.Data obtained in cassava samples dryed in a cabinet dryer were used to design and construct a

household dryer. Some information about cassava is provided: physical properties, chemical

composition, toxicology, drying process in different conditions and equipment. It was proved that

all process could be analysed as one falling rate period. Two kinds of effective diffusioncoefficients were determined, considering a liquid phase and considering a gas phase. Tryals at

three different temperatures and three different air rates, lead to established their effect on dryingtime until to reach low moisture contents which make sure product stability. Adequate dryingprocess conditions were established using this information and a hosehold cabinet dryer was

designed. Microbiological and chemical analysis were made to compare two groups of cassava

slices. One group were dryed in a laboratory oven and the other group were dryed in the shown

household cabinet dryer.Several recomendations for cassava dehydration using dryers adapted in common firebox, were

provided. It could be applied in regions without civil infrastructure to mantain product quality.

The book of 524 pages, has the Contents at the begining, and the Index at the end. The Index were

made to makes easier searching of technical data and physical properties of specific foods.

This book was published with the support of ORGANIZATION OF AMERICAN STATES,

Department of Scientific and Technological Affairs, Multinational Proyect of Biotechnology and

Food Technology.

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Principios En