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ENGINEERING PRINCIPLES

APPLIED IN FOODS

SUMMARY -RESUMEN

Juan deDiosAlvarado

INGENIERO EN ALIMENTOS, M. en C.

FACULTAD DE CIENCIAEINGENIERÍA EN ALIMENTOS

UNIVERSIDAD TÉCNICA DE AMBATO - ECUADOR

SECRETARIA GENERAL DE LA ORGANIZACIÓN DE LOS ESTADOS AMERICANOS PROGRAMA REGIONAL DE DESARROLLO CIENTIFICO Y TECNOLÓGICO PROYECTO MULTINACIONAL DE BIOTECNOLOGIA Y TECNOLOGÍA DE ALIMENTOS

Engineering Principles

Applied in Foods

SUMMARY

Most of the scientific information including technical articles and book s have been published in English. Iberoamerican contribution, specially Food Engineering texts, i s 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 engineeri ng and food technology undergraduates students. Information about several kinds of foods are pr ovided and specially physical properties could be considered. In addition, certain topics lik e heat process calculations and heat transfer coeficients determination could help in postgraduation studies. Nine chapters and thirty eigth topics are presented. It is important to point out experimental data presentation in whole topics because they were obtained acuting skills t o use a small quantity of available equipments andreactives.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. Book includes basic principles first, food physical properties later and proc ess which involve former knowledge finally. In addition, a nomenclature for each chapter and inde xes of subjects and products are presented to make it usefull. Whole topics are structurated to be use like independient experimental u nits. They include: Introduction and Bibliography, which consider cognitive aspects of teach ing.Materials and Methods and Experimental Results, which explain procedure aspects. Objec tives and Questions, in order to cover formative aspects the main purpose of educational acti vity.

Chapters and topics consider are:

Chapter 1. Material and Energy Balances.

Use of these balances is fundamental for development of several engineer ings including Food

Engineering.

Considering a didactic view, this chapter indicates basic concepts. Dete rminations and calculations aremade to demonstrate balances compliment. The chapter emphasizes the impo rtance and frequent use of balancesinmany practical experiences during processing and food study. Topic 1.1. is about application of material balance during jelly prepara tion. It presents the fundamental equation and limits to identify total mass entering and leav ing the system. Refractometric data are used to quantify soluble solids changes during c oction, this values are compared with the established by solids balance in order to prove their concordance. Jelly,marmelade and jam preparation is common in food industry and when refrac tometers are not available these balances could be used by technitians to control pro cess and its conditions for preparing these products or similar ones. Obteined results demonstrate t he 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 includes the most usefull equations and models for calculation of this t hermal property. Hwang and Hayakawa method to evaluate experimentally specific heat, without di rect contact between product and environment, are based in energy balances. This method and i ts fundamentals are described giving details. Cow milk experimental data obteined with calorimeters made from commerci al thermos bottles are provided. They are similar to those ones reported in technical publi cations 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 mul tiple applications of energy balances. Possibly, the most common chart used by engineers is the psychrometric c hart, this graphic presents relationships obtained by material and energy balances establis hed in air and vapour mixtures. Psychrometric chart calculations and their elaboration are pre sented 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 inclu des the possibility to introduce changes in calculation due to variability of pressure caused b y the differentaltitudabove 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 p rogram 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 k nowledge about these mechanisms is necessary to improve or design more reliable and safer pro cesses. To understand a particular mechanism is important to know about thermodynamic, kinetic , transport and other fundamental principles related to the mechanism. Three cases are conside red 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 rel evant point is to inculcate in students the importance of a real knowledge of this phenomenons for f ood engineering development. Topic 2.1. discusses about Fourier law. This law quantifies thermal ener gy that flows perpendiculary to isothermic surfaces in motionless and isotropic materi als. 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 experiment al 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 comp uters 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 det ermined using tubes made of glass and the minimal quantity of equipments. These values are compar ed 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 diffus ion from liquid to air, 2

SUMMARY

is pointed out because it allows to explore important aspects like molecula r mass effect and molecular collisions. Topic 2.3. discusses conjunctly tire mechanism of heat transfer by conve ction and radiation. It is i ndicated that Newton law is important to determine convective heat trans fer coefficient. Some of the most useful equations to quantify heat flow due to free convection i n cylinders in contact with air,were chooses and transcribed. Some of these are dimensionless, others ha ve dimension numbers and they are specific for materials in contact with air. Experiments using horizontal and vertical ice cylinders were done An ana logy between heat and mass transfer was applied to determined heat transfer coefficients of fr ee convection to air. These values were compared with those ones calculated by chosenequations. Relationship between different mechanisms of heat and mass transfer is important if they are considered simultaneously. Chapter 3. Kinetic of Reactions that Occur in Foods. The most important characteristic of foods is their continued tranformationin the whole time. Chemical and biochemical reactions happen all around and they cause posi tive and negative changes. This Chapter treated about the quantification of these changes. Fundamental models of chemical kinetic are studied. Reactions and common changes that occur in foods are characterized by kinetic equations. The main purpose is inc ulcate into students the worth of kinetics study to understand food complexity and to applied thi s principles in practical situations, for example shelf life determination. Topic 3.1. is about oil stability. A general model to determine kinetic reactions of any order was derived. Peroxid index changes were determined, they are a measurement of fatty a cid oxidation. Data were obtained from samples of linseed, lupin and pumpkin seeds. The kine tic model was used to determined the reaction order, kinetic equations and its reaction rates. A possible application of these values to select antioxidant substances are mentioned. Fatty acid composition of seventeen oils and fats are included. Browning is a complex process and occurs as a result of chemical and bio chemical reactions.

Topic 3.2. are related to banana browning.

With the purpose to calculate the order of the kinetic reaction, based o n shelf life concept, an equation was established. Easy measurements as spectrophotometric ones were made in samples of fou r banana varieties. Some samples were under natural conditions and others were under thermal treatment to destroy enzymes. After that, all samples were mantained at 20° and 30°C. T hese values were used to determine the reaction order. In all of the cases, the reaction order wa s near to 1, that correspond to a first order kinetic. A little higher values were registered in samp les put down thermal treatment.

Shelf life concept are indicated. A

Q10 value, which quantifies sensibility of the changes due to temperature, was calculated. As a result of the thermal treatment, treat ed samples had Q10 values higher than the others, specially in"Seda"and "Cavendish" commercial varieties. Food nutritional aspects and nutritive value must be consider always dur ing food process. Topic

3.3. discusses ascorbic acid or vitamin C degradation because it is a ty

pical thermolabil compound. Several researches made about this are mentioned. Ascorbic acid content was determined by a chemical method in fresh juices of lime, lemon, tangerine and grapefruit . Samples were maintained under aerobic conditions and four temperatures during different times. Itwas proved that degradation follows a first order kinetic, equations and its practical applications are included.Arrhenius equation, which are used to calculate activation energy, descr ibes temperature effect on this reaction from 20° to 92°C. The highest value of activation energy

ENGINEERING PRINCIPLES APPLIED IN FOODS

3 correponds to grapefruit juice, 58.09 [kj/mol], and the lowest one was obtained in Sierra lemon juice, 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 indispensabl e for engineering calculations. Laboratories are generally equiped to do basic measurement s and they could be modify in order to determine physical properties of foods. As a result o f the big amountof products several laboratory experiences, including methods and techncas to measure mechanical properties. are presented. In the seven developed topics it was pretended students remember basic c oncepts 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 numerica l exercise. Topic 4.1. is about an application Arquimedes Principle. Basic principle s about impulse and flotation are indicated . Published information about the relationship b etween specific gravity and solids content of potatoes, are reported. Weigth determinations in samples of four potatoe varieties was made in a ir 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 informationofall varieties is compared with other published equations. It is indicated that this method could be used to predict the yield duri ng 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 oflupino,six varietiesof corn, wheat and rice; flours of corn, wheat, rice, barley, peas, faba bean, an d 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 caus es. It contents many experimental data and empirical equations established with them. Many technical articles related to milk, oils, fruit and vegetable juice s are mentioned. The relationships between density and temperature, density and main soli d components as fat, carbohidrates and protein; are studied in raw and pasteurised cow milk. Polinomic equations to determine juice density in twenty six different f ruits 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, naranjill a, nectarine, orange, papaw, passion-flower, passion-fruit, pear, peach, pineapple, plum, pric kilypear, starfruit, strawberry, tangerine, tomato, treetomato, watermelon. A general equation obtained by multiple correlation analysis, which desc ribes density changes with temperature a range of 10° to 40°C and with soluble solids content within 5° to 25°Brix, is presented.Values of volumetric thermal expansion coefficients are included for all of these products. In addition, density values for juices of ajipepper, peas, sugar-cane, o nion, cauliflower, bean, faba bean, potatoe, gherkin, green pepper, radish,beet, cassava, carrot, pumpkin; which were 4

SUMMARY

determined in a range of 10 to 70°C, are provided. Inverse linear relationship between density and temperature was determin ed in oils of avocado, cotton, lupino, sunflower, linseed, corn, peanut, olive, castor-oil, soybean and sesame; and fats of cocoa, coconut, almond and palm pulp. Calculation of volumetric therm al expansion coefficients was made easily and data are included. The terms of linear equations and data of density as a function of temperature are presented. A multiple correlation equation w as obtained, it establish density changes with average molecular weight, iodine index and temperat ure. Topic 4.4. is related to surface tension of fruit juices and provides it s principal theorical aspects. Surface tension data of twenty two different fruit juices were determine d in a range of 10 to 50°C. Equations obtained by linear correlation analysis between surface tensio n and temperature, are included. Soluble solids content as °Brix and moisture content values are presented. By a multiple correlation an equation was established between juice surf ace tension, °Brix and temperature. Elastic or Young's module, a typical property of solid materials, is ref ered in Topic 4.5. This topic provides a revision of principal concepts and it describes a measu rement 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's 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 ob tain information about meat mechanical resistance. The principles of strength of food materials and tension-deformation dia grams used are included. Cylindrical muscule portions of beef and pork meat were lengthened by co nsecutive and alternative addition of weigths. Making use of tension-reformation diagrams, next mecanic properties were determined:

proporcionality limit, elastic limit, fluency limit, rupturelimite,resiliencelimite,tenacity modulus,

striction and yield point elongation. Notorial differences prove that be ef is more ressistant to strength 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 fou rteen oils and fats in a range of 10°to 80°C for oils and of 30° to 80°C for fats. Linear equations establish ed between refractive index and temperature for each product are included. Iodine i ndex is a measurement of fatty acids insaturation, this property was related with refractive inde x using a linear correlation with all data at 40°C. Another linear relationship between refractive index and average molecular weigth is provided. Average molecular weigth is considered as an indicat ive of fatty acids length. In addition, a multiple correlation equation between the refractive inde x 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 f ood process problems which include tranportation. Most of the food products are non-Newtonian and r heology is indispensable to understand their behaviour. Chapter fifth provides a review of basic principles, for example, Newton viscosity law and non-

ENGINEERING PRINCIPLES APPLIED IN FOODS

5 Newtonian models, in both cases temperature and composition effect are a nalyzed. Procedures to measure viscosity using capillary viscosimeters and to measure rheologic al parameters using rotational viscosimeters or rheometers are detailled. In the formative p art, two points are considered, the first one is to emphatize the need of using specific dat a and equations for calculations 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 stru cture 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 transc ription of the derivation of Poiseuille equation. Specific equations; which describe temperature and composition effect over viscosity of cow milk, fruit juices and oils; are included. Cow milk viscosity data measured by Ostwald viscometer at different temp eratures and lactose concentrations are presented. Activation energy average calculated by Ar rhenius equation was 18.1 [kJ/mol]for cow milk flow. Rotational viscometer were used to determine viscosit y 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. [k

J/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 70°C. Inverse effec t of temperature over viscosity is described by linear equations. All viscosity values are similar except f or castor-oil whose behaviour is anomalous. Activation energy values are in a range of 22.2 to 29.0 [kJ/mol], but castor-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 solid s for example chesse. Topic 5.2. provides basic information about rheological treatment for no n-Newtonian fluids, time effect is analysed. This topic also provides specific references for cre am, yoghurt, butter and a sweetened milk. Pseudoplastic behaviour of whipped cream was proved using a rotational v icometer 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 as pseudoplastic fluids. It is indicated the importance to consider raw mat erials and process influence over rheological parameters. These parameters could be used as control i ndexes for final products or during fermentation. According to the behaviour of butter samples, they could be defined as B ingham fluids. Yield stress could be used as a control index for unctuosity. Thixotrophy was proved in sweetened milk. This product becomes weaker an d loses consistence with the time of shear stress. Liquated pulps of apple, avocado, blackberry, cherry-black, guanabana, g uava, mango, naranjilla, papaw, passion-flower, passion-fruit, pear, peach, plum, starfruit, stra wberry, tomato and treetomato were considered to determine fluid behavior index and consite ncy coefficient by the Power fluid law equation, for pseudoplastic fluids. Six samples were considered and measurements were made in duplicate. Rot ation speed was increased and decreased using a simple cylinder, concentric cylinder geo metry 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: banan a, banana-little,

SUMMARY

"maqueño" and "limeho",were determined using a rotational rheometer. Two model were considered, the power law model for pseudoplastic behaviour and the Hers chel-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 f or flow are provided. It ispointed 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 Eng ineering these properties are used in heat transfer calculations and to established critical contr ol points during processes. These properties are also employed in Food Techonology as control indexe s and to compare the efficiency of equipment and industrial plants. In addition, they are use d to control raw materials, materials during process and final products. A review of essencial aspects and concepts are provided. Simple methods to determine thermal properties are described. In a formative way, it is pointed out the conv enience of specific technical data availability as a fundamental basis to study or applyed heat transf er process. In the topic 6.1. are included the development of an equation which rela ted specific heat and sugar content of syrups. Several published equations for specific heat calcula tion 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 ju ice of apple, avocado, babaco, banana, banana-little, blackberry, cherry-black, grape, grapefru it, guava, lemon, lime, mango, muskmelon, naranjilla, nectarine, orange, papaw, passion-flower, passion-fruit, pear, peach, pineapple, plum, prickilypear, strawberry, tangerine, tomato, tre etomato, watermelon. A linear correlation was established between 'Brix and specific heat and i t could be used by processers of this kind of preserved food. Several publications are mentioned in Topic 6.2., they are about fruit t hermal diffusivity and equations for it calculation. A simple method to determine thermal diffu sivity, 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, avocad o, banana (four varieties), blackberry, cherry-black, cucumber, grape, grapefruit, lime, naranjilla, nectarine, melon, orange, papaw, passion-flower, pear, peach, pineapple, plum, prickilypear, starf ruit, strawberry, tangerine, tomato, treetomato and watermelon.When a specific product is analysed, the compliment of a linear relationship between thermal diffusivity and mois ture content occurs. However, it was established that this relation is not satisfactory if se veral 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 va lue were aproximately

0.5 [W/m. K] in all samples.

In foods, whose geometry is similar to spheric form, Fourier law for uns teady-state heat transfer, could be applied in order to determine thermal conductivity . Data measu red in potato and orange

ENGINEERING PRINCIPLES APPLIED IN FOODS

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 su rface of samples and heat conductivity determination, are provided. Results are similar to published ones. It was observed that the method i s 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 p rocess 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. Exper imental values determined in water, milk and carrot juice are presented. Calorimetric m ethod worth is confirmed. Latent heat of vaporization for foods is only a little higher that the v alue determined in pure water. To evaporate water from food with a low moisture content, is required mo re quantity of energy than the required to evaporate water from foods with a high moisture con tent. Data obtanined in samples of wheat products, by measuring vapour pressure and applying Oth mer law, are shown in Topic 6.6. A review of thermodynamics principles which explain fugacity and activit y concepts and their relationship with the Othmer law. This Topic also includes data obtained in milling samples of biscuits, noodles and bread by measuring of vapour pressure at three tem peratures. Water activity values are calculated and presented. Employing mathematical technics, va lues of latent heat of vaporization were determined, they are higher than pure water values unt il 23%.

Chapter 7. Heat Transfer.

This is one of the most important chapters, because in any moment of foo d processes exist changes of 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 emplo yment in control performance. It is important that new professionals apply these knowledg e in layout and construction 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 i ncresed 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 f low and heat transfer cofficients inside and outside of surface pipes. Corrections made for th ese equipments and resistance due to fouling are analyzed.

The Characteristics of laborato

r y equipment used in experimentation, main accesories and its properties were included. It is presented an example of heat transfer coefficients calculation usi ng experimental data in order to determine the overall heat transfer coefficient. Four conditions of v apour pressure in the shell and water flow inside pipes were considered. A computer program to calculate heat transfer coefficients and resistanc e due to fouling are provided. It employs different equations and selects the best one accord ing 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. 8

SUMMARY

Topic 7.2. is about plate heat exchangers, one of the most popular exchangers in food process industries.Main technical characteristics and general conditions to use them properly are included. Exchanger sections are described, and they are regeneration, heating, cooling, pump, homogenization, regulation tank, holding, tmalcooling and control system. Published relationships to determined heat transfer in these exchangers and to calculate resistance due to encrustations, are transcribed. A plate heat exchanger of laboratory was employed to shown tire operation of these equipments. Data obtained in a water-water system, with flow of cold and hot liquid in concurrent or counter current flow, are presented. Three different flow rates were considered to determine dimensionless numbers like: Reynolds, Prandtl, Peclet and Nusselt. The last one is used to calculate heat transfer coefficients employing experimental values or equations. Heat transfer determination and its overall heat transfer coefficients calculation could be used to quantify milk fouling influence over heat transfer. Topic 7.3. is about steam jacketed kettle. Main resistances offered to heat transfer by internal surface, are analyzed. Main equations to calculate heat transfer coefficients for condensed steam and boiling water, were compiled. Information about thermal conductivity of stainless steel plates, is included. Trials are done employing two jacketed kettlers, one is old and spheric, the second is new and cylinderc. Steam at different pressures as the heating agent and boiling water were consider to calculate overall heat transfer coefficient. Comparison of calculated values with experimental data measuring condensed steam mass and evaporated water mass, was used to calculate resistance due to encrustations. A computer program in BASIC language to simplify resistance calculation due to encrustacions, is included. Based on condensed steam in contact with inner surface, heat tranfer coefficient were determined, these data were compared with heat transfer coefficient calculated using the equations proposed by Brown, Charm, Perry and Green. This comparison leads to propose the last equation use to contol these equipments and to stablished cleaning and maintenance periods according to encrustations. Heat transfer coefficient data determined in boiling water contained in the kettle, are compared with those calculated by Rohsenow equation. It deserves particular attention that heat transfer coefficients could be used as control and layout tools, accordig to the methodology described in this Topic. The importance of heat transfer in unsteady-state during food processing is emphatized in Topic 7.4. This topic included, published equations for spherical and similar forms, which consider three

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

and finite surface and internal resistance to heat transfer. Heating and cooling trials in potatoes at air and maintained in cartoon boxes without protection, lead to confirm published equations and charts used to calculate temperature changes with time, when external heat transfer conditions are steady.

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

air, are analyzed. Experimental data were used to determine this film coefficient by trial and error procedure. Sterilization process of canning foods was accomplished in retorts. In this situation, heat transfer occurs in unsteady-state and through the finite cylinders, with initial nonuniform temperature. It is a quite complex situation of heat transfer and it could be analysed by the numerical method of finite differences,whose principles and equations are presented in Topic 7.5. An example presented by Charm was used to make a computer program in BASIC languaje, which calculates temperature changes in different points in the can during process. The program includes thermal properties calculation as function of temperature employing Choi and Okos model. It calculates mass average temperature inside the can for different times. Employing this information, time of

ENGINEERING PRINCIPLES APPLIED IN FOODS

9 cooling could be determined. Data obtained using a small can and a product whose proximate compositio n was known, are presented.

Chapter 8. Thermal Process.

This is a very important Chapter because it contents the principles whic h lead Food Engineering become 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 conditi ons are calculated based on the microbial growth, enzyme activity and food nutrition value. Curre nt computer programs could calculate process conditons exactly and easily. A procedimental pa rt 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, consumme rs would get sick or die. For that reason, aresponsableprofessional, who undesrtant process importance, is required. Two topics are discussed: the first one is about juice and milk pasteuri zation 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 t he time of pasteurizarion for cow milk and orange juice including its mathematic principles, are d evelopment 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 c ould be obtained analyzing data of thermal death time curve. A method suggested by Toledo to determ ine food pasteurization time in liquids that flow inside tube heaters exchangers, is described. Tu be heaters exchangers are commonly employed in aseptic processing. Temperature data registered in heating and cooling trials with cow milk and orange juice, are included. Consideringletalrate ofSalmonellaspp. andCostridium botulinumTipe E, a simple form to applied the General Method using experimental data and temperatu re secuence, are provided. A similar procedure was used for orange juice but consideringCandid tropicalisand

Rodotorularubraletalrate.

Pasteurization times calculated in both cases correspond to specific cha racteristics of each product. Boilmilk during five or more minutes is a common practice. Obtained results employing household instruments, proved that a correct pasteurization process requ ire 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 "broke n" heating curves; are included. The nomographic method description and graphics employing are enclosed. Experimentaldata obtained in canned prawns during sterilization in retort, lead to s how Ball method application and nomograph utilization considering thermal resista nce ofClostridium botulinumandBacillus 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 simul taneous. Considering this, many equipments was developed and constructed. In few cases their descri ption is included. 10

SUMMARY

This chapter provided fundamental concepts of water activity and equatio ns which describe drying kinetic at different conditions. Experimental methods to study this proc ess as an industrial one or in a general manner are presented. Simple drying methods were pointed ou t because they could be applied in rural regions. Obtained results were used for equipment la yout, one of the most important activities for Food Engineers. Water activity rather than moisture content influences biological reacti ons. 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 v alues and no-electrolitic constituens of binary solutions are reported, as well as specific equati ons 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 N orrishequation worth. Correlations, between soluble solid content (°Brix) and water activ ity, lead to establish differences between the effect of monosaccharide and disaccharide addition. Monosacc harides are more efficient to reduce water activity than disaccharides. Presented equations allow tocalculewater 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 g rapes to make raisins. Equations for water activity calculation in systems of multicompounds wi th high moisture content, are provided. Equations to describe water and solute flow during osmotic process are also included. Results of tryals at 35° and 45°C with bananas, are used to compar e process, by measuring of product weight and water content reduction. Water activity values were c alculated at different ti mes during osmotic dehydration to establish permeability constants, whic h are used to evalue process 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 th ree temepratures. Coefficients of heat and mass transfer, which were determined after osmo tic preconcentration in drying trials employing air, were presented. Four sorption isotherms for raisins at 20°C with different sugar contents are included. Solar drying is the most employed method to preserve foods in rural regi ons. Advantages of solar radiations at the Equator zone are discussed and equations for drying so lar energy calculation are presented in Topic 9.3. Different kinds of solar dryers and equations fo r 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 im proved, 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 -2°C, using sodium hidrox ide as hygroscopic agent and air movement inside of the chamber, were used to determine overall h eat transfer value and Heldman an Hohner number. According to this number it was established th at the process is

ENGINEERING PRINCIPLES APPLIED IN FOODS

11 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 value s and rehydration tests, lead to establish high quality of the product protein and very small phy sical damaged, specially when it is obtained by atmospheric freeze-drying. Potato slices dehydrat ed employing cooling temperatures showed considerable physical changes and microorganism grow th, however this method with a proper control, could be consider as an alternative for fo od preservation, specially in mountain regions whose temperature is nearly to the water freezing po int. Topic 9.5. is about Fick's second law application to control drying proc ess which are very common in vegetable products industries. First part, includes basic equations which established a relationship be tween heat transfer and water transfer when drying rate is constant. A detailed description of F ick's second law derivation is included. Equations obtained, which describe moisture changes with ti me considering diffusivity as the main mechanism of mass transfer in the falling rate period, are e nclosed. Equations to establish temperature effect on the diffusion coefficient and multiple e quations which also consider relative humidity and air velocity influence, are compiled. Equations pu blished by several authors to analyse drying process, when there are more than one falling rate per iod, 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 corre lation equations for diffusivity.Values of activation energy for drying were calculated usingArrhenius 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 d esign and construct a household dryer. Some information about cassava is provided: physical pr operties, chemical composition, toxicology, drying process in different conditions and equi pment. It was proved that allprocess could be analysed as one falling rate period. Two kinds of effec tive diffusion coefficients were determined, considering a liquid phase and considering a gas phase. Tryals at three different temperatures and three different air rates, lead to esta blished their effect on drying ti me until to reach low moisture contents which make sure product stabilit y. Adequate drying process conditions were established using this information and a hosehol d 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 c ommon 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 a t 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 STA TES, Department of Scientific and Technological Affairs, Multinational Proyec t of Biotechnology and

Food Technology.

12

SUMMARY