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Food Engineering Introduction Dr. Asma Eskhan 1  The Food Process A ‘process’ is defined as a set of actions in a specific sequence, to a specific end. A manufacturing process starts with raw materials and ends with products and by- products. The ‘actions’ that...

Food Engineering Introduction Dr. Asma Eskhan 1  The Food Process A ‘process’ is defined as a set of actions in a specific sequence, to a specific end. A manufacturing process starts with raw materials and ends with products and by- products. The ‘actions’ that constitute a process may be grouped in a relatively small number of operations governed by the same basic principles and serving essentially similar purposes. Some of the unit operations of the food processing industry are listed in Table I.1 : 2 3 4 While the type of unit operations and their sequence vary from one process to another, certain features are common to all food processes: Material balances and energy balances are based on the universal principle of the conservation of matter and energy Practically every operation involves exchange of material, momentum and/or heat between the different parts of the system. These exchanges are governed by rules and mechanisms, collectively known as transport phenomena In any manufacturing process, adequate knowledge of the properties of the materials involved is essential. The principal distinguishing peculiarity of food processing is the outstanding complexity of the materials treated and of the chemical and biological reactions induced. This characteristic reflects strongly on issues related to process design and product quality and it calls for the extensive use of approximate models. Mathematical – physical modeling is indeed particularly useful in food engineering. Of particular interest are the physical properties of food materials and the kinetics of chemical reactions. 5  One of the distinguishing features of food processing is the concern for food safety and hygiene. This aspect constitutes a fundamental issue in all the phases of food engineering, from product development to plant design, from production to distribution The importance of packaging in food process engineering and technology cannot be overemphasized. Research and development in packaging is also one of the most innovative areas in food technology today Finally, common to all industrial processes, regardless of the materials treated and the products made, is the need to control. The introduction of modern measurement methods and control strategies is, undoubtedly, one of the most significant advances in food process engineering of the last years. 6  Batch and Continuous Processes Processes may be carried-out in batch, continuous or mixed fashion. In batch processing, a portion of the materials to be processed is separated from the bulk and treated separately. The conditions such as temperature, pressure, composition etc. usually vary during the process. The batch process has a definite duration, and, after its completion, a new cycle begins, with a new portion of material. The batch process is usually less capital intensive but may be more costly to operate and involves costly equipment dead-time for loading and unloading between batches. It is easier to control and lends itself to intervention during the process. It is particularly suitable for small-scale production and to frequent changes in product composition and process conditions. A typical example of a batch process would be the mixing of flour, water, yeast and other ingredients in a bowl mixer to make a bread dough. After having produced one batch of dough for white bread, the same mixer can be cleaned and used to make a batch of dark dough. 7  In continuous processing, the materials pass through the system continuously, without separation of a part of the material from the bulk. The conditions at a given point of the system may vary for a while at the beginning of the process, but ideally, they remain constant during the best part of the process. In engineering terms, a continuous process is ideally run at steady state for most of its duration. Continuous processes are more difficult to control, require higher capital investment, but provide better utilization of production capacity, at lower operational cost. They are particularly suitable for lines producing large quantities of one type of product for a relatively long duration. A typical example of a continuous process would be the continuous pasteurization of milk. Pasteurization is a process by which milk is heated to a specific temperature for a set period of time to kills harmful bacteria that can lead to diseases like as listeriosis, typhoid fever, tuberculosis, diphtheria and brucellosis. 8  Mixed processes are composed of a sequence of continuous and batch processes. An example of a mixed process would be the production of strained infant food. In this example, the raw materials are first subjected to a continuous stage consisting of washing, sorting, continuous blanching or cooking, mashing and finishing (screening). Batches of the mashed ingredients are then collected in formulation tanks where they are mixed according to formulation. Usually, at this stage, a sample is sent to the quality assurance laboratory for evaluation. After approval, the batches are pumped, one after the other, to the continuous homogenization, heat treatment and packaging line. Thus, this mixed process is composed of one batch phase between two continuous phases. To run smoothly, mixed processes require that buffer storage capacity be provided between the batch and continuous phases. Buffer storage areas or systems are used to temporarily store items that will soon be needed for production or orders. Normally buffer storage does not have fixed storage positions for specific items. 9 Pallet flow racks act as a storage buffer, supplying production with the materials required  Flow diagrams, also called flow charts or flow sheets, serve as the standard graphical representation of processes. In its simplest form, a flow diagram shows the major operations of a process in their sequence, the raw materials, the products and the byproducts. Additional information, such as flow rates and process conditions such as temperatures and pressures may be added. Because the operations are conventionally shown as rectangles or ‘blocks’ , flow charts of this kind are also called block diagrams. Figure I.1 shows a block diagram for the manufacture of chocolate. 10  A more detailed description of the process provides information on the main pieces of equipment selected to perform the operations. Standard symbols are used for frequently utilized equipment items such as pumps, vessels, conveyors, centrifuges, filters etc. ( Figure I.2 ). 11  Other pieces of equipment are represented by custom symbols, resembling fairly the actual equipment or identified by a legend. Process piping is schematically included. The resulting drawing is called an equipment flow diagram. A flow diagram is not drawn to scale and has no meaning whatsoever concerning the location of the equipment in space. A simplified pictorial equipment flow diagram for the chocolate production process is shown in Figure I.3. The next step of process development is the creation of an engineering flow diagram. In addition to the items shown in the equipment flow diagram, auxiliary or secondary equipment items, measurement and control systems, utility lines and piping details such as traps, valves etc. are included. The engineering flow diagram serves as a starting point for the listing, calculation and selection of all the physical elements of a food plant or production line and for the development of a plant layout. 12 Figure I.3 Pictorial flow diagram of chocolate manufacturing process (Courtesy of Bühler AG)  Food Engineering Chapter 1 Physical Properties of Food Materials Dr. Asma Eskhan 1  1.1 Introduction The physical properties of foods can be defined as those properties that lend themselves to description and quantification by physical rather than chemical means. This seemingly obvious distinction between physical and chemical properties reveals an interesting historical fact. Indeed, until the 1960s, the chemistry and biochemistry of foods were by far the most active areas of food research. The systematic study of the physical properties of foods (often considered a distinct scientific discipline called ‘food physics’ or ‘physical chemistry of foods’ ) is of relatively recent origin. The physical properties of foods are of utmost interest to the food engineer, mainly for two reasons: 1. Many of the characteristics that define the quality (e.g. texture, structure, appearance) and stability (e.g. water activity) of a food product are linked to its physical properties. 2 2. Quantitative knowledge of many of the physical properties, such as thermal conductivity, density, viscosity, specific heat, enthalpy and many others, is essential for the rational design and operation of food processes and for the prediction of the response of foods to processing, distribution and storage conditions. These are sometimes referred to as ‘ engineering properties ’ , although most physical properties are significant both from the quality and engineering points of view. 3  1.2 Mechanical Properties 1.2.1 Definitions By mechanical properties, we mean those properties that determine the behavior of food materials when subjected to external forces. As such, mechanical properties are relevant both to processing (e.g. conveying, size reduction) and to consumption (texture, mouth feel). The forces acting on the material are usually expressed as stress, i.e. intensity of the force per unit area (N.m −2 or Pa). The dimensions and units of stress are like those of pressure. Very often, but not always, the response of materials to stress is deformation, expressed as strain. Strain is usually expressed as a dimensionless ratio, such as the elongation as a percentage of the original length. The relationship between stress and strain is the subject matter of the science known as rheology. 4 We define three ideal types of deformation: Elastic deformation : deformation appears instantly with the application of stress and disappears instantly with the removal of stress. For many materials, the strain is proportional to the stress, at least for moderate values of the deformation. The condition of linearity, called Hooke’s law is formulated in Eq. (1.1): where E = Young’s modulus, Pa F = force applied, N A0 = original cross-sectional area ΔL = elongation, m L0 = original length. 5  Plastic deformation: deformation does not occur as long as the stress is below a limit value known as yield stress. Deformation is permanent, i.e. the body does not return to its original size and shape when the stress is removed. Viscous deformation : deformation (flow) occurs instantly with the application of stress, and it is permanent. The rate of strain is proportional to the stress (Chapter 2). The types of stress are classified according to the direction of the force in relation to the material. Normal stresses are those that act in a direction perpendicular to the material’s surface. Normal stresses are compressive if they act towards the material and tensile if they act away from it. Shear stresses act in a direction parallel (tangential) to the material’s surface ( Figure 1.1 ). The increase in the deformation of a body under constant stress is called creep. Figure 1.1 Types of stress The decay of stress with time, under constant strain, is called relaxation. 6 7  1.2.2 Rheological models The stress–strain relationship in food materials is usually complex. It is therefore useful to describe the real rheological behavior of foods with the help of simplified, approximate models. Those models are constructed by connecting ideal elements (elastic, viscous, friction, rupture etc.) in series, in parallel or in combinations of both. Some of these models are shown in Figure 1.2. The physical models are useful in the development of mathematical models (equations) for the description and prediction of the complex rheological behavior of foods. The rheological characteristics of fluids are discussed in some detail in a subsequent section (Chapter 2, Section 2.3). Figure 1.2 Three rheological models 8 1.3 Thermal Properties Almost every process in the food industry involves thermal effects such as heating, cooling or phase transition. The thermal properties of foods are therefore of considerable relevance in food process engineering. The following properties are of particular importance: thermal conductivity, thermal diffusivity, specific heat, latent heat of phase transition and emissivity. In addition, theoretical or empirical methods have been developed for the prediction of these properties in the light of the chemical composition and physical structure of food materials. Specific heat cp (kJ.kg−1.K−1 ) is among the most fundamentals of thermal properties. It is defined as the quantity of heat (kJ) needed to increase the temperature of one unit mass (kg) of the material by one degree (°K) at constant pressure. The specification of ‘at constant pressure’ is relevant to gases where the heat input needed to cause a given increase in temperature depends on the process. It is practically irrelevant in the case of liquids and solids. 9 The definition of specific heat can be formulated as follows: The specific heat of a material can be determined experimentally by static (adiabatic) calorimetry or differential scanning calorimetry or calculated from measurements involving other thermal properties. It can be also predicted quite accurately with the help of a number of empirical equations. The simplest model for solutions and liquid mixtures assumes that the specific heat of the mixture is equal to the sum of the pondered contribution of each component. The components are grouped in classes: water, salts, carbohydrates, proteins, lipids. The specific heat, relative to water, is taken as: salts 0.2; carbohydrate 0.34; proteins 0.37; lipids 0.4; water 1. The specific heat of water is 4.18 kJ.kg−1.K−1. 10 The specific heat of a solution or liquid mixture is therefore: where X represents the mass fraction of each of the component groups (Rahman, 1995 ). For mixtures that approximate solutions of sugar in water (e.g. fruit juices), Eq. (1.3) becomes: Another frequently used model assigns to the total dry matter of the mixture a single relative specific value of 0.837. The resulting approximate empirical expressions for temperatures above and below freezing are given in Eq. (1.5): 11 1.4 Electrical Properties The electrical properties of foods are particularly relevant to microwave and ohmic heating of foods and to the effect of electrostatic forces on the behavior of powders. The most important properties are electrical conductivity and the dielectric properties. These are discussed in Chapter 3, in relation with ohmic heating and microwave heating. 1.5 Structure Very few foods are truly homogeneous systems. Most foods consist of mixtures of distinct physical phases, in close contact with each other. The heterogeneous nature of foods may be visible to the naked eye or perceived only when examined under a microscope or electron microscope. In foods, the different phases are seldom in complete equilibrium with each other and many of the desirable properties of ‘ fresh ’ foods are due to the lack of equilibrium between the phases. 12 Following are some of the different structural elements in foods: 1. Cellular structures: vegetables, fruits and muscle foods consist in large part of cellular tissue. The characteristics of the cells and, more particularly, of the cell walls determine the rheological and transport properties of cellular foods. One of the characteristics particular to cellular foods is turgidity or turgor pressure. Turgor is the intracellular pressure resulting from osmotic differences between the cell content and the extracellular fluid. This is the factor responsible for the crisp texture of fruits and vegetables and for the ‘fleshy’ appearance of fresh meat and fish. Cellular food structures may also be created artificially. Wheat bread consists of gas-filled cells with distinct cell walls. The numerous puffed snacks and breakfast cereals produced by extrusion owe their particular crispiness to their cellular structure with brittle cell walls. 13 2. Fibrous structures: in this context we refer to physical fibers, i.e. to solid structural elements with one dimension much larger than the other two and not to ‘dietary fiber’. The most obvious of the fibrous foods is meat. Indeed, protein fibers are responsible for the chewiness of muscle foods. The creation of a man- made fibrous structure is the main challenge of the meat analog developer. 3. Gels: gels are macroscopically homogeneous colloidal systems, where dispersed particles (generally polymeric constituents such as polysaccharides or proteins) have combined with the solvent (generally water) to create a semi-rigid solid structure. Gels are usually produced by first dissolving the polymer in the solvent, then changing the conditions (cooling, concentration, cross-linking) so that the solubility is decreased. Gelation is particularly important in the production of set yogurt, dairy desserts, custard, tofu, jams and confectionery. The structural stability of food gels subjected to shear and certain kinds of processing (e.g. freezing– thawing) is an important consideration in product formulation and process design. The gelling agent in the production of jam is pectin (a complex starch). 14 4. Emulsions: emulsions are intimate mixtures of two mutually immiscible liquids, where one of the liquids is dispersed as fine globules in the other ( Figure 1.3 ). In the case of foods the two liquid media are, in most cases, fats and water. Two possibilities exist for emulsions consisting of oil and water: a. The dispersed phase is oil (oil-in-water, o/w emulsions). This is the case of milk, cream, sauces and salad dressings. Figure 1.3 Schematic structure of oil- b. The dispersed phase is water (water-in-oil, w/o in-water and water-in-oil emulsions emulsions). Butter and margarine are w/o emulsions. Emulsions are not thermodynamically stable systems. They do not form spontaneously. Emulsification requires energy Depending on the agent used, a particular input (mixing, homogenization) in order to shear one of the emulsifier may be more soluble in one phase phases into small globules and disperse them in the continuous or another. If an emulsifier is more soluble phase. Emulsions tend to break apart as the result of in water, it is more likely to facilitate the creation of oil-in-water emulsions. coalescence (fusion of the disperse droplets into larger ones) Conversely, oil-soluble emulsifiers tend to and creaming (separation of the original emulsion into a more encourage the formation of water-in-oil emulsions. concentrated emulsion or cream, and some free continuous phase). Emulsions are stabilized with the help of surface-active agents known as emulsifiers. 15 4. Foams: foams are cellular structures consisting of gas (air) filled cells and liquid cell walls. Due to surface forces, foams behave like solids. Ice cream is essentially frozen foam, since almost half of its volume is air. Foams with specific characteristics (bubble size distribution, density, stiffness, stability) are important in milk-containing beverages and beer. On the other hand, the spontaneous excessive foaming of some liquid products (e.g. skim milk) during transportation and processing may create serious engineering problems. Undesired foaming is controlled by proper design of the equipment, mechanical foam breakers or through the use of food grade chemical antifoaming (prevention) and defoaming (foam abatement) agents such as oils and certain silicone based compounds. 5. Powders: solid particles, 10 to 1000 micrometers in size, are defined as powders. Smaller particles are conventionally called ‘ dust ’and larger particles are ‘ granules ’. Some food products and many of the raw materials of the food industry are powders. Powders are produced by size reduction, precipitation, crystallization or spray drying. One of the main issues related to powders in food engineering is the flow and transportation of particulate materials. 16  Food Engineering Chapter 1 Physical Properties of Food Materials Dr. Asma Eskhan 1 1.6 Water Activity 1.6.1 The importance of water in foods Water is the most abundant constituent in most foods. Indicative values of water content in a number of food products are shown in Table 1.1. Classification of foods into three groups according to their water content (high, intermediate and low moisture foods) has been suggested ( Franks, 1991 ). Fruits, vegetables, juices, raw meat, fish and milk belong to the high moisture category. Bread, hard cheeses and sausages are examples of intermediate moisture foods, while the low moisture group includes dehydrated vegetables, grains, milk powder and dry soup mixtures. The functional importance of water in foods goes far beyond its mere quantitative presence in their composition. On one hand, water is essential for the good texture and appearance of fruits and vegetables. In such products, loss of water usually results in lower quality. On the other hand, water, being an essential requirement for the occurrence and support of chemical reactions and microbial growth, is often responsible for the microbial, enzymatic and chemical deterioration of food. 2 3  It is now well established that the effect of water on the stability of foods cannot be related solely to the quantitative water content. As an example, honey containing 23% water is perfectly shelf stable while dehydrated potato would undergo rapid spoilage at a moisture content half as high. To explain the influence of water, a parameter that reflects both the quantity and the ‘effectiveness’ of water is needed. This parameter is water activity. 1.6.2 Water activity, definition and determination Water activity, aw , is defined as the ratio of the water vapor pressure of the food to the vapor pressure of pure water at the same temperature. where: p = partial pressure of water vapor of the food at temperature T p0= equilibrium vapor pressure of pure water at temperature T 4 The same type of ratio also defines the relative humidity of air, RH (usually expressed as a percentage): where: p′ = partial pressure of water vapor in air. If the food is in equilibrium with air, then p = p′. It follows that the water activity of the food is equal to the relative humidity of the atmosphere in equilibrium with the food. For this reason, water activity is sometimes expressed as the equilibrium relative humidity, ERH. Many of the methods and instruments for the determination of water activity are based on Eq. (1.8). A sample of the food is equilibrated with a small head-space of air in a close chamber and then the relative humidity of the headspace is measured by an appropriate hygrometric method such as the ‘chilled mirror’ technique ( Figure 1.4 ).5 Typical water activity values of some food products are given in Table 1.2. Figure 1.4 Measurement of water activity 1.6.3 Water activity: prediction The principal mechanisms responsible for the depression of vapor pressure of water in foods are solvent–solute interaction, binding of water molecules to the polar sites of polymer constituents (e.g. polysaccharides and proteins), adsorption of water on the surface of the solid matrix and capillary forces ( Le Maguer, 1987 ). In high moisture foods, such as fruit juices, the depression may be attributed entirely to water– solute interaction. If such foods are assumed to behave as ‘ideal solutions’, then their water vapor pressure obeys Raoult’s law (see Section 13.2), as in Eq. (1.9): where xw is the water content (in molar fraction) of the food. It follows that the water activity of an ideal aqueous solution is equal to the molar concentration of water xw. The water activity of high moisture foods (with aw of 0.9 or higher) can be calculated quite accurately by this method. 7 EXAMPLE 1.1 Estimate the water activity of honey. Consider honey as an 80% w/w aqueous solution of sugars (90% hexoses, 10% disaccharides). Solution: The composition of 100 g of honey is: Assuming Raoult’s law, the water activity of honey is equal to the molar fraction of water: 8  As the water content is reduced, water binding by the solid matrix and capillary forces become increasingly significant factors and overshadow water–solute interaction. Furthermore, the assumption of ideal solution behavior can no longer be applied because of the reduced concentration of the liquid phase. The relationship between water content and water activity, aw = f(X) becomes more complex. This is discussed in the next section. Water activity is temperature dependent. Considering the definition of water activity, as given in Eq. (1.6), one would be tempted to conclude the opposite. Temperature affects both p and p0 in the same manner (the Law of Clausius-Clapeyron), therefore their ratio should not be affected by the temperature. This is true for the liquid phase and, indeed, the water activity of high moisture foods is affected by temperature very slightly, if at all. The situation is different at lower levels of water content. Temperature affects not only the water molecules but also the solid matrix interacting with water. Therefore, temperature affects water activity at low moisture levels where adsorption and capillary effects are strong. The direction and intensity of temperature effects are not predictable. 9 1.6.4 Water vapor sorption isotherms The function representing the relationship between water content (e.g. as grams of water per gram of dry matter) and water activity at constant temperature is called the ‘water vapor sorption isotherm’ or a ‘moisture sorption isotherm’ of a food. The general form of a hypothetical sorption isotherm is shown in Figure 1.5. Sorption isotherms of a large number of foods have been compiled by Iglesias and Chirife (1982). Sorption isotherms are determined experimentally. Basically, samples of the food are equilibrated at constant temperature with atmospheres at different known relative humidities. After equilibration, the samples are analyzed for water (moisture) content. Each pair of ERH–moisture content data give one point on the isotherm. The experimental methods for the determination of sorption isotherms fall into two groups, namely, static and dynamic procedures. In static methods, weighed samples of food are placed in jars, over saturated aqueous solutions of different salts and left to equilibrate at constant temperature. At constant temperature, the concentration of saturated solutions is constant and so is their water vapor pressure. 10 Figure 1.5 General form of a sorption isotherm 11 12  The relative humidity of the atmosphere in equilibrium with saturated solutions of some salts is given in Table 1.3. In dynamic methods, the sample is equilibrated with a gas stream, the relative humidity of which is continuously changed. The quantity of moisture adsorbed or desorbed is determined by recording the change in the weight of the sample. The two curves shown in Figure 1.5 indicate the phenomenon of ‘hysteresis’ , often encountered. One of the curves consists of experimental data points where the food sample came to equilibrium by losing moisture (desorption). The other curve represents points obtained by the opposite path, i.e. by gain of moisture (adsorption). The physical explanation of the sorption hysteresis has been the subject of many studies. Generally, hysteresis is attributed to the condensation of some of the water in the capillaries ( Labuza, 1968 ; Kapsalis, 1987 ; deMann 1990 ). The observation that, depending on the path of sorption, food can have two different values of water activity at the same moisture content casts doubt on the thermodynamic validity of the concept of sorption equilibrium ( Franks, 1991 ). 13  Numerous attempts have been made to develop mathematical models for the prediction of sorption isotherms ( Chirife and Iglesias, 1978 ). Some of the models developed are based on physical theories of adsorption (see Chapter 12). Others are semi-empirical expressions developed by curve fitting techniques. One of the best-known models is the Brunauer-Emmett-Teller (BET) equation. The basic assumptions on which the BET model is based are discussed in Section 12.2. Applied to water vapor sorption, the BET equation is written as follows: where: X = water content, grams water per gram of dry matter Xm = a parameter of the equation, interpreted as the value of X for the saturation of one monomolecular layer of water on the adsorbing surface (the BET monolayer) C = a constant, related to the heat of adsorption. 14 To find Xm and C from experimental sorption data, the BET equation is written as follows: If the group Φ is plotted against aw, a straight line is obtained ( Figure 1.6 ). Xm and C are calculated from the intercept and the slope. The BET model has been found to fit well sorption isotherms, up to water activity values of about 0.45. 15 EXAMPLE 1.2 Following are 3 points from the sorption isotherm of potato at 20°C: Estimate the monolayer value of potato, based on the data. Solution: We calculate the group Φ in Eq. (1.11), as a function of aw. We find: 16 The linear plot of Φ versus a w is found to be: Φ = 16.8 aw + 0.57 Solving for C and Xm we find C = 30.47 and Xm 0.058 Figure 1.6 Linearization of the BET equation with 3 experimental points 17 Another equation which is often used to predict sorption isotherms is the Guggenheim- Anderson-de Boer (GAB) model shown below: where C and K are constants, both related to the temperature and heat of adsorption. The range of applicability of the GAB equation is wider than that of the BET model. 18 1.6.5 Water activity: effect on food quality and stability Bacterial growth does not occur at water activity levels below 0.9. With the exception of osmophilic species, the water activity limit for the growth of molds and yeasts is between 0.8 and 0.9. Most enzymatic reactions require water activity levels of 0.85 or higher. The relationship between water activity and chemical reactions (Maillard browning, lipid oxidation) exhibit more complex behavior with maxima and minima (Figure 1.7). Figure 1.7 Relative rate of deterioration mechanisms as affected by water activity. A: Lipid oxidation; B: Maillard browning; C: Enzymatic activity; D: Mold growth; E: Bacteria growth. The Maillard reaction is an organic chemical reaction in which reducing sugars react with amino acids to form a complex mixture of compounds. This reaction is responsible for the characteristic flavor and aroma of browned food. 19 Lipids (also known as fats) are components of plant (e.g., vegetable oils) and animal tissues (e.g., meat, eggs, milk). 20  Food Engineering Chapter 1 Physical Properties of Food Materials Dr. Asma Eskhan 1 1.7 Phase Transition Phenomena in Foods 1.7.1 The glassy state in foods With few exceptions, foods should be regarded as metastable systems capable of undergoing change. Stability is a consequence of the rate of change. In turn, the rate of change depends on molecular mobility. In recent years, molecular mobility has become a subject of strong interest among food scientists. The subject is particularly important in solid and semi-solid foods with low to intermediate water content. In the majority of foods belonging to this category, the interaction between polymeric constituents, water and solutes is the key issue in connection with molecular mobility, diffusion and reaction rates. Accordingly, concepts and principles developed by polymer scientists are now being applied to foods ( Slade and Levine, 1991, 1995 ). 2  Consider a liquid food product, such as honey, consisting of a concentrated aqueous solution of sugars. The physical properties and stability of such a solution depend on two variables: concentration and temperature. If the concentration is increased by slowly removing some of the water and the temperature is lowered gradually, solid crystals of sugar will be formed. If the process of concentration and cooling is carried out under different conditions, crystallization will not take place, but the viscosity of the solution will increase until a rigid, transparent, glass-like material will be obtained. The familiar transparent hard candy is an example of glassy (vitreous) food. The glassy state is not limited to sugar–water systems. Intermediate and low moisture foods often contain glassy regions consisting of polymer materials (e.g. gelatinized starch) and water. The phenomenon of passage from the highly viscous, rubbery semi-liquid to the rigid glass is called ‘glass transition’ and the temperature at which that occurs is the glass transition temperature, Tg. 3  Physically, a glass is an amorphous solid. It is also sometimes described as a supercooled liquid of extremely high viscosity. Conventionally, the viscosity assigned to a glass is in the order of 1011 to 1013 Pa.s., although it is practically impossible to verify this convention experimentally. The molecules of a glass do not have an orderly arrangement as in a solid crystal, but they are sufficiently close and sufficiently immobile to possess the characteristic rigidity of solids. Because of the negligible molecular mobility, the rate of chemical and biological reactions in glassy material is extremely low. The rigidity of the glassy regions affects the texture of the food. Staling of bread is due to the transition of the starch–water system from rubbery to glassy state. The crunchiness of many snack products is due to their glassy structure. Staling is a chemical and physical process in bread and similar foods that reduces their palatability. Staling is not simply a drying-out process due to evaporation. One important mechanism is the migration of moisture from the starch granules into the interstitial spaces, degelatinizing the starch; stale bread's leathery, hard texture results from the starch amylose and amylopectin molecules realigning themselves causing 4 recrystallisation. 1.7.2 Glass transition temperature The different physical states of an aqueous solvent–solute system capable of forming an amorphous solid and the processes of passage from one state to another have been described by Roos and Karel (1991a) in a frequently cited diagram ( Figure 1.8 ). Figure 1.8 State diagram of a carbohydrate solution. (Adapted from Roos and Karel, 1991a.) 5  Boiling of a liquid or melting of a crystal are ‘thermodynamic’ phase transitions, also known as ‘first order transitions’. They occur at a fixed, definite set of conditions (temperature, pressure), independent of rate. The phases in transition are mutually in equilibrium. In contrast, glass transition is of kinetic nature. It does not involve large step changes in properties and does not require a considerable latent heat of transition. The glass transition temperature of a given rubbery material is not a fixed point. It varies somewhat with the rate and direction of the change (e.g. rate of heating or cooling), therefore the procedure for its determination has to be specified exactly. Glass transition temperatures of pure dry sugars are given in Table 1.4. Glass transition temperature is strongly dependent on concentration. Dilute solutions have lower Tg. This led Roos and Karel (1991b) to conclude that water acts on the amorphous food as a plasticizer in a polymer system. The effect of concentration on Tg is shown in Figure 1.9 for a solution of sucrose. 6 7 Figure 1.9 Effect of temperature on T g of sucrose solutions 8  It has been suggested that the glass transition temperature of a blend can be predicted, using the Gordon–Taylor equation, borrowed from polymer science and shown in Eq. (1.13): where: Tg = glass transition temperature of the mixture Tg1, Tg2 = absolute glass transition temperatures (°K) of component 1 and 2, respectively. The subscript 2 refers to the component with the higher Tg. w1 , w2 = weight fractions of component 1 and 2, respectively k = a constant. A simpler approximate expression is the Fox equation ( Schneider, 1997 ): 9  According to Johari et al. (1987) , the Tg of water is 138°K or −135°C. Glass transition temperatures of some carbohydrates are shown in Table 1.4. The viscosity of solutions increases sharply as Tg is approached. Near Tg, the effect of temperature on viscosity does not comply with the Arrhenius law but follows the Williams–Landel–Ferry (WLF) relationship ( Roos and Karel, 1991c), shown in Eq. (1.15): where μ and μg stand for the viscosity at the temperatures T and Tg respectively. Just as water activity, glass transition temperature has become a key concept in food technology, with applications in quality assessment and product development. Since the glassy state is considered as a state where molecular mobility is at a minimum, it has become a custom to study food properties and stability, not as a function of the temperature T, but as a function of the difference T − Tg ( Simatos et al., 1995 ). 10  Several methods exist for the determination of Tg. The results may vary somewhat, depending on the method used. The most commonly applied method is differential scanning calorimetry (DSC). DSC measures and records the heat capacity (i.e. the amount of heat necessary to increase the temperature by 1 degree Celsius) of a sample and of a reference as a function of temperature. A sharp increase or decrease in heat capacity indicates an endothermic or exothermic phase transition at the temperature where this occurs. In the case of first order transitions, such as melting, the amplitude of the change is considerable. In contrast, glass transition is detected as a subtle inflexion in the heat capacity curve ( Figure 1.10 ). Since the change occurs over a temperature range and not sharply at one temperature, the decision where to read Tg is subject to interpretation. The two most common conventions are the mid-point of the step and the point representing the onset of the transition ( Simatos et al. 1995 ) 11 Figure 1.10 Glass transition temperature from DSC plot 12 Food Engineering Chapter 2 Fluid Flow Dr. Asma Eskhan 1  2.1 Introduction The majority of industrial food processes involve fluid movement. Liquid foods such as milk and juices have to be pumped through processing equipment or from one container to another. In a blast freezer, a rapid stream of cold air is blown over the food. In a wheat mill, the grain, the milled intermediates and the final products are most often conveyed in a stream of air (pneumatic conveying). Essential process service media (utilities) such as water, steam and various gases have to be distributed about the plant in properly designed pipelines. A number of important unit operations such as filtration, pressing and mixing are, essentially, particular applications of fluid flow. The mechanism and rate of energy and mass transfer are strongly dependent on flow characteristics. Finally, the sensory quality of many liquid and semi-liquid foods depends, to a large extent, on the flow properties of the product. 2  This chapter consists of four parts. The first part deals with the study of fluids in motion. This is the realm of a discipline known as ‘fluid dynamics’. The second part is about the flow and deformation properties of fluids. This is the subject matter of the science called ‘rheology’. Technical elements such as pumps and piping, used for conveying fluids are discussed in the third part. The fourth part deals with flow and flow-related phenomena involving particulate solids. 3 2.2 Elements of Fluid Dynamics 2.2.1 Viscosity Consider a mass of fluid confined between two flat plates ( Figure 2.1 ). The lower plate is held stationary. The upper plate moves in the x direction at a constant velocity vx. Assume that the liquid layer in immediate contact with each plate moves at the velocity of that plate (no slippage). What has been described is clearly a ‘shearing’ action on the fluid. Newton’s law states that the shearing force Fx required to maintain the upper plate in movement is proportional to the area of the plate A and to the velocity gradient (dvx /dz). It is assumed that there is no movement other than in the x direction. Figure 2.1 Definition of viscosity 4  The proportionality factor μ is called viscosity. Viscosity is a property of the fluid and represents the resistance of the fluid to shearing action. Its units in the SI system are Pascal.second (Pa.s). The traditional c.g.s unit is the poise, after the French physicist Poiseuille (1799–1869). The conversion factor is: The viscosity of liquids is strongly temperature-dependent and almost pressure independent. The viscosity of gases increases with pressure and decreases slightly with increasing temperature. Viscosity data for various materials of interest in food process engineering are given in the Appendix. The shearing force per unit area is the shear stress, shown by the symbol τ. The velocity gradient (dvx /dz) is called the shear rate, represented by the symbol γ. Eq. (2.1) can now be written as: 5  For many fluids, the viscosity is independent of the shear rate and Eq. (2.2) is linear. Such fluids are qualified as ‘Newtonian ’. Gases, water, milk and dilute solutions of low molecular weight solutes are practically Newtonian. Other fluids, such as solutions of polymers and concentrated suspensions are non- Newtonian. Their viscosity depends on the shear rate. For non-Newtonian liquids Eq. (2.2) is not linear. 6 2.3 Flow Properties of Fluids 2.3.1 Types of fluid flow behavior With respect to flow properties, fluids are classified according to the relationship between shear stress τ and shear rate γ. Shear stress–shear rate curves for typical classes of fluids are shown in Figure 2.8. Newtonian fluids: the τ – γ relationship is linear with a zero intercept. The constant slope of the straight line is the viscosity. Gases, low molecular weight liquids and dilute solutions behave like Newtonian fluids. Bingham fluids: the τ – γ relationship is linear with a non-zero positive intercept called yield stress, τ0. The fluid does not flow if the shear Figure 2.8 Types of rheological behavior of fluids stress is less than τ0. Some paste-like concentrated suspensions behave like Bingham fluids (also called Bingham plastic materials).  Shear thinning or pseudoplastic fluids: the τ – γ relationship is non-linear with a zero intercept. The curve is concave, i.e. the viscosity decreases with increasing shear, hence the qualification of shear thinning. Fruit juice concentrates and solutions of proteins or hydrocolloid gums exhibit pseudoplastic behavior. Shear thickening or dilatant fluids: the τ – γ relationship is non-linear with a zero intercept but the curve is convex. The viscosity increases with increasing shear. Some types of honey have been reported to have this type of flow behavior ( Steffe, 1996 ). Herschel-Bulkley fluid: this type is non-linear with a positive yield stress. The models of flow behavior listed above are ‘ time-independent ’ because the response of shear stress to a change in shear rate is immediate. It can be said that these models ‘have no memory ’. In contrast, the flow behavior of certain fluids is time-dependent. At constant shear, their viscosity changes with time. If shear is applied to certain fluids, qualified as thixotropic, their viscosity decreases gradually with time. 8  If shear is stopped, their viscosity increases gradually. Tomato ketchup and some gels are thixotropic. Fluids which show the opposite behavior are called rheopectic. Rheopectic behavior is not common ( Steffe, 1996 ). Several mathematical models have been proposed for the representation of the types of flow behavior described above. The Herschel-Bulkley model, shown in Eq. (2.27) is most commonly used. For a Newtonian fluid, τ0 = 0, s = 1 For a Bingham fluid, τ0 > 0, s = 1 For a shear thinning fluid, τ0 = 0, 0 < s < 1 For a shear thickening fluid, τ0 = 0, s > 1 For a Herschel-Buckley fluid, τ0 > 0, s > 0. 9  Another expression, known as the Casson equation is given in Eq. (2.28) Apparent viscosity is τ/γ. In a Newtonian fluid, the apparent viscosity is also the true (Newtonian) viscosity, independent of shear rate. For all the types of nonNewtonian fluids, apparent viscosity depends on the shear rate. For shear thinning and shear thickening fluids (so-called Power law fluids), the apparent viscosity is: 10 Food Engineering Chapter 2 Fluid Flow Dr. Asma Eskhan 1 2.4 Transportation of Fluids 2.4.2 Pumps: Types and operation A pump is, essentially, a device that uses energy to increase the total head of a fluid. By this general definition, not only pumps for moving liquids but also gas compressors, vacuum pumps, blowers and fans are considered pumps. Pumps are classified in two major groups, namely, kinetic pumps and positive displacement pumps. Kinetic pumps: kinetic pumps impart to the fluid a velocity, hence kinetic energy, which is then converted to pressure according to the Bernoulli law. The most widespread type of kinetic pump is the centrifugal pump. The fluid is admitted at the center of the pump housing. A rapidly rotating impeller (rotor) imparts to the fluid a rotational movement at high velocity. The fluid moves in a radial direction away from the center to the periphery under the action of centrifugal forces Figure 2.12 Principle of operation of a centrifugal pump  Positive displacement pumps: in a positive displacement pump, a portion of the fluid is confined in a chamber and mechanically moved (displaced) forward. The main types of positive displacement pumps are: the reciprocating (piston, diaphragm) pumps, the rotary (lobe, gear) pumps, the progressing cavity (Moyno) pumps and the peristaltic pumps. Figure 2.20 Principle of operation of the progressing cavity pump Figure 2.15 Operation of reciprocating piston pump Figure 2.17 Air operated double diaphragm valve Figure 2.19 Progressing cavity pump. (Courtesy of NETZSCH) 4 Figure 2.18 Lobe pump. (Courtesy of Viking Pumps) Figure 2.21 Peristaltic pump. (Courtesy of the Dept of Biotechnology and Food Engineering, Technion, I.I.T.) Peristaltic pumps are extensively used in the laboratory, in medical applications and in some industrial operations requiring fluid dispensing at a low but precisely controllable flow rate. 5 2.4.3 Pump selection From the short and necessarily partial description above, it is obvious that the number of different kinds of pumps is very large ( Philby and Stewart, 1984 ; Nesbitt, 2006 ). Pumps are selected in the light of process requirements (volumetric capacity and its controllability, inlet and discharge pressure), process conditions (viscosity, temperature, corrosiveness and erosiveness of the fluid to be pumped), power requirement, cost and additional conditions dictated by the specific usage. For pumping food materials, compliance with sanitary requirements (ease of cleaning in-place, no stagnant regions, building materials suitable for contact with food, no risk of contamination with lubricants, no leakage through seals etc.) is essential. Generally, centrifugal pumps are used for high volume, low head applications with liquids of relatively low viscosity. Positive displacement pumps, on the contrary, are more frequently used in high pressure, low-capacity applications and can handle highly viscous liquids. As a rule, centrifugal pumps are simpler and less expensive. Pumps suitable for use in food processes are available in both categories ( Figures 2.22 to 2.24 ).  Excessive agitation, high viscosities, impact and shear may not be acceptable in the case of fluids with a ‘ structure ’ , materials prone to foaming and, of course, liquids carrying fragile solid particles. Special pumps are available for such applications. Among the most remarkable examples are the fish pumps capable of pumping fairly large live fish in water without causing damage to the animals. When high discharge pressure is required, the best choice is a reciprocating piston pump. This is the type of pump used, for example, in high pressure homogenizers for forcing fluids through the homogenizing head at pressures of 10 to 70 MPa. Variable (controllable) flow rate is often a process requirement. With centrifugal pumps, this is easily achieved by installing a valve, preferably after the discharge. Centrifugal pumps usually operate at constant speed, which is usually the speed of the motor directly coupled with the pump. Positive displacement pumps run at lower speed and their shaft is connected to the motor through a speed reducing gearbox. Variable flow rate is achieved by using a variable speed motor or a drive transmission with speed variability. A device for changing the length of piston travel is found in some reciprocating piston pumps. Figure 2.22 Sanitary centrifugal pump. Figure 2.23 Impellers and rotors for sanitary pumps. (Courtesy of Alva-Laval) (Courtesy of Alva-Laval) 8  A Sanitary pump is a type of pump which is easily cleanable and has smooth internal surfaces to prevent bacterial growth and therefore prevents contamination of the product. The most used material in Industry is Stainless Steel 316L. The sanitary pump versions work in the same way as Industrial pump versions. There is no difference in how they operate. A Sanitary pump is commonly used in the Food and Beverage Industries, Biopharma/Pharma for Active Ingredients & products, traditionally purified water and/or WFI (Water for Injection). Figure 2.24 Sanitary rotating lobe pump. (Courtesy of Alva-Laval) 9 2.4.5 Piping The pipelines of a food plant fall into two categories: Pipelines for the transportation of utility fluids such as cold and hot water (except water to be used in product formulation), steam, compressed air, liquid or gas fuels etc. Pipelines for the transportation of products, semi-products and raw materials used in product formulation, including water. The pipes and fittings of utility pipelines are usually made of steel. Their dimensions (inside and outside diameter, wall thickness) and modes of connection are specified by official standards and listed in detailed tables. Pipelines for steam and hot water are thermally insulated. For esthetic and sanitary reasons, the utility pipelines are laid-out following mutually perpendicular planes. Product pipelines consist of tubes and fittings made of stainless steel. The design diameters of the tubes are calculated according to the flow rate, pressure drop and fluid properties, but tubing with diameters below about 20 mm are seldom used for foods.  The connections between tubes and fittings may be dismantlable or permanent (welded). The former can be taken apart for cleaning and inspection and provide the flexibility required for changing layouts. A considerable number of different and mutually incompatible ‘systems’ of stainless-steel piping connections are available. This lack of standardization must be taken into consideration in designing a dismantlable product pipeline and in specifying the type of connections between the piping and equipment items. Permanent product pipelines are less expensive and particularly adequate for plants with a fixed layout, such as large dairy plants. With the development of reliable clean in place (CIP) systems, the use of welded stainless-steel pipelines is becoming more widespread. 2.5 Flow of Particulate Solids (Powder Flow) 2.5.2 Flow properties of particulate solids Powder flow is the displacement of powder particles in relation to each other, under the effect of some directional force (gravity, entrainment in a fluid stream, mechanical force of an auger, scraper, vibrator, agitator, mixer etc. and occasionally electrostatic forces). The resistance of the powder to such displacement is called powder strength. Powder strength is a consequence of inter-particle forces. The main types of adhesive forces between particles (or between particles and solid surfaces) are: 1. van der Waals forces 2. Liquid bridge forces 3. Electrostatic forces. In an absolutely dry powder and under conditions found in common food applications, van der Waals forces are the main cause of particle–particle adhesion. However, even in an apparently dry powder, a thin film of liquid may often exist on the surface of particles. In this case, liquid bridge forces become at least as important as van der Waals forces.  The van der Waals force (after Johannes Diderik van der Waals, 1837–1923, Dutch physicist, 1910 Nobel Prize in Physics) is a universal force of attraction between material bodies. It is highly dependent on the distance between the bodies and significant only when that distance is very small (say 100 nm). Its origin is electromagnetic. Liquid bridge forces ( Figure 2.27 ) are related to capillary forces. The analysis of powder flow in terms of the forces acting between individual particles is difficult. An alternative approach to powder flow considers the powder bulk as a continuous solid and applies to powder flow the concepts and methods of solid mechanics. At a ‘macro’ scale, inter-particle forces are treated as friction, since friction is the force that opposes movement of two touching bodies with respect to each other. 14 2.5.4 Pneumatic transport of particulate solids Particulate solids in bulk can be transported in a stream of gas (pneumatic conveying) or in a flowing liquid (hydraulic conveying). Both methods are used in the food industry. Grain, flour and other mill products, sugar etc. are often transported pneumatically. Hydraulic conveying is common in vegetable processing (peas, tomatoes). Pumping fish with sea water from submerged nets up to the fi shing boat may be considered hydraulic transport. The basic principles of the two processes are the same, but the physical setups and equipment used are obviously different. The advantages of pneumatic transport, over other types of conveying are: 1. The transport takes place in a closed system, protecting the product against contamination and protecting the environment against pollution 2. The product is conveyed in a pipeline that can be easily disassembled and reassembled and can even contain flexible sections as hoses. Pneumatic conveyors are particularly suitable for serving changing points of pick-up and delivery 3. Solids can be transported over long distances, through complicated pathways 4. The pneumatic conveyor is compact. Hydraulic conveyor for tomatoes 16  The disadvantages of pneumatic transport are: 1. Energy consumption per unit weight of solids transported is considerably higher than in mechanical conveyors 2. Due to the intimate contact with air, drying and oxidation of the product may occur. Systems for pneumatic conveying in pipes are classified into two categories: pressure systems and vacuum systems. In the pressure system, the air blower is installed upstream and blows air into the system. The pipeline is under positive pressure. In the vacuum system, the blower is installed downstream and sucks air through the system. The pipeline is under negative pressure. Pressure (or positive pressure) conveyors are suitable for systems with a single pick-up point and multiple delivery points (Figure 2.38). Pressure systems can be used with higher solid-to-gas ratio. Introducing the solid into a system at positive pressure requires special devices such as locks. Escape of dust through leakages in the system may be a problem. Vacuum systems are suitable for multiple pick-up–single delivery setups (Figure 2.39). Because of the limited pressure difference available for transport, vacuum systems cannot convey solids over long distances. 18 19  The pressure drop resulting from flow of a mixture of gas and solid particles cannot be calculated by the methods used for air alone. In addition to the usual variables (pipe diameter, velocity, gas viscosity, gas density), parameters of the solid (particle size and shape, solid-to-gas ratio and the gravity acceleration g) must be included in the dimensional analysis. In one of the methods of calculation described by McCabe and Smith (1956) , a ratio β is defined as follows: where (ΔP/L)s and (ΔP/L)g are the pressure drop per unit length for the suspension and for the gas alone, flowing at the same velocity. An empirical equation for the calculation of β is proposed, as follows where: D = diameter of the pipe r = mass ratio of solids to fluid a,K parameters of the equation, functions of the other variables of the equation and of the direction of flow (vertical or horizontal). The most important design variable in pneumatic transport is the velocity of the air. Insufficient air velocity may result in settling of particles in horizontal sections and in elbows, tees and sudden expansion. Usual air velocities are in the range of 20–30 m.s−1.