Chapter 1 Separation processes-an overview A S GRANDISON and M. J. LEWIS, Department of Food Science and Technology, The University of Reading, Whiteknights, PO Box 226, Reading, RG6 6AP 1.1 FOODS- THE RAW MATERIAL Food and drink play a vital role in all our lives, providing us with the nutrients essential for all our daily activities, including cell maintenance, growth and reproduction Although foods are commonplace and much taken for granted, their composition and tructure are by no means simple. Firstly, all foods are chemical in nature. For most foods the principal component is water and this water plays an important role in the overall behaviour of that food. One of the most important branches of separation is the removal of water, to save transportation costs and improve microbial stability The other components can be classified into major components, such as protein, fat or lipid, sugars, starch and fibre. The minor components include the minerals, which are known collectively as ash, vitamins and organic acids. Information on food composition and the amounts of major and minor components can be found in the Composition of Foods Tables(Paul and Southgate, 1978). Table 1. I illustrates just some of the compo- sition data that is available for a selection of food Food composition tables are useful in that they provide an average composition However, some of their limitations are illustrated below. taking milk as an example. It be noted that similar points could be made about most oth Milk is extremely complex in terms of its chemical composition, containing protein fat, carbohydrate, minerals and vitamins. There are many different proteins, which can be subdivided into the whey proteins, which are in true solution in the aqueous phase, and the caseins, which are in the colloidal form. The fat itself is a complex mixture of triglycerides and, being immiscible with water, is dispersed as small droplets, stabilised by a membrane, within the milk. The vitamins are classified as water or fat soluble, depending on which phase they most associate with. Some of the minerals, such as calcium and phosphorus, partition between the aqueous phase and the colloidal casein and play a major role in the stability of the colloidal dispersion. In addition, there are many other components present in trace amounts, which may affect its delicate flavour
Chapter 1 Separation processes - an overview A. S. GRANDISON and M. J. LEWIS, Department of Food Science and Technology, The University of Reading, Whiteknights, PO Box 226, Reading, RG6 6AP 1.1 FOODS - THE RAW MATERIAL Food and drink play a vital role in all our lives, providing us with the nutrients essential for all our daily activities, including cell maintenance, growth and reproduction. Although foods are commonplace and much taken for granted, their composition and structure are by no means simple. Firstly, all foods are chemical in nature. For most foods the principal component is water and this water plays an important role in the overall behaviotir of that food. One of the most important branches of separation is the removal of water, to save transportation costs and improve microbial stability. The other components can be classified into major components, such as protein, fat or lipid, sugars, starch and fibre. The minor components include the minerals, which are known collectively as ash, vitamins and organic acids. Information on food composition and the amounts of major and minor components can be found in the Composition of Foods Tables (Paul and Southgate, 1978). Table 1.1. illustrates just some of the composition data that is available, for a selection of foods. Food composition tables are useful in that they provide an average composition. However, some of their limitations are illtistrated below, taking milk as an example. It should be noted that similar points could be made about most other foods. Milk is extremely complex in terms of its chemical composition, containing protein, fat, carbohydrate, minerals and vitamins. There are many different proteins, which can be subdivided into the whey proteins, which are in true solution in the aqueous phase, and the caseins, which are in the colloidal form. The fat itself is a complex mixture of triglycerides and, being immiscible with water, is dispersed as small droplets, stabilised by a membrane, within the milk. The vitamins are classified as water or fat soluble, depending on which phase they most associate with. Some of the minerals, such as calcium and phosphorus, partition between the aqueous phase and the colloidal casein and play a major role in the stability of the colloidal dispersion. In addition, there are many other components present in trace amounts, which may affect its delicate flavour
2 A. S. Grandison and M.J. Lewis Table 1. 1 Composition of foods(weight/100 g) Milk Apple Peas Flour Beef water(g) 87.6(878)85.678.513.074.0 82.1 protein (g fat(g) 38(3.9) tr 1.2 gar(g 1.7 0.0 0.0 starch(g) 00(0.0 6.678.4 fibre(g) 0.0(0.0) 5.2 potassium(mg) sodium (mg 50(55) 6 iron(mg) 0.05(006)0.3 95(92 6 130 180 170 vitamin C(mg) vitamin BI(mg) 0.04(0.06)0.040.320.330.07008 vitamin B6(mg) 0.04(0.06)0.030.160.150.32 033 vitamin D(ug) 0.03(0.03) vitamin E (mg) 0.10(0.09)0.2tr 0.15 0.44 flour- household pla These values are taken from Paul and Southgate (1978). Figures in parentheses are for milk, taken from MeCance and widdowson's Composition of Foods Tables (Sth edn)(1991), Royal Society of Chemistry. MAFF. There are slight differences between the reported results and processing characteristics and nutritional value, such as trace minerals, organic acids and non-protein nitrogen compounds such as peptides, urea and amino acids. Walstra and Jenness(1984)have listed over 60 components present in milk, at levels that can be readily detected. Milk is also potentially a very unstable material. For example the pro- tein can be made to coagulate by a variety of methods, including heating, addition of the enzyme rennet, acid, salts and ethanol. Also the fat globules rise to the surface under the influence of gravity Superimposed on this complex composition is the fact that it is subject to wide variation, Milks from different species differ markedly, and many types of milk other than cows are consumed worldwide, e.g. sheep, goat, buffalo, camel. Within the same pecies there are large differences between breeds, and even between individual animals in the same herd. In addition to this, and of prime importance to the milk-processing industry, milk from the same animals is subject to wide seasonal variation, reflecting the change in the animals'diet throughout the year, and the stage of lactation. Factors relating to the handling of milk, such as the pH or the amount of dissolved oxygen, are also important to its stability Foods may also be contaminated with matter from their production environment, i.e soil, water and farmyard. For example milk may be contaminated with dirt, straw, anti biotics, growth hormones, heavy metals, or radionuclides
2 A. S. Grandison and M. J. Lewis Table 1.1. Composition of foods (weight/100 g) Milk Apple Peas Flour Beef Cod water (g) 87.6 (87.8) 85.6 78.5 13.0 74.0 82.1 protein (g) 3.3 ( 3.2) 0.3 5.8 9.8 20.3 17.4 fat (€9 3.8 ( 3.9) tr. 0.4 1.2 4.6 0.7 starch (g) 0.0 ( 0.0) 0.4 6.6 78.4 - sugar (g) 4.7 ( 4.8) 9.2 4.0 1.7 0.0 0.0 fibre (8) 0.0 ( 0.0) 2.4 5.2 3.4 - sodium (mg) 50( 55) 2 1 2 61 77 - potassium (mg) 150(140) 120 340 140 350 320 calcium (mg) 120 (1 15) 4 15 150 7 16 iron (mg) 0.05 (0.06) 0.3 1.9 2.2 2.1 0.3 phosphorus (mg) 95 ( 92) 16 100 130 180 170 vitamin C (mg) 1.50 (1.0) 15 25 vitamin B 1 (mg) 0.04 (0.06) 0.04 0.32 0.33 0.07 0.08 vitamin B6 (mg) 0.04 (0.06) 0.03 0.16 0.15 0.32 0.33 - - - vitamin D (ug) 0.03 (0.03) - - - tr tr vitamin E (mg) 0.10 (0.09) 0.2 tr tr 0.15 0.44 * flour - household plain tr - trace These values are taken from Paul and Southgate (1978). Figures in parentheses are for milk, taken from McCance and Widdowson’s Coniposiiion of Foods Tables (5th edn) (1991), Royal Society of Chemistry, MAFF. There are slight differences between the reported results. and processing characteristics and nutritional value, such as trace minerals, organic acids and non-protein nitrogen compounds such as peptides, urea and amino acids. Walstra and Jenness (1984) have listed over 60 components present in milk, at levels that can be readily detected. Milk is also potentially a very unstable material. For example the protein can be made to coagulate by a variety of methods, including heating, addition of the enzyme rennet, acid, salts and ethanol. Also the fat globules rise to the surface under the influence of gravity. Superimposed on this complex composition is the fact that it is subject to wide variation. Milks from different species differ markedly, and many types of milk other than cow’s are consumed worldwide, e.g. sheep, goat, buffalo, camel. Within the same species there are large differences between breeds, and even between individual animals in the same herd. In addition to this, and of prime importance to the milk-processing industry, milk from the same animals is subject to wide seasonal variation, reflecting the change in the animals’ diet throughout the year, and the stage of lactation. Factors relating to the handling of milk, such as the pH or the amount of dissolved oxygen, are also important to its stability. Foods may also be contaminated with matter from their production environment, i.e. soil, water and farmyard. For example milk may be contaminated with dirt, straw, antibiotics, growth hormones, heavy metals, or radionuclides
In chemical terms alone, there is a great deal of scope for separating the components in milk and some examples are listed ater removal to produce evaporated or dried products fat separation to produce creams and butter; protein separation to produce cheese or protein concentrates calcium removal to improve stability lactose removal, as a specialised ingredient or for low-lactose products removal of components responsible for tainting raw milk or the cooked flavour of heat-treated milk prodi removal of radionuclides from milk In plant products pesticides and herbicides may additionally be present. Some foods, particularly of plant origin, also contain natural toxins, for example oxalic acid in rhubarb, and trypsin inhibitors, phytates and haemagglutinins in many legumes cyanogenic glycosides in cassava and glucosinolates in rapeseed(Watson, 1987; Jones. 1992). However, the activity of most of these is reduced during normal processing and king metho Foods also contain active enzyme systems. For example, raw milk contains phosphatase, lipases and proteases, xanthine oxidase and many others. Fruits and vegetables contain polyphenol oxidases and peroxidases, both of which cause colour changes in foods, particularly browning, and lipoxygenases, which produce rancid off lavours(Nagodawithana and Reed, 1993) Therefore foods and wastes produced during food processing provide the raw material for extraction of enzymes and other important biochemicals with a range of applications especially in the food and pharmaceuticals industries. Some examples are listed in Table 1. 2. In the biotechnology industry, similar components may be produced by fermentation or enzymatic reactions and require extraction and purification. Perhaps the simplest example is alcohol, produced by a yeast fermentation, where the alcohol concentration that can be produced is limited to about 15 to 20%0, as it inhibits further yeast metabolism Alcohol can be recovered and concentrated by distillation. For low-alcohol or alcohol free beers and wines, there is a requirement to remove alcohol, Again distillation or membrane techniques can be used a wide range of food additives and medical compounds are produced by fermentation these include many enzymes, such as proteases for milk clotting or detergent cleaners, amino acids such as glutamic acid for monosodium glutamate(msG) production, aspartic acid and phenylalanine for aspartame, and lysine for nutritional supplements, organic acids such as citric, gluconic and lactic, and hydrocolloids, such as xanthan gum for stabilising or thickening foods, and a wide range of antibiotics and other medicinal compounds In most cases it is necessary to purify these materials from dilute raw materials, which often requires sophisticated separation techniques. In fact a large proportion of the activities of the biotechnology industry is concerned with separations of this nature, which is known as downstream processing. In general, the products produced by bio- processing applications are more valuable than food products, and it is economicall feasible to apply more complex separation techniques
Separation processes - an overview 3 In chemical terms alone, there is a great deal of scope for separating the components in milk and some examples are listed: water removal to produce evaporated or dried products; fat separation to produce creams and butter; protein separation to produce cheese or protein concentrates; calcium removal to improve stability; lactose removal, as a specialised ingredient or for low-lactose products; removal of components responsible for tainting raw milk or the cooked flavour of heat-treated milk products; removal of radionuclides from milk. In plant products pesticides and herbicides may additionally be present. Some foods, particularly of plant origin, also contain natural toxins, for example oxalic acid in rhubarb, and trypsin inhibitors, phytates and haemagglutinins in many legumes, cyanogenic glycosides in cassava and glucosinolates in rapeseed (Watson, 1987; Jones, 1992). However, the activity of most of these is reduced during normal processing and cooking methods. Foods also contain active enzyme systems. For example, raw milk contains phosphatase, lipases and proteases, xanthine oxidase and many others. Fruits and vegetables contain polyphenol oxidases and peroxidases, both of which cause colour changes in foods, particularly browning, and lipoxygenases, which produce rancid offflavours (Nagodawithana and Reed, 1993). Therefore foods and wastes produced during food processing provide the raw material for extraction of enzymes and other important biochemicals with a range of applications, especially in the food and pharmaceuticals industries. Some examples are listed in Table 1.2. In the biotechnology industry, similar components may be produced by fermentation or enzymatic reactions and require extraction and purification. Perhaps the simplest example is alcohol, produced by a yeast fermentation, where the alcohol concentration that can be produced is limited to about 15 to 20%, as it inhibits further yeast metabolism. Alcohol can be recovered and concentrated by distillation. For low-alcohol or alcoholfree beers and wines, there is a requirement to remove alcohol. Again distillation or membrane techniques can be used. A wide range of food additives and medical compounds are produced by fermentation; these include many enzymes, such as proteases for milk clotting or detergent cleaners, amino acids such as glutamic acid for monosodium glutamate (MSG) production, aspartic acid and phenylalanine for aspartame, and lysine for nutritional supplements, organic acids such as citric, gluconic and lactic, and hydrocolloids, such as xanthan gum for stabilising or thickening foods, and a wide range of antibiotics and other medicinal compounds. In most cases it is necessary to purify these materials from dilute raw materials, which often requires sophisticated separation techniques. In fact a large proportion of the activities of the biotechnology industry is concerned with separations of this nature, which is known as downstream processing. In general, the products produced by bioprocessing applications are more valuable than food products, and it is economically feasible to apply more complex separation techniques
4 A.S. Grandison and M.J. Lewis Table 1. 2. Biochemicals extracted from foods and by products Produc Application Papaya Papain Meat tenderization Beer haze removal Calf stomach Re Barley Glucose syrup production Pa Control of diabetes Connective tissue Gelatin Gelling agent pr Emulsifier Horseradish peroxidase E Ovotransferrin Antibacterial Most foods also come contaminated with microorganisms, derived from the environ- ment where they are produced, such as soil, water or the farmyard. These will cause food to spoil or decay, or in the case of pathogenic organisms, cause food poisoning, either directly or by producing toxins. Their activity needs to be controlled. Foods can be pasteurised, blanched, sterilised, and irradiated to control such activity. For liquid products microorganisms can also be removed by membrane sterilisation techniques However, it is not only the chemical nature of the food that is important; the organisation and structure of components, and hence the physical properties, are vit considerations to the application of separation techniques. For example, the composition of apples as shown in Table 1. 1 appears to be relatively simple. However, to fabricate (create)an apple in the laboratory from these components would be technicall impossible. Large differences occur between apples in terms of their colour, flavour and texture which are not apparent from composition tables. Similar considerations apply to many other raw materials. Unfortunately for the food processor, nature does not provide materials of uniform chemical or physical properties. Foods have important physical properties, which will influence the separation technique that is to be selected; some of ese are listed in Table 1. 3. In addition, the structure of both raw materials and processed foods is very varied. They may exist as emulsions or colloids. They may be non homogeneous on a macroscopic or microscopic scale, possessing fibrous structure and cellular structure or layered structures such as areas of fat in meat Foods are found as solids or liquids, but gas is frequently incorporated. This may be desirable, as in processed foods such as ice cream, bread or carbonated drinks. However, it may be desirable ove dissolved gases from liquids such as oxygen or cellular gases from fruit and vegetables before certain processing operations This brief introduction has aimed to illustrate the diverse nature of foods and related biological materials, and give an insight into their composition and structure. It is this
4 A. S. Grandison and M. J. Lewis Table 1.2. Biochemicals extracted from foods and by products Source Product Application Papaya Papain Meat tenderisation Beer haze removal Calf stomach Rennet Cheesemaking Barley Amylase Glucose syrup production Pancreas Insulin Control of diabetes Connective tissue Gelatin Gelling agent Egg Lysozyme Food preservative Soybean Lecithin Emulsifier Horseradish Peroxidase Diagnostics Milk Lactoperoxidase Antibacterial Egg Ovotransferrin Antibacterial Baking Most foods also come contaminated with microorganisms, derived from the environment where they are produced, such as soil, water or the farmyard. These will cause food to spoil or decay, or in the case of pathogenic organisms, cause food poisoning, either directly or by producing toxins. Their activity needs to be controlled. Foods can be pasteurised, blanched, sterilised, and irradiated to control such activity. For liquid products microorganisms can also be removed by membrane sterilisation techniques. However, it is not only the chemical nature of the food that is important; the organisation and structure of components, and hence the physical properties, are vital considerations to the application of separation techniques. For example, the composition of apples as shown in Table 1.1 appears to be relatively simple. However, to fabricate (create) an apple in the laboratory from these components would be technically impossible. Large differences occur between apples in terms of their colour, flavour and texture which are not apparent from composition tables. Similar considerations apply to many other raw materials. Unfortunately for the food processor, nature does not provide materials of uniform chemical or physical properties. Foods have important physical properties, which will influence the separation technique that is to be selected; some of these are listed in Table 1.3. In addition, the structure of both raw materials and processed foods is very varied. They may exist as emulsions or colloids. They may be nonhomogeneous on a macroscopic or microscopic scale, possessing fibrous structure and cellular structure, or layered structures such as areas of fat in meat. Foods are found as solids or liquids, but gas is frequently incorporated. This may be desirable, as in processed foods such as ice cream, bread or carbonated drinks. However, it may be desirable to remove dissolved gases from liquids such as oxygen or cellular gases from fruit and vegetables before certain processing operations. This brief introduction has aimed to illustrate the diverse nature of foods and related biological materials, and give an insight into their composition and structure. It is this
Separation processes-an overview 5 complexity and diversity which provides the scope and potential for separating selected Table 1.3. Examples of physical properties of foods, and separation processes to which they relate Physical property Separation technique Size, size distribution, shape Screening, air classification Density Centrifugation Liquid extraction processes rheologica Surface properties Froth flotation Thermal properties Evaporation, Electrical Electrostatic sorting Membrane separations Solubility Solvent extraction Thermal denaturation Optical Reflectance(colour)sorting 1.2 SEPARATION TECHNIQ 1. 2.1 Introduction Separation of one or more components from a complex mixture is a requirement for many operations in the food and biotechnology industries. The components in question range from particulate materials down to small molecules, The separations usually aim to achieve removal of specific components, in order to increase the added value of the products, which may be the residue, the extracted components or both. All separations rely on exploiting differences in physical or chemical properties of the mixture of compo- nents. Some of the more common properties involved in separation processes are partick or molecular size and shape, density, solubility and electrostatic charge. These properties re discussed in more detail elsewhere(Mohsenin, 1980, 1984: Lewis, 1990). In some operations, more than one of these properties are involved. However, most of the processes involved are of a physical nature Separation from solids or liquids involves the transfer of selected components across the boundary of the food. In many processes another stream or phase is involved, for example in extraction processes. However, this is not always so, for example expression, centrifugation or filtration. In expression, fruit juice or oil is squeezed from the food by application of pressure. In centrifugation, fat can be separated from water due to thei density differences, by the application. of a centrifugal force, In filtration there is a physical barrier to the transfer of certain components and the liquid is forced through the barrier by pressure, whilst the solids are retained. The resistance to flow will change throughout the filtration process, due to solids build-up. It can be seen that main driving
Separation processes - an overview 5 complexity and diversity which provides the scope and potential for separating selected components from foods. Table 1.3. Examples of physical properties of foods, and separation processes to which they relate Physical property Separation technique Size, size distribution, shape Screening, air classification Density Centrifugation Viscosity Liquid extraction processes Rheological Expression Surface properties Froth flotation Thermal properties Evaporation, drying Electrical Electrostatic sorting Diffusional Extraction Solubility Solvent extraction Optical Reflectance (colour) sorting Membrane separations Thermal denaturation 1.2 SEPARATION TECHNIQUES 1.2.1 Introduction Separation of one or more components from a complex mixture is a requirement for many operations in the food and biotechnology industries. The components in question range from particulate materials down to small molecules. The separations usually aim to achieve removal of specific components, in order to increase the added value of the products, which may be the residue, the extracted components or both. All separations rely on exploiting differences in physical or chemical properties of the mixture of components. Some of the more common properties involved in separation processes are particle or molecular size and shape, density, solubility and electrostatic charge. These properties are discussed in more detail elsewhere (Mohsenin, 1980, 1984; Lewis, 1990). In some operations, more than one of these properties are involved. However, most of the processes involved are of a physical nature. Separation from solids or liquids involves the transfer of selected components across the boundary of the food. In many processes another stream or phase is involved, for example in extraction processes. However, this is not always so, for example expression, centrifugation or filtration. In expression, fruit juice or oil is squeezed from the food by application of pressure. In centrifugation, fat can be separated from water due to their density differences, by the application of a centrifugal force. In filtration there is a physical barrier to the transfer of certain components and the liquid is forced through the barrier by pressure, whilst the solids are retained. The resistance to flow will change throughout the filtration process, due to solids build-up. It can be seen that main driving
6 A.S. Grandison and M. J. Lewis forces in these applications are pressure and density differences. As for all processes separation rates are very important and these are affected by the size of the driving forces involved In situations where a second phase or stream is involved, mass-transfer considerations become important; these involve the transfer of components within the food to the oundary, the transfer across the boundary and into the bulk of the extraction solvent. It is also important to increase the interfacial area exposed to the solvent. Therefore, size eduction, interfacial phenomena, turbulence and diffusivities all play a role in these processes. In many applications this additional stream is a liquid, either water or an organic solvent; more recently supercritical fluids, such as carbon dioxide, have been investigated(see Chapter 2). However, in hot-air drying the other phase is hot air, which supplies the energy and removes the water. Mass-transfer considerations are important also in some membrane applications and adsorption processes, where the additional ream is a solid. Other examples of driving force are concentration differences and chemical potential, which are involved in these operations (Loncin and Merson, 1979; Gekas, 1992) In some processes, both heat and mass transfer processes are involved. This is especially so for separation reactions involving a change of phase, such as evaporation or sublimation. Heat is required to cause vaporisation for evaporation, dehydration and distillation processes. Water has a much higher latent heat of vaporisation(2257 kJ/kg) han most other organic solvents. With solid foods the rate of heat transfer through the food may limit the overall process; for example in freeze-drying the process is usually limited by rate of heat transfer through the dry layer. Separation processes may be batch or continuous. A single separation process, for example a batch extraction, involves the contact of the solvent with the food. Initially concentration gradients are high and the rate of extraction is also high. The extraction rate falls exponentially and eventually an equilibrium state is achieved when the rate becomes zero. The extraction process may be accelerated by size reduction, inducing turbulence nd increasing the extraction temperature, Equilibrium is achieved either when all the material has been extracted. in situations where the volume of solvent is well in excess of the solute or when the solvent becomes saturated with the solute, i.e. when the solubility limit has been achieved when there is an excess of solute over the solvent however the attainment of equilibrium may take some considerable time. Batch reactions may operate far away from equilibrium or close to it. Equilibrium data is important in that it provides information on the best conditions that can be achieved at the prevailing conditions. Equilibrium data is usually determined at fixed conditions of temperature and pressure. Some important types of equilibrium data are: solubility data for extraction processe apour/liquid equilibrium data for fractional distillation partition data for selective extraction processes rater sorption data for drying Continuous processes may be single-or multiple-stage processes, The stages them- elves may be discrete entities, for example a series of stirred tank reactors, or there may
6 forces in these applications are pressure and density differences. As for all processes, separation rates are very important and these are affected by the size of the driving forces involved. In situations where a second phase or stream is involved, mass-transfer considerations become important; these involve the transfer of components within the food to the boundary, the transfer across the boundary and into the bulk of the extraction solvent. It is also important to increase the interfacial area exposed to the solvent. Therefore, size reduction, interfacial phenomena, txbulence and diffusivities all play a role in these processes. In many applications this additional stream is a liquid, either water or an organic solvent; more recently supercritical fluids, such as carbon dioxide, have been investigated (see Chapter 2). However, in hot-air drying the other phase is hot air, which supplies the energy and removes the water. Mass-transfer considerations are important also in some membrane applications and adsorption processes, where the additional stream is a solid. Other examples of driving force are concentration differences and chemical potential, which are involved in these operations (Loncin and Merson, 1979; Gekas, 1992). In some processes, both heat and mass transfer processes are involved. This is especially so for separation reactions involving a change of phase, such as evaporation or sublimation. Heat is required to cause vaporisation for evaporation, dehydration and distillation processes. Water has a much higher latent heat of vaporisation (2257 kJ/kg) than most other organic solvents. With solid foods the rate of heat transfer through the food may limit the overall process; for example in freeze-drying the process is usually limited by rate of heat transfer through the dry layer. Separation processes may be batch or continuous. A single separation process, for example a batch extraction, involves the contact of the solvent with the food. Initially concentration gradients are high and the rate of extraction is also high. The extraction rate falls exponentially and eventually an equilibrium state is achieved when the rate becomes zero. The extraction process may be accelerated by size reduction, inducing turbulence and increasing the extraction temperature. Equilibrium is achieved either when all the material has been extracted, in situations where the volume of solvent is well in excess of the solute or when the solvent becomes saturated with the solute, i.e. when the solubility limit has been achieved, when there is an excess of solute over the solvent. However, the attainment of equilibrium may take some considerable time. Batch reactions may operate far away from equilibrium or close to it. Equilibrium data is important in that it provides information on the best conditions that can be achieved at the prevailing conditions. Equilibrium data is usually determined at fixed conditions of temperature and pressure. Some important types of equilibrium data are: solubility data for extraction processes; vapour/liquid equilibrium data for fractional distillation; partition data for selective extraction processes; water sorption data for drying. Continuous processes may be single- or multiple-stage processes. The stages themselves may be discrete entities, for example a series of stirred tank reactors, or there may A. S. Grandison and M. J. Lewis
Separation processes-an overview 7 many stages built into one unit of equipment, for example a distillation column or a screw extractor. The flow of the two streams can either be co-current or counter-current although counter-current is normally favoured as it tends to give a more uniform driving force over the length of the reactor as well as a higher average driving force over the reactor. In some instances a combination of co-current and counter-current may be used for example in hot air drying the initial process is co-current to take advantage of the high initial driving rates, whereas the final drying is counter-current to permit drying to a lower moisture content Continuous equipment usually operates under steady state conditions, i.e. the driving force changes over the length of the equipment, but at any particular location it remains constant with time. However, when the equipment is first started, it may take some time achieve steady-state. During this transition period it is said to be operating under unsteady state conditions. In continuous processes the flow may be either streamline or turbulent Consideration should be taken of residence times and distribution of residence mes within the separation process; the two extremes of behaviour are plug flow, with no istribution of residence times, through to a well-mixed situation, with an infinite distribution of residence times. More detailed analysis of residence time distributions is by Levenspiel (1972) How close the process operates to equilibrium depends upon the operating conditions, flow rates of the two phases, time and temperature. These conditions affect the efficiency of the process, such as the recovery and the size of equipment required Finally, all rates of reaction are temperature dependent. Physical processes are no exception, although activation energies are usually much lower than for chemical reaction rates. Using higher temperatures normally increases separation rates However, there are limitations with biological materials: higher temperatures increase degradation reactions, causing colour and flavour changes, enzyme inactivation, protein denaturation, loss of functionality and a reduction in nutritional value. Safety issues with respect to microbial growth may also need to be considered A brief overview of separation methods is now considered in this chapter, base primarily on the nature of the material or stream subjected to the separation process, i.e whether it is solid, liquid or gaseous. Other possible classifications are based on unit operations; e. g. filtration, evaporation, dehydration etc. or types of phase contact, such as lid-liquid or gas-liquid ng pre fore detailed descriptions of conventional techniques can be found elsewhere -e.g Brennan et al. (1990), Perry and Green(1984), King(1982) 1.2.2 Separations from solids Most solid foods are particulate in nature, with particles having a large variety of shapes and sizes. Separations involving solids fall into two categories. The first is where it is required to separate or segregate the particles; such processes are classified as solid-solid separations. The second is where the requirement may be to selectively remove one or veral components from the solid matrix. Other processes involving solids are concerned with the removal of discrete solid particles from either liquids or gases and vapours(but these will be discussed in other sections)
Separation processes -an overview 7 be many stages built into one unit of equipment, for example a distillation column or a screw extractor. The flow of the two streams can either be co-current or counter-current, although counter-current is normally favoured as it tends to give a more uniform driving force over the length of the reactor as well as a higher average driving force over the reactor. In some instances a combination of co-current and counter-current may be used; for example in hot air drying, the initial process is co-current to take advantage of the high initial driving rates, whereas the final drying is counter-current to permit drying to a lower moisture content. Continuous equipment usually operates under steady state conditions, i.e. the driving force changes over the length of the equipment, but at any particular location it remains constant with time. However, when the equipment is first started, it may take some time to achieve steady-state. During this transition period it is said to be operating under unsteady state conditions. In continuous processes the flow may be either streamline or turbulent. Consideration should be taken of residence times and distribution of residence times within the separation process; the two extremes of behaviour are plug flow, with no distribution of residence times, through to a well-mixed situation, with an infinite distribution of residence times. More detailed analysis of residence time distributions is provided by Levenspiel (1972). How close the process operates to equilibrium depends upon the operating conditions, flow rates of the two phases, time and temperature. These conditions affect the efficiency of the process, such as the recovery and the size of equipment required. Finally, all rates of reaction are temperature dependent. Physical processes are no exception, although activation energies are usually much lower than for chemical reaction rates. Using higher temperatures normally increases separation rates. However, there are limitations with biological materials: higher temperatures increase degradation reactions, causing colour and flavour changes, enzyme inactivation, protein denaturation, loss of functionality and a reduction in nutritional value. Safety issues with , respect to microbial growth may also need to be considered. A brief overview of separation methods is now considered in this chapter, based primarily on the nature of the material or stream subjected to the separation process, i.e. whether it is solid, liquid or gaseous. Other possible classifications are based on unit operations; e.g. filtration, evaporation, dehydration etc. or types of phase contact, such as solid-Iiquid or gas-liquid contacting processes. More detailed descriptions of conventiopal techniques can be found elsewhere -e.g. Brennan et at. (1990), Perry and Green (1984), King (1982). 1.2.2 Separations from solids Most solid foods are particulate in nature, with particles having a large variety of shapes and sizes. Separations involving solids fall into two categories. The first is where it is required to separate or segregate the particles; such processes are classified as solid-solid separations. The second is where the requirement may be to selectively remove one or several components from the solid matrix. Other processes involving solids are concerned with the removal of discrete solid particles from either liquids or gases and vapours (but these will be discussed in other sections)
8 A, S. Grandison and M. J. Lewis Solid-solid separations Separations can be achieved on the basis of particle size from the sorting of large food units down to the molecular level. Shape, and other factors such as electrostatic charge or degree of hydration, may also affect these separations. Screening of materials through perforated beds(e. g. wire mesh or silk screens) produces materials of more uniform particle size. Screening contributes to sorting and grading of many foods, in particular fruits, vegetables and cereals. Cleaning of particulate materials or powders in the dry state can be achieved using screens in two ways. Dedusting is the removal of undersize contaminants from larger particles, e. g. beans or cereals. Scalping is the removal of oversize contaminants from powders or small particulate materials, e.g. sugar, flour. A wide range of geometric designs exists, including flat bed and rotary fixed aperture screens, and numerous variable aperture designs are available(Slade, 1967; Brennan et Differences in aerodynamic properties can be exploited in the cleaning, sorting and grading of particulate food raw materials(e. g. cereals, peas, nuts, flour)in the dry state Many designs of aspirator have evolved in which the feed is applied to controlled velocity air streams where separation into two or more fractions is effected. Alternatively differences in hydrodynamic properties can be used in the sorting of foods such as apples A combination of particle size and density may be used to separate solids by settle ment. If the solids are suspended in a fluid (liquid or gas), separation may be achieved on the basis that larger, more dense particles will settle more rapidly than smaller, less dense ones. This may be aided by the application of centrifugal force in air classification, as discussed in Chapter 9. Differences in buoyancy between solid particles is the basis of flotation washing of some foods. For example, heavy debris, such as stones or bruised and rotten fruit, may be removed from sound fruit by fluming the dirty produce over a series of weirs Froth flotation depends on the differential wetting of particles. In the case of separat- ing peas from weed seeds, the mixture is immersed in a dilute mineral oil emulsion through which air is blown. The contaminating seeds float on the foam and may be skimmed off. On a smaller scale the method can be used to separate materials which react selectively with a surfactant, such as heavy metals, from a mixture. Surface active agents, such as proteins and other foam- inducing materials, can be separated in a similar anner. These techniques are commonly used in effluent treatment Operations where the outer surface of the food is removed also fall into this category Examples include dehulling of cereals and legumes and peeling of fruit and vegetables (see Chapter 9). Cereals may be cleaned and sorted on the basis of shape to remove contaminants of similar size. Examples of this are disc and cylinder sorters which employ indentations of particular shape to pick up the corresponding food particles A range of equipment is also available to separate food units on the basis of photo- metric, magnetic and electrostatic properties. Red and green tomatoes, or blackened beans or nuts may be separated automatically on the basis of reflectance properties Magnetic cleaning is used to remove ferrous metal particles from foods, and thus to protect both the consumer and processing equipment. Electrostatic properties may be
8 Solid-solid separations Separations can be achieved on the basis of particle size from the sorting of large food units down to the molecular level. Shape, and other factors such as electrostatic charge or degree of hydration, may also affect these separations. Screening of materials through perforated beds (e.g. wire mesh or silk screens) produces materials of more uniform particle size. Screening contributes to sorting and grading of many foods, in particular fruits, vegetables and cereals. Cleaning of particulate materials or powders in the dry state can be achieved using screens in two ways. Dedusting is the removal of undersize contaminants from larger particles, e.g. beans or cereals. Scalping is the removal of oversize contaminants from powders or small particulate materials, e.g. sugar, flour. A wide range of geometric designs exists, including flat bed and rotary fixed aperture screens, and numerous variable aperture designs are available (Slade, 1967; Brennan et al., 1990). Differences in aerodynamic properties can be exploited in the cleaning, sorting and grading of particulate food raw materials (e.g. cereals, peas, nuts, flour) in the dry state. Many designs of aspirator have evolved in which the feed is applied to controlled velocity air streams where separation into two or more fractions is effected. Alternatively, differences in hydrodynamic properties can be used in the sorting of foods such as apples or peas. A combination of particle size and density may be used to separate solids by settlement. If the solids are suspended in a fluid (liquid or gas), separation may be achieved on the basis that larger, more dense particles will settle more rapidly than smaller, less dense ones. This may be aided by the application of centrifugal force in air classification, as discussed in Chapter 9. Differences in buoyancy between solid particles is the basis of flotation washing of some foods. For example, heavy debris, such as stones or bruised and rotten fruit, may be removed from sound fruit by fluming the dirty produce over a series of weirs. Froth flotation depends on the differential wetting of particles. In the case of separating peas from weed seeds, the mixture is immersed in a dilute mineral oil emulsion through which air is blown. The contaminating seeds float on the foam and may be skimmed off. On a smaller scale, the method can be used to separate materials which react selectively with a surfactant, such as heavy metals, from a mixture. Surface active agents, such as proteins and other foam-inducing materials, can be separated in a similar manner. These techniques are commonly used in effluent treatment. Operations where the outer surface of the food is removed also fall into this category. Examples include dehulling of cereals and legumes and peeling of fruit and vegetables (see Chapter 9). Cereals may be cleaned and sorted on the basis of shape to remove contaminants of similar size. Examples of this are disc and cylinder sorters which employ indentations of particular shape to pick up the corresponding food particles. A range of equipment is also available to separate food units on the basis of photometric, magnetic and electrostatic properties. Red and green tomatoes, or blackened beans or nuts may be separated automatically on the basis of reflectance properties. Magnetic cleaning is used to remove ferrous metal particles from foods, and thus to protect both the consumer and processing equipment. Electrostatic properties may be A. S. Grandison and M. J. Lewis
exploited in separating seeds which may be of similar size and shape, or in the cleaning More detailed information on solid-solid separations is provided in Chapter 9, Separation from the solid matrix Many plant materials contain valuable liquid components such as oils or juices in the cellular structure. These may be separated from the pulped raw material by the use of presses, in a process known as expression. Batch type hydraulic systems or continuous roller, screw or belt systems are available for different applications such as fruit juice, wine and cane sugar production, or extraction of oil from seeds. Expression of fruit juices may be aided by the use of enzymes to improve efficiency of expression and to control the pectin level. Some of the physical properties related to expression processes are discussed by Schwartzberg(1983) An alternative system to recover components from within a solid matrix is extraction which relies on the use of differential solubilities for extraction of soluble solids such as sugar from sugar beet, coffee from roasted ground beans, juices from fruit and vegetables and from materials during the manufacture of instant tea. The most common extraction material is hot or superheated water. However, organic solvents are used, e. g. hexane for oil extraction and methylene chloride to extract caffeine from tea and coffee. The use of supercritical fluids such as carbon dioxide is covered in detail in Chapter 2. Extraction processes as equilibrium stage processes are covered in more detail by Brennan et al 1990), Loncin and Merson(1979), Perry and green(1984). Many oil extraction processes employ expression, followed by solvent extraction,to obtain a high recovery of oil. The crude oil is then subjected to a series of refining processes, involving degumming, decolorisation and deodorisation to remove undesirable Water, the most common component of most foods, can be removed from sol the process of dehydration; in this case thermal energy is required to effect evaporation of the water, and this is usually supplied by hot air. Hot air drying is classified as liquid phase drying and results in shrinkage and case-hardening and loss of some volatiles of foods, Types of drier include overdraught, throughdraught, fluidised bed and pneumatic driers. These are described in more detail by Brennan et al.(1990), Mujumdar(1987) Freeze-drying, whereby the food is frozen and then subjected to a vacuum, provides a method which reduces shrinkage, case-hardening and flavour loss. Sublimation occurs during freeze-drying. Here conditions are controlled such that water is removed directly from its solid phase to its vapour phase, without passing through the liquid state. To achieve this, the water vapour pressure must be kept below the triple point pressure (4.6 mm Hg)(Mellor, 1978; Dalgleish, 1990) The removal of air from fruit and vegetables, prior to heat treatment in sealed to prevent ex 如mm ng. This is accomplished by blanching, using steam mised using steam (Selman
Separation processes - an overview 9 exploited in separating seeds which may be of similar size and shape, or in the cleaning of tea. More detailed information on solid-solid separations is provided in Chapter 9. Separation from the solid matrix Many plant materials contain valuable liquid components such as oils or juices in the cellular structure. These may be separated from the pulped raw material by the use of presses, in a process known as expression. Batch type hydraulic systems or continuous roller, screw or belt systems are available for different applications such as fruit juice, wine and cane sugar production, or extraction of oil from seeds. Expression of fruit juices may be aided by the use of enzymes to improve efficiency of expression and to control the pectin level. Some of the physical properties related to expression processes are discussed by Schwartzberg (1983). An alternative system to recover components from within a solid matrix is extraction, which relies on the use of differential solubilities for extraction of soluble solids such as sugar from sugar beet, coffee from roasted ground beans, juices from fruit and vegetables and from materials during the manufacture of instant tea. The most common extraction material is hot or superheated water. However, organic solvents are used, e.g. hexane for oil extraction and methylene chloride to extract caffeine from tea and coffee. The use of supercritical fluids such as carbon dioxide is covered in detail in Chapter 2. Extraction processes as equilibrium stage processes are covered in more detail by Brennan et al. (1990), Loncin and Merson (1979), Perry and Green (1984). Many oil extraction processes employ expression, followed by solvent extraction, to obtain a high recovery of oil. The crude oil is then subjected to a series of refining processes, involving degumming, decolorisation and deodorisation to remove undesirable components. Water, the most common component of most foods, can be removed from solids by the process of dehydration; in this case thermal energy is required to effect evaporation of the water, and this is usually supplied by hot air. Hot air drying is classified as liquid phase drying and results in shrinkage and case-hardening and loss of some volatiles of foods. Types of drier include overdraught, throughdraught, fluidised bed and pneumatic driers. These are described in more detail by Brennan et al. (1990), Mujumdar (1987). Freeze-drying, whereby the food is frozen and then subjected to a vacuum, provides a method which reduces shrinkage, case-hardening and flavour loss. Sublimation occurs during freeze-drying. Here conditions are controlled such that water is removed directly from its solid phase to its vapour phase, without passing through the liquid state. To achieve this, the water vapour pressure must be kept below the triple point pressure (4.6 mm Hg) (Mellor, 1978; Dalgleish, 1990). The removal of air from fruit and vegetables, prior to heat treatment in sealed containers, is of paramount importance to prevent excessive strain on the seams during the sterilisation and subsequent cooling. This is accomplished by blanching, using steam or hot water. Nutrient losses due to leaching are minimised using steam (Selman, 1987)
10 A S. Grandison and M. j. Lewis 1.2.3 Separations from liquid This section will cover those situations where the separation takes place from a fluid, i.e a substance which flows when it is subject to a shear stress. An important physical property is the viscosity of the fluid, which is defined as the ratio of the shear stress to shear rate. Viscosity and its measurement is discussed in more detail by Lewis (1990 e Solid components may be present dissolved in the liquid, in a colloidal dispersion or in suspension. For example, milk contains lactose, minerals and whey proteins in true solution, casein and calcium phosphate as a colloidal dispersion and fat globules dispersed in the aqueous phase. There may also be sediment resulting from other contaminants of the milk. The objective of the separation may be to remove any of these Liquid-solid separations Liquid-solid separation applies to operations where discrete solids are removed from the liquid. There are a number of ways of achieving this and these will be briefly Conventional filtration systems separate suspended particles of solids from liquids on the basis of particle size. The liquid component is passed through a porous membrane or ptum which retains the solid material either as a filter cake on the upstream surface, or Filter media may be rigid, such as wire mesh or porous ceramics, or flexible, such as woven textiles, and are available in a variety of geometric shapes and pore sizes. In practice, the flow of filtrate may be brought about by gravity the application of pressure greater than atmospheric upstream of the filter(pressure filtration), applying a vacuum downstream (vacuum filtration)or by means of centrifugal force(centrifugal filtration). The theory and equipment for industrial filtration are full described by Brennan et aL. (1990). Applications can be divided into those where a slurry containing large amounts of insoluble solids is separated into a solid cake and a liquid, either of which may be the desired product; alternatively clarification is the removal of small quantities(<2%)of suspended solids from a valuable liquid Filtration finds applications throughout the food and biotechnology industries. Sugar juices from cane or beet are filtered to remove high levels of insoluble solids, and are frequently clarified at a later stage. Filtration is employed at various stages during the refining of edible oils. In the brewing industry filtration of mash, yeast recovery after manufacture of numerous other foods, e. g. vinegar, starch and sugar syrups, Mui. o fermentation and clarification of beer are carried out. Filtration is used during the wine, canning brines In biotechnology, filtration is carried out to clarify and recover cells from fermentation broths More recently, membranes with much smaller pores have been introduced. Micro- filtration involves the removal of very fine particles or the separation of microorganisms and sterilisation of fluids(see Chapter 5) Ultrafiltration membranes permit the passage of water and components of low molecular weight in a fluid but reject macromolecules such as protel starch Solids may be separated from liquids on the basis of particle size and density using settlement, or using centrifugation. Settlement is a slow process because it relies on the influence of gravity, but is widely used in water and effluent treatment processes. In
10 1.2.3 Separations from liquids This section will cover those situations where the separation takes place from a fluid, i.e. a substance which flows when it is subject to a shear stress. An important physical property is the viscosity of the fluid, which is defined as the ratio of the shear stress to shear rate. Viscosity and its measurement is discussed in more detail by Lewis (1990). Solid components may be present dissolved in the liquid, in a colloidal dispersion or in suspension. For example, milk contains lactose, minerals and whey proteins in true solution, casein and calcium phosphate as a colloidal dispersion and fat globules dispersed in the aqueous phase. There may also be sediment resulting from other contaminants of the milk. The objective of the separation may be to remove any of these components. Liquid-solid separations Liquid-solid separation applies to operations where discrete solids are removed from the liquid. There are a number of ways of achieving this and these will be briefly reviewed. Conventional filtration systems separate suspended particles of solids from liquids on the basis of particle size. The liquid component is passed through a porous membrane or septum which retains the solid material either as a filter cake on the upstream surface, or within its structure, or both. Filter media may be rigid, such as wire mesh or porous ceramics, or flexible, such as woven textiles, and are available in a variety of geometric shapes and pore sizes. In practice, the flow of filtrate may be brought about by gravity, the application of pressure greater than atmospheric upstream of the filter (pressure filtration), applying a vacuum downstream (vacuum filtration) or by means of centrifugal force (centrifugal filtration). The theory and equipment for industrial filtration are fully described by Brennan et al. (1990). Applications can be divided into those where a slurry containing large amounts of insoluble solids is separated into a solid cake and a liquid, either of which may be the desired product; alternatively clarification is the removal of small quantities (<2%) of suspended solids from a valuable liquid. Filtration finds applications throughout the food and biotechnology industries. Sugar juices from cane or beet are filtered to remove high levels of insoluble solids, and are frequently clarified at a later stage. Filtration is employed at various stages during the refining of edible oils. In the brewing industry filtration of mash, yeast recovery after fermentation and clarification of beer are carried out. Filtration is used during the manufacture of numerous other foods, e.g. vinegar, starch and sugar syrups, fruit juices, wine, canning brines. In biotechnology, filtration is carried out to clarify and recover cells from fermentation broths. More recently, membranes with much smaller pores have been introduced. Microfiltration involves the removal of very fine particles or the separation of microorganisms and sterilisation of fluids (see Chapter 5). Ultrafiltration membranes permit the passage of water and components of low molecular weight in a fluid but reject macromolecules such as protein or starch. Solids may be separated from liquids on the basis of particle size and density using settlement, or using centrifugation. Settlement is a slow process because it relies on the influence of gravity, but is widely used in water and effluent treatment processes. In A. S. Grandison and M. J. Lewis