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