9 Heat- induced changes in milk 9.1 Introduction In modern dairy technology, milk is almost always subjected to a heat treatment; typical examples are Thermization eg65C×15s Pasteurization LTLT (low temperature, long time) 63C X 30 min HTST(high temperature, short time) 72C X 15s Forewarning(for sterilization) eg90C×2-10min 120°C×2min Sterilization UHT (ultra-high temperatur 130-140°C×3-5s 110-115°C×1020min The objective of the heat treatment varies with the product being produced. Thermization is generally used to kill temperature-sensitive micro-organisms, e.g. psychrotrophs, and thereby reduce the microflora of milk for low-temperature storage. The primary objective of pasteurization is to kill pathogens but it also reduces the number of non-pathogenic micro-organisms which may cause spoilage, thereby standardizing the milk as a raw material for various products. Many indigenous enzymes, e.g lipase, are also inactivated, thus contributing to milk stability. Forewarming (preheating) increases the heat stability of milk for subsequent sterilization (as discussed in section 9.7. 1). Sterilization renders milk shelf-stable for very long periods, although gelation and flavour changes occur during storage, especially of UHT-sterilized milk Although milk is a very complex biological fluid containing comple protein, lipid, carbohydrate, salt, vitamins and enzyme systems in soluble, colloidal or emulsified states, it is a very heat-stable system, which allows it to be subjected to severe heat treatments with relatively minor changes in comparison to other foods if subjected to similar treatments. However, numerous biological, chemical and physico-chemical changes occur in milk during thermal processing which affect its nutritional, organoleptic and or technological properties. The temperature dependence of these changes
9 Heat-induced changes in milk 9.1 Introduction In modern dairy technology, milk is almost always subjected to a heat treatment; typical examples are: Thermization Pasteurization e.g. 65°C x 15 s LTLT (low temperature, long time) 63°C x 30 min HTST (high temperature, short time) 72°C x 15 s Forewarming (for sterilization) e.g. 90°C x 2-10 min, Sterilization 120°C x 2 min UHT (ultra-high temperature) 130-140°C x 3-5s In-container 110-115°C x 10-20min The objective of the heat treatment varies with the product being produced. Thermization is generally used to kill temperature-sensitive micro-organisms, e.g. psychrotrophs, and thereby reduce the microflora of milk for low-temperature storage. The primary objective of pasteurization is to kill pathogens but it also reduces the number of non-pathogenic micro-organisms which may cause spoilage, thereby standardizing the milk as a raw material for various products. Many indigenous enzymes, e.g. lipase, are also inactivated, thus contributing to milk stability. Forewarming (preheating) increases the heat stability of milk for subsequent sterilization (as discussed in section 9.7.1). Sterilization renders milk shelf-stable for very long periods, although gelation and flavour changes occur during storage, especially of UHT-sterilized milks. Although milk is a very complex biological fluid containing complex protein, lipid, carbohydrate, salt, vitamins and enzyme systems in soluble, colloidal or emulsified states, it is a very heat-stable system, which allows it to be subjected to severe heat treatments with relatively minor changes in comparison to other foods if subjected to similar treatments. However, numerous biological, chemical and physico-chemical changes occur in milk during thermal processing which affect its nutritional, organoleptic and/or technological properties. The temperature dependence of these changes
DAIRY CHEMISTRY AND BIOCHEMISTRY log f( min) 13 h 80 Figure 9.1 The time needed (r)at various temperatures(T)to inact enzymes and bacteria and spores: to cause a certain invert 1% of lactose to lactulose: to cause heat coagulation; to reduce and to make 10%and 75% of the whey proteins insoluble at pH 4.6(fr Table 9. 1 Approximate values for the temperature dependence of some reactions in heated milk (modified from Walstra and Jennes, 1984 Reaction Activation energy (kJ mol") Qoat100°C Many chemical reactions 20-30 Many enzyme-catalysed reactions Autoxidation of lipids 14-2.5 aillard reactions(browning) 100-180 24-5.0 2.6-28 leat coagulation of milk Heat denaturation of p 200-600 60-1750 Typical enzyme inactiv (plasmin) Killing vegetative bacteria 200-600 Killing of spores 250-330 9.0-170 varies widely, as depicted in general terms in Figure 9. 1 and Table 9.1.The ost significant of these changes, with the exception of the killing of bacteria, will be discussed below. In general, the effect(s) of heat on the principal constituents of milk will be considered individually, although there are interactions between constituents in many cases
348 DAIRY CHEMISTRY AND BIOCHEMISTRY h min. T("C) Figure 9.1 The time needed (1') at various temperatures (T) to inactivate some enzymes and cryoglobulins; to kill some bacteria and spores; to cause a certain degree of browning; to convert 1% of lactose to lactulose; to cause heat coagulation; to reduce available lysine by 1%; and to make 10% and 75% of the whey proteins insoluble at pH 4.6 (from Walstra and Jenness, 1984). Table 9.1 Approximate values for the temperature dependence of some reactions in heated milk (modified from Walstra and Jennes, 1984) Reaction Activation energy (kJ mol- ') Qlo at 100°C Many chemical reactions Many enzyme-catalysed reactions Autoxidation of lipids Maillard reactions (browning) Dephosphorylation of caseinate Heat coagulation of milk Degradation of ascorbic acid Heat denaturation of protein Typical enzyme inactivation Inactivation of milk proteinase Killing vegetative bacteria Killing of spores (plasmin) 80-130 40 - 60 40-100 100-180 110-120 150 60-120 200-600 450 75 200-600 250-330 2.0-3.0 1.4-1.7 1.4-2.5 2.4-5.0 2.6-2.8 3.7 1.7-2.8 6.0-175.0 50.0 1.9 6.0-175.0 9.0- 17.0 varies widely, as depicted in general terms in Figure 9.1 and Table 9.1. The most significant of these changes, with the exception of the killing of bacteria, will be discussed below. In general, the effect(s) of heat on the principal constituents of milk will be considered individually, although there are interactions between constituents in many cases
HEAT-INDUCED CHANGES IN MILK 9.2 Lipids Of the principal constituents, the lipids are probably the least affected by heat.However, significant changes do occur in milk lipids, especially in their physical properties, during heating. 9.2.1 Physicochemical changes Creaming. The chemical and physicochemical aspects of the lipids in milk were discussed in Chapter 3. The principal effect of heat treatments on milk lipids is on creaming of the fat globules. As discussed in Chapter 3, the fat in milk exists as globules, 0.1-20 um in diameter(mean, 3-4 um). The globules are stabilized by a complex membrane acquired within the secre tory cell and during excretion from the cell. Owing to differences in density between the fat and aqueous phases, the globules float to the surface to form a cream layer. In cows'milk, the rate of creaming is far in excess of that predicted by Stokes'law, owing to aggregation of the globules which is promoted by cryoglobulins(a group of immunoglobulins). Buffalo, ovine or caprine milks do not undergo cryoglobulin-dependent agglutination of fat globules and cream very slowly with the formation of a compact cream When milk is heated to a moderate temperature(e.g 70C x 15 min), the cryoglobulins are irreversibly denatured and hence the creaming of milk is mpaired or prevented; HTST pasteurization(72 C x 15s) has little or effect on creaming potential but slightly more severe conditions have an adverse effect(Figure 9.2) Homogenization, which reduces mean globule diameter to below 1 um, retards creaming due to the reduction in globule size but, more importantly, to the denaturation of cryoglobulins which prevents agglutination. In fact, there are probably two classes of cryoglobulin, one of which is denatured by heating, the other by homogenization Changes in the fat globule membrane. The milk fat globule membrane (MFGM)itself is altered during thermal processing. Milk is usually agitated during heating, perhaps with foam formation. Agitation, especially of warm ilk in which the fat is liquid, may cause changes in globule size due to disruption or coalescence: significant disruption occurs during direct UHT processing Foaming probably causes desorption of some membrane material and its replacement by adsorption of skim-milk proteins. In these ases, it may not be possible to differentiate the effect of heating from the total effect of the process Heating per se to above 70c denatures membrane proteins, with the xposure and activation of various amino acid residues, especially cysteine
HEAT-INDUCED CHANGES IN MILK 349 9.2 Lipids Of the principal constituents, the lipids are probably the least affected by heat. However, significant changes do occur in milk lipids, especially in their physical properties, during heating. 9.2.1 Physicochemical changes Creaming. The chemical and physicochemical aspects of the lipids in milk were discussed in Chapter 3. The principal effect of heat treatments on milk lipids is on creaming of the fat globules. As discussed in Chapter 3, the fat in milk exists as globules, 0.1-20pm in diameter (mean, 3-4pm). The globules are stabilized by a complex membrane acquired within the secretory cell and during excretion from the cell. Owing to differences in density between the fat and aqueous phases, the globules float to the surface to form a cream layer. In cows’ milk, the rate of creaming is far in excess of that predicted by Stokes’ law, owing to aggregation of the globules which is promoted by cryoglobulins (a group of immunoglobulins). Buffalo, ovine or caprine milks do not undergo cryoglobulin-dependent agglutination of fat globules and cream very slowly with the formation of a compact cream layer. When milk is heated to a moderate temperature (e.g. 70°C x 15 min), the cryoglobulins are irreversibly denatured and hence the creaming of milk is impaired or prevented; HTST pasteurization (72°C x 15 s) has little or no effect on creaming potential but slightly more severe conditions have an adverse effect (Figure 9.2). Homogenization,, which reduces mean globule diameter to below 1 pm, retards creaming due to the reduction in globule size but, more importantly, to the denaturation of cryoglobulins which prevents agglutination. In fact, there are probably two classes of cryoglobulin, one of which is denatured by heating, the other by homogenization. Changes in the fat globule membrane. The milk fat globule membrane (MFGM) itself is altered during thermal processing. Milk is usually agitated during heating, perhaps with foam formation. Agitation, especially of warm milk in which the fat is liquid, may cause changes in globule size due to disruption or coalescence; significant disruption occurs during direct UHT processing. Foaming probably causes desorption of some membrane material and its replacement by adsorption of skim-milk proteins. In these cases, it may not be possible to differentiate the effect of heating from the total effect of the process. Heating per se to above 70°C denatures membrane proteins, with the exposure and activation of various amino acid residues, especially cysteine
DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 9.2 Time-temperature for the destruction of berculosis (.), inactivation of alkaline phosphatase ()and creaming ability of milk (--)(from Webb and Johnson, 1965) This may cause the release of H, S (which can result in the development of an off-flavour) and disulphide interchange reactions with whey proteins, leading to the formation of a layer of denatured whey proteins on the fat globules at high temperatures(>100.C). The membrane and or whey proteins may participate in Maillard browning with lactose and the cysteine may undergo B-elimination to dehydroalanine, which may then react with lysine to form lysinoalanine or with cysteine residues to form lanthionine, leading to covalent cross linking of protein molecules(section 9.6.3). Mem brane constituents, both proteins and pl lost from the membrane to the aqueous phase at high temperatures. Much of the indigenous copper in milk is associated with the MFGM and some of it is transferred to the serum on heat processing. Thus, severe heat treatment of cream improves the oxidative stability of butter made from it as a result of the reduced concentration of pro-oxidant Cu in the fat phase and the antioxidant effect of exposed sulphydryl groups The consequences of these changes in the MFGM have been the subject of little study, possibly because severely heated milk products are usually homogenized and an artificial membrane, consisting mainly of casein and some whey proteins, is formed; consequently, changes in the natural mem brane are not important. Damage to the membrane of unhomogenized products leads to the formation of free(non-globular) fat and consequently to 'oiling-off and the formation of a cream plug'( Chapter 3)
3 50 DAIRY CHEMISTRY AND BIOCHEMISTRY 50 60 70 80 Temperature ("C) Figure 9.2 Time-temperature curves for the destruction of M. tuberculosis (. . .), inactivation of alkaline phosphatase (-) and creaming ability of milk (---) (from Webb and Johnson, 1965). This may cause the release of H,S (which can result in the development of an off-flavour) and disulphide interchange reactions with whey proteins, leading to the formation of a layer of denatured whey proteins on the fat globules at high temperatures (> l0OT). The membrane and/or whey proteins may participate in Maillard browning with lactose and the cysteine may undergo p-elimination to dehydroalanine, which may then react with lysine to form lysinoalanine or with cysteine residues to form lanthionine, leading to covalent cross-linking of protein molecules (section 9.6.3). Membrane constituents, both proteins and phospholipids, are lost from the membrane to the aqueous phase at high temperatures. Much of the indigenous copper in milk is associated with the MFGM and some of it is transferred to the serum on heat processing. Thus, severe heat treatment of cream improves the oxidative stability of butter made from it as a result of the reduced concentration of pro-oxidant Cu in the fat phase and the antioxidant effect of exposed sulphydryl groups. The consequences of these changes in the MFGM have been the subject of little study, possibly because severely heated milk products are usually homogenized and an artificial membrane, consisting mainly of casein and some whey proteins, is formed; consequently, changes in the natural membrane are not important. Damage to the membrane of unhomogenized products leads to the formation of free (non-globular) fat and consequently to 'oiling-off and the formation of a 'cream plug' (Chapter 3)
HEAT-INDUCED CHANGES IN MILK Severe heat treatment, as is encountered during roller drying and to a lesser extent spray drying, results in at least some demulsification of milk fat, with the formation of free fat, which causes(Chapter 3) the appearance of fat droplets when such products are used in tea or increased susceptibility of the fat to oxidation, since it is not protected by a membrane: reduced wettability/dispersibility of the powder a tendency of powders to clump 9.2.2 Chemical change Severe heat treatments, e.g. frying, may convert hydroxyacids to lactones which have strong, desirable flavours and contribute to the desirable attributes of milk fat in cooking Release of fatty acids and some interesterification may also occur, but such changes are unlikely during the normal processing of milk Naturally occurring polyunsaturated fatty acids are methylene- interrup- ted but may be converted to conjugated isomers at high temperatures. Four 是A,人人 CAT 10,:12c Figure 93 Isomers of conjugated linoleic acid
HEAT-INDUCED CHANGES IN MILK 351 Severe heat treatment, as is encountered during roller drying and to a lesser extent spray drying, results in at least some demulsification of milk fat, with the formation of free fat, which causes (Chapter 3): the appearance of fat droplets when such products are used in tea or coffee; increased susceptibility of the fat to oxidation, since it is not protected by a membrane; reduced wettability/dispersibility of the powder; a tendency of powders to clump. 9.2.2 Chemical changes Severe heat treatments, e.g. frying, may convert hydroxyacids to lactones, which have strong, desirable flavours and contribute to the desirable attributes of milk fat in cooking. Release of fatty acids and some interesterification may also occur, but such changes are unlikely during the normal processing of milk. Naturally occurring polyunsaturated fatty acids are methylene-interrupted but may be converted to conjugated isomers at high temperatures. Four & R1 12 10 9 - R2 12 in Rl R2 9.c- 11.1 9.1-1 1, t 13 12 10 9 Linoleic acid - - - Rl water 9,C-12.C Liirolric acid Rl R1 12 10 R2 11 R2 I(l,t - 12.c 111,t - 12, t Figure 9.3 Isomers of conjugated linoleic acid