《乳品生物化学》（英文版） 9 Heat-induced changes in milk

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 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 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). Mem￾brane 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 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)

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-interrup￾ted 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

352 DAIRY CHEMISTRY AND BIOCHEMISTRY Table 9.2 Concentration of conjugated linoleic acid ( CLA) isomers in selected foods(modified from Ha, Grimm and Pariza, ng CLA/kg Fat content CLA in fat Sample food 623±150 Cheddar cheese 325±1.7 Romano cheese 3569±63 32.1+0.8 11119 1693+8 49 574.1±24.8 318±1. 18053 3345±13.3 355±1.0 l8150±90.3 206±1,1 10.7 283±19 4.0±0.3 707.5 eurized whole Ground beef grilled 9940±309 7±0.3 92897 uncooked 561.7±220 74±02 isomers of conjugated linoleic acid(CLA)are shown in Figure 9.3. It claimed that CLa has anticarcinogenic properties. The mechanism of CLA formation in foods in general is not clear but heat treatment, free radical type oxidation and microbial enzymatic reactions involving linoleic and linolenic acids in the rumen are thought to be major contributors. Rather high concentrations of Cla have been found in heated dairy products specially processed cheese(Table 9.2). It has been suggested that whey proteins catalyse isomerization 9.3 Lactose The chemistry and physicochemical properties of lactose, a reducing disac- haide containing galactose and glucose linked by a B(1-4)-bond, were escribed in Chapter 2 When severely heated in the solid or molten state, lactose, like other sugars,undergoes numerous changes, including mutarotation, various isomerizations and the formation of numerous volatile compounds, includ- ing acids, furfural, hydroxymethylfurfural, CO2 and CO. In solution under strongly acidic conditions, lactose is degraded on heating to monosacchar- ides and other products, including acids. These changes do not normally occur during the thermal processing of milk. However, lactose is relatively unsta ble under mild alkaline conditions at moderate ter undergoes the Lobry de Bruyn-Alberda van Ekenstein rearrangement of Doses to ketoses(Figure 9.4)

352 DAIRY CHEMISTRY AND BIOCHEMISTRY Table 9.2 Concentration of conjugated linoleic acid (CLA) isomers in selected foods (modified from Ha, Grimm and Parka, 1989) Sample mg CLA/kg Fat content CLA in fat food (YO.) (mg kg-') Parmesan cheese Cheddar cheese Romano cheese Blue cheese Processed cheese Cream cheese Blue spread Cheese whiz Milk pasteurized whole non-pasteurized whole Ground beef grilled uncooked 622.3 f 15.0 440.6 f 14.5 356.9 f 6.3 169.3 f 8.9 574.1 f 24.8 334.5 f 13.3 202.6 & 6.1 1815.0 90.3 28.3 f 1.9 34.0 f 1.0 994.0 f 30.9 561.7 f 22.0 32.3 f 0.9 32.5 f 1.7 32.1 f 0.8 30.8 f 1.5 31.8 f 1.1 35.5 f 1.0 20.2 0.8 20.6 & 1.1 4.0 f 0.3 4.1 i 0.1 10.7 f 0.3 27.4 f 0.2 1926.7 1355.7 1111.9 549.8 1805.3 942.3 1003.0 8810.7 707.5 829.3 9289.7 2050.0 isomers of conjugated linoleic acid (CLA) are shown in Figure 9.3. It is claimed that CLA has anticarcinogenic properties. The mechanism of CLA formation in foods in general is not clear but heat treatment, free radical￾type oxidation and microbial enzymatic reactions involving linoleic and linolenic acids in the rumen are thought to be major contributors. Rather high concentrations of CLA have been found in heated dairy products, especially processed cheese (Table 9.2). It has been suggested that whey proteins catalyse isomerization. 9.3 Lactose The chemistry and physicochemical properties of lactose, a reducing disac￾charide containing galactose and glucose linked by a p( l-4)-bond, were described in Chapter 2. When severely heated in the solid or molten state, lactose, like other sugars, undergoes numerous changes, including mutarotation, various isomerizations and the formation of numerous volatile compounds, includ￾ing acids, furfural, hydroxymethylfurfural, CO, and CO. In solution under strongly acidic conditions, lactose is degraded on heating to monosacchar￾ides and other products, including acids. These changes do not normally occur during the thermal processing of milk. However, lactose is relatively unstable under mild alkaline conditions at moderate temperatures where it undergoes the Lobry de Bruyn- Alberda van Ekenstein rearrangement of aldoses to ketoses (Figure 9.4)

Lactose lactulose Organicacids+galactose Epilactose Epilactose =4-0-p-D-galactopyranosyl-D-mannopyrand actulose=4-0-p-D-galactopysanosyl-D-fructofuranose Figure 9.4 Heat-induced changes in lactose under mild alkaline conditio

I + I I 0 P c .- 0 0 s m I

354 DAIRY CHEMISTRY AND BIOCHEMISTRY Lactose undergoes at least three heat- induced changes during the pro cessing and storage of milk and milk products 9.3. Formation of lactulose On heating at low temperatures under slightly alkaline conditions, the glucose moiety of lactose is epimerized to fructose with the formation of actulose, which does not occur in nature The significance of lactulose has been discussed in Chapter 2. Lactulose is not formed during HTST process- ing but is formed during UHT sterilization(more during indirect than direct heating)and especially during in-container sterilization; therefore, the con- centration of lactulose in milk is a useful index of the severity of the heat treatment to which the milk has been subjected(see Figure 2.19). The concentration of lactulose is probably the best index available at present for differentiating between UHT and in-container sterilized milks and a number of assay procedures have been developed, using HPLC or enzymatic/ spectrophotometric principles 9.3.2 Formation of acids Milk as secreted by the cow contains about 200 mg CO2I-. Owing to its low concentration in air, CO2 is rapidly and, in effect, irreversibly lost from milk on standing after milking; its loss is accelerated by heating, agitation Figure 9.5 Changes in titratable acidity (O) lactic acid () and lactose(D)on heating milk(from Gould, 1945.)

3 54 DAIRY CHEMISTRY AND BIOCHEMISTRY Lactose undergoes at least three heat-induced changes during the pro￾cessing and storage of milk and milk products. 9.3. I Formation of lactulose On heating at low temperatures under slightly alkaline conditions, the glucose moiety of lactose is epimerized to fructose with the formation of lactulose, which does not occur in nature. The significance of lactulose has been discussed in Chapter 2. Lactulose is not formed during HTST process￾ing but is formed during UHT sterilization (more during indirect than direct heating) and especially during in-container sterilization; therefore, the con￾centration of lactulose in milk is a useful index of the severity of the heat treatment to which the milk has been subjected (see Figure 2.19). The concentration of lactulose is probably the best index available at present for differentiating between UHT and in-container sterilized milks and a number of assay procedures have been developed, using HPLC or enzymatic/ spectrophotometric principles. 9.3.2 Formation of acids Milk as secreted by the cow contains about 200 mg CO, 1-'. Owing to its low concentration in air, CO, is rapidly and, in effect, irreyersibly lost from milk on standing after milking; its loss is accelerated by heating, agitation 2 m u u .r - 9 0 1 2 3 Heating period at 116°C (h) Figure 9.5 Changes in titratable acidity (O), lactic acid (0) and lactose (0) on heating homogenized milk in sealed cans at 116°C. Titratable acidity expressed as mg lactic acid/100 g milk (from Gould, 1945.)

HEAT-INDUCED CHANGES IN MILK Temperature of heating (C) Figure 9.6 Eect of temperature on the rate of heat- induced production of acid in milk(from Jenness and Patton, 1959) and vacuum treatment. This loss of CO, causes an increase in ph of about 0. 1 unit and a decrease in the titratable acidity of nearly 0.02%/, expressed as lactic acid. Under relatively mild heating conditions, this change in pH is hore or less offset by the release of H+ on precipitation of Ca3(PO4)2,as discussed in section 9. 4 On heating at temperatures above 100C, lactose is degraded to acids with a concomitant increase in titratable acidity(Figures 9.5, 9.6). Formic acid is the principal acid formed; lactic acid represents only about 5% of the acids formed. Acid production is significant in the heat stability of milk, e.g. when assayed at 130.C, the pH falls to about 5. 8 at the point of coagulation (after about 20 min)( Figure 9.7). About half of this decrease is due to the formation of organic acids from lactose; the remainder is due to the precipitation of calcium phosphate and dephosphorylation of casein, as discussed in section 9. 4 In-container sterilization of milk at 115.C causes the ph to decrease to about 6 but much of this is due to the precipitation of calcium phosphate the contribution of acids derived from lactose has not been quantified cause insignificant degradation of lactose to acids.g UHT sterilization, ccurately. Other commercial heat treatments, inclue

HEAT-INDUCED CHANGES IN MILK 355 7- 6- c 2 .d 3 8 5- 2 4- 5 3- u CJ z - E 2- I I 1 90 100 110 120 Temperature of heating ("C) Figure 9.6 Effect of temperature on the rate of heat-induced production of acid in milk (from Jenness and Patton, 1959). and vacuum treatment. This loss of CO, causes an increase in pH of about 0.1 unit and a decrease in the titratable acidity of nearly 0.02%, expressed as lactic acid. Under relatively mild heating conditions, this change in pH is more or less offset by the release of H+ on precipitation of Ca,(PO,),, as discussed in section 9.4. On heating at temperatures above lOO"C, lactose is degraded to acids with a concomitant increase in titratable acidity (Figures 9.5, 9.6). Formic acid is the principal acid formed; lactic acid represents only about 5% of the acids formed. Acid production is significant in the heat stability of milk, e.g. when assayed at 130"C, the pH falls to about 5.8 at the point of coagulation (after about 20min) (Figure 9.7). About half of this decrease is due to the formation of organic acids from lactose; the remainder is due to the precipitation of calcium phosphate and dephosphorylation of casein, as discussed in section 9.4. In-container sterilization of milk at 115°C causes the pH to decrease to about 6 but much of this is due to the precipitation of calcium phosphate; the contribution of acids derived from lactose has not been quantified accurately. Other commercial heat treatments, including UHT sterilization, cause insignificant degradation of lactose to acids

356 DAIRY CHEMISTRY AND BIOCHEMISTRY 68 62 60 5.6 period at130°Cmin) Figure 9.7 The ph of samples of milk after heating for various periods at 130C with air(O) O,(O)or N,(A)in the headspace above the milk; t, coagulation time (from Sweetsur and white. 1975). 9.3.3 Maillard browning The mechanism and consequences of the maillard reaction were discussed in Chapter 2. The reaction is most significant in severely heat-treated products, especially in-container sterilized milks. However, it may also occur to a significant extent in milk powders stored under conditions of high humidity and high temperature, resulting in a decrease in the solubility of the powder. If cheese contains a high level of residual lactose or galactose (due to the use of a starter unable to utilize galactose; Chapter 10), it is susceptible to Maillard browning, especially during cooking on pizza, e. g Mozzarella(Pizza)cheese. Browning may also occur in grated cheese during storage if the cheese contains residual sugars; in this case, the water activity of the cheese(aw s0.6)is favourable for the Maillard reaction. Poorly washed casein and especially whey protein concentrates (which contain 30-60% lactose)may undergo Maillard browning when used as ingredients in heat-treated foods 1. The final polymerization products(melanoidins)are brown and hence dairy products which have undergone Maillard browning are discoloured and aesthetically unacceptable

356 DAIRY CHEMISTRY AND BIOCHEMISTRY r E .- L 0 5.6 0 10 20 30 40 Heating period at 130°C (min) Figure 9.7 The pH of samples of milk after heating for various periods at 130°C with air (O), 0, (0) or N, (A) in the headspace above the milk; T, coagulation time (from Sweetsur and White, 1975). 9.3.3 Maillard browning The mechanism and consequences of the Maillard reaction were discussed in Chapter 2. The reaction is most significant in severely heat-treated products, especially in-container sterilized milks. However, it may also occur to a significant extent in milk powders stored under conditions of high humidity and high temperature, resulting in a decrease in the solubility of the powder. If cheese contains a high level of residual lactose or galactose (due to the use of a starter unable to utilize galactose; Chapter lo), it is susceptible to Maillard browning, especially during cooking on pizza, e.g. Mozzarella (Pizza) cheese. Browning may also occur in grated cheese during storage if the cheese contains residual sugars; in this case, the water activity of the cheese (a, - 0.6) is favourable for the Maillard reaction. Poorly washed casein and especially whey protein concentrates (which contain 30-60% lactose) may undergo Maillard browning when used as ingredients in heat-treated foods. Maillard browning in milk products is undesirable because: 1. The final polymerization products (melanoidins) are brown and hence dairy products which have undergone Maillard browning are discoloured and aesthetically unacceptable