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 radicaltype 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 disaccharide 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, including acids, furfural, hydroxymethylfurfural, CO, and CO. In solution under strongly acidic conditions, lactose is degraded on heating to monosaccharides 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 processing 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 processing but is formed during UHT sterilization (more during indirect than direct heating) and especially during in-container sterilization; therefore, the concentration 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