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 cysteineHEAT-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