114 DAIRY CHEMISTRY AND BIOCHEMISTRY pressure pump ◆ homogenized 品° e Figure 3. 23 Diagram of a milk homogenizer. Reducing the average diameter of the fat globules to 1 um results in a four- to sixfold increase in the fat/plasma interface. There is insufficient natural membrane to completely coat the newly formed surface or insuffi- cient time for complete coverage to occur and consequently the globules in homogenized milk are coated by a membrane which consists mostly of casein(93% of dry mass, with some whey proteins, which are adsorbed less efficiently than the caseins)(Figure 3. 25). The membrane of homogenized lilk contains 2.3 g protein per 100 g fat (10 mg protein m-2), which is very considerably higher than the level of protein in the natural membrane (0.5-0.8g per 100 g fat), and is estimated to be about 15 nm thick. the casein content in the serum phase of homogenized milk is reduced by about Homogenization causes several major changes in the properties of milk: Homogenized milk does not cream naturally and the fat is recovered only poorly by mechanical separation. This is due in part to the smaller average size of the fat globules but failure of the globules in homogenized milk to form aggregates, due mostly to the agitation induced denatura- tion of some immunoglobulins, is mainly responsible for the failure to cream
114 DAIRY CHEMISTRY AND BIOCHEMISTRY Milk from highpressure pump n Spring-loaded valve Figure 3.23 Diagram of a milk homogenizer. Reducing the average diameter of the fat globules to 1 pm results in a four- to sixfold increase in the fat/plasma interface. There is insufficient natural membrane to completely coat the newly formed surface or insufficient time for complete coverage to occur and consequently the globules in homogenized milk are coated by a membrane which consists mostly of casein (93% of dry mass, with some whey proteins, which are adsorbed less efficiently than the caseins) (Figure 3.25). The membrane of homogenized milk contains 2.3 g protein per lOOg fat (10mg proteinm-'), which is very considerably higher than the level of protein in the natural membrane (0.5-0.8g per 1OOg fat), and is estimated to be about 15nm thick. The casein content in the serum phase of homogenized milk is reduced by about 6-8%. Homogenization causes several major changes in the properties of milk: 1. Homogenized milk does not cream naturally and the fat is recovered only poorly by mechanical separation. This is due in part to the smaller average size of the fat globules but failure of the globules in homogenized milk to form aggregates, due mostly to the agitation-induced denaturation of some immunoglobulins, is mainly responsible for the failure to cream
MILK LIPIDS 115 Unh Globule diameter(um) enization on the size( volume distribution) of fat globules in milk (modified from Mulder and Walstra, 1974). 2. As discussed in section 3. 10. 1, homogenized milk is very susceptible to hydrolytic rancidity because the artificial membrane does not isolate the fat from the lipase; consequently, homogenized milk must be pasteurized prior to or immediately after homogenization. h ized milk is also more susceptible to sunlight oxidized flavour, which is due to the production of methional from methionine, but is less susceptible to metal-catalysed lipid oxidation; the latter is presumably because the phospholipids, which are very susceptible to oxidation (highly un- saturated) and are located largely in the natural membrane (which contains pro-oxidants, e.g. xanthine oxidase and metals) are more uni- formly distributed after homogenization and, therefore, are less likely to propagate lipid oxidation 3. Homogenized milk is whiter due to finer dispersion of the fat(and thus greater light scattering) and its flavour is more bland
MILK LIPIDS 115 2 4 6 Globule diameter (um) Figure 3.24 Effect of homogenization on the size (volume distribution) of fat globules in milk (modified from Mulder and Walstra, 1974). 2. As discussed in section 3.10.1, homogenized milk is very susceptible to hydrolytic rancidity because the artificial membrane does not isolate the fat from the lipase; consequently, homogenized milk must be pasteurized prior to or immediately after homogenization. Homogenized milk is also more susceptible to sunlight oxidized flavour, which is due to the production of methional from methionine, but is less susceptible to metal-catalysed lipid oxidation; the latter is presumably because the phospholipids, which are very susceptible to oxidation (highly unsaturated) and are located largely in the natural membrane (which contains pro-oxidants, e.g. xanthine oxidase and metals) are more uniformly distributed after homogenization and, therefore, are less likely to propagate lipid oxidation. 3. Homogenized milk is whiter due to finer dispersion of the fat (and thus greater light scattering) and its flavour is more bland
116 DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 3. 25 Schematic representation of the membrane of fat globules in homogenized milk (modified from Walstra, 1983) 4. The heat stability of whole milk is reduced by homogenization, as is the strength(curd tension)of rennet-induced gels; these changes will be discussed in more detail in Chapters 9 and 10. Viscosity is increased for unidentified reasons, probably independent of size changes. Homogenized milk has improved foaming characteristics, a feature which may be due to the release of foam-promoting proteins from the natural membrane or to reduction in fat globule size-small globules are less likely to damage foam lamellae Homogenization reduces surface tension, possibly due to inclusion of very surface-active proteins in the artificial membrane and to changes in the fat globule surface Homogenized milk drains cleanly from the sides of a glass bottle or drinking glass. Milk for homogenization should be clarified to avoid sedimentation of leucocytes The efficiency of homogenization may be assessed by microscopic exam ination or more effectively by a particle sizer, e.g. Malvern Mastersizer 3.10.4 Heating Normal HTST pasteurization causes very little change in the fat globule membrane or in the characteristics of milk fat dependent on the membrane
116 DAIRY CHEMISTRY AND BIOCHEMISTRY 7 PLASMA \\ Whey protein FAT Figure 3.25 Schematic representation of the membrane of fat globules in homogenized milk (modified from Walstra, 1983). 4. The heat stability of whole milk is reduced by homogenization, as is the strength (curd tension) of rennet-induced gels; these changes will be discussed in more detail in Chapters 9 and 10. Viscosity is increased for unidentified reasons, probably independent of size changes. Homogenized milk has improved foaming characteristics, a feature which may be due to the release of foam-promoting proteins from the natural membrane or to reduction in fat globule size - small globules are less likely to damage foam lamellae. Homogenization reduces surface tension, possibly due to inclusion of very surface-active proteins in the artificial membrane and to changes in the fat globule surface. Homogenized milk drains cleanly from the sides of a glass bottle or drinking glass. Milk for homogenization should be clarified to avoid sedimentation of leucocytes. The efficiency of homogenization may be assessed by microscopic examination or more effectively by a particle sizer, e.g. Malvern Mastersizer. 3.10.4 Heating Normal HTST pasteurization causes very little change in the fat globule membrane or in the characteristics of milk fat dependent on the membrane
MILK LIPIDS However, excessively high pasteurization temperatures denature the cryog d aggregation of the fat globules and creaming are impaired or revented. Severe treatments, e.g. 80C x 15 min, remove lipid and protein material from the membrane, the fat globules are partially denuded and may coalesce, forming large clumps of fat and resulting in defects such as cream plug in milk or cream(section 3. 11) Processes such as thermal concentration also cause membrane damage, especially since many of these treatments also involve vigorous agitation in high velocity heating systems. Since milk for concentrated and dehydrated milk products is normally homogenized, damage to the natural membrane is of little significance 3.11 Physical defects in milk and cream In addition to the favour defects initiated or influenced by damage to the t globule membrane, such damage also results in a variety of physical defects in milk and especially in cream. The more important of these are oiling- of,, 'cream plug andage thickening Oiling-off, characterized by the appearance of globules of oil or fat on the surface of coffee or tea when milk, and especially cream, is added, is due to membrane damage during processing, resulting in 'free fat'; low pressure homogenization re- emulsifies the free fat and eliminates the defect Cream plug' is characterized by the formation of a layer of solid fat the surface of cream or milk in bottles; the defect is due to a high level of free fat which forms interlocking crystals on cooling and is most common in high-fat creams. Cream plug is common in unhomogenized, pasteurized late lactation milk, presumably due to a weak mfGm Age thickening is due essentially to a high level of free fat, especially in crystals of free fat Two somewhat related instability problems are feathering'and'bitty cream 'Feathering is characterized by the appearance of white flecks when Ik or cream is poured on hot coffee and is a form of heat-induced coagulation; the white flecks are mainly destabilized protein. The heat tability of cream and its resistance to feathering are reduced by e single-stage homogenization high homogenization pressure at low temperatur high concentrations of Ca+in the cream or water a high ratio of fat to serum solids, i.e. high-fat creams; high temperature and low pH of the coffee Protein-lipid interaction is enhanced by homogenization, while high tem- peratures, low pH and high divalent cation concentration induce aggrega tion of the casein-coated fat globules into large visible particles. Stability
MILK LIPIDS 117 However, excessively high pasteurization temperatures denature the cryoglobulins and aggregation of the fat globules and creaming are impaired or prevented. Severe treatments, e.g. 80°C x 15 min, remove lipid and protein material from the membrane, the fat globules are partially denuded and may coalesce, forming large clumps of fat and resulting in defects such as cream plug in milk or cream (section 3.11). Processes such as thermal concentration also cause membrane damage, especially since many of these treatments also involve vigorous agitation in high velocity heating systems. Since milk for concentrated and dehydrated milk products is normally homogenized, damage to the natural membrane is of little significance. 3.11 Physical defects in milk and cream In addition to the flavour defects initiated or influenced by damage to the fat globule membrane, such damage also results in a variety of physical defects in milk and especially in cream. The more important of these are ‘oiling-off, ‘cream plug’ and ‘age thickening’. ‘Oiling-off, characterized by the appearance of globules of oil or fat on the surface of coffee or tea when milk, and especially cream, is added, is due to membrane damage during processing, resulting in ‘free fat’; low pressure homogenization re-emulsifies the free fat and eliminates the defect. ‘Cream plug’ is characterized by the formation of a layer of solid fat on the surface of cream or milk in bottles; the defect is due to a high level of ‘free fat’ which forms interlocking crystals on cooling and is most common in high-fat creams. Cream plug is common in unhomogenized, pasteurized, late lactation milk, presumably due to a weak MFGM. ‘Age thickening’ is due essentially to a high level of free fat, especially in high-fat creams; the product becomes very viscous due to interlocking of crystals of free fat. Two somewhat related instability problems are ‘feathering’ and ‘bitty’ cream. ‘Feathering’ is characterized by the appearance of white flecks when milk or cream is poured on hot coffee and is a form of heat-induced coagulation; the white ‘flecks are mainly destabilized protein. The heat stability of cream and its resistance to feathering are reduced by: 0 single-stage homogenization; 0 high homogenization pressure at low temperature; 0 high concentrations of Caz+ in the cream or water; 0 a high ratio of fat to serum solids, i.e. high-fat creams; 0 high temperature and low pH of the coffee. Protein-lipid interaction is enhanced by homogenization, while high temperatures, low pH and high divalent cation concentration induce aggregation of the casein-coated fat globules into large visible particles. Stability