《乳品生物化学》（英文版） 3.9 Stability of the milk fat emulsion.pdf_大学文库

DAIRY CHEMISTRY AND BIOCHEMISTRY views on the structure of the MfGM and note that complete information on the structure is still not available. Since the MFGM is a dynamic, unstable structure, it is probably not possible to describe a structure which is applicable in all situations and conditions 3.9 Stability of the milk fat emulsion The stability, or instability, of the milk fat emulsion is very significant with respect to many physical and chemical characteristics of milk and dairy products. The stability of the emulsion depends strongly on the integrity of the MfGm and as discussed in section 3. 8. 7, this membrane is quite fragile and is more or less extensively changed during dairy processing operations In the following, some of the principal aspects and problems related to or arising from the stability of the milk fat emulsion are discussed. Some of these relate to the inherent instability of emulsions in general, others are pecifically related to the milk system 3.9.1 Emulsion stability in general Lipid emulsions are inherently unstable systems due to 1. The difference in density between the lipid and aqueous phases(c. 0.9 and 1.036gcm 3, respectively, for milk), which causes the fat globules to float or cream according to Stokes equation 2r2(p1-p2)g where V is the rate of creaming: r, the radius of fat globules; P, and p2, the densities of the continuous and dispersed phases, respectively; g, acceleration due to gravity; and n, viscosity of the system. If creaming not accompanied by other changes, it is readily reversible by gentle agitation 2. The interfacial tension between the oil and aqueous phases. Although interfacial tension is reduced by the use of an emulsifier, the interfacial film may be imperfect. When two globules collide, they may adhere (flocculate), e.g. by sharing emulsifier, or they may coalesce due to the aplace principle which states that the pressure is greater inside small globules than inside large globules and hence there is a tendency for large fat globules (or gas bubbles) to grow at the expense of smaller ones Taken to the extreme. this will lead to the formation of a continuous mass Destabilization processes in emulsions are summarized schematically in Figure 3. 19. The rate of destabilization is influenced by the fat content, shear rate(motion), liquid: solid fat ratio, inclusion of air and globule size

104 DAIRY CHEMISTRY AND BIOCHEMISTRY views on the structure of the MFGM and note that complete information on the structure is still not available. Since the MFGM is a dynamic, unstable structure, it is probably not possible to describe a structure which is applicable in all situations and conditions. 3.9 Stability of the milk fat emulsion The stability, or instability, of the milk fat emulsion is very significant with respect to many physical and chemical characteristics of milk and dairy products. The stability of the emulsion depends strongly on the integrity of the MFGM and, as discussed in section 3.8.7, this membrane is quite fragile and is more or less extensively changed during dairy processing operations. In the following, some of the principal aspects and problems related to or arising from the stability of the milk fat emulsion are discussed. Some of these relate to the inherent instability of emulsions in general, others are specifically related to the milk system. 3.9.1 Emulsion stability in general Lipid emulsions are inherently unstable systems due to: 1. The difference in density between the lipid and aqueous phases (c. 0.9 and 1.036 g cm-3, respectively, for milk), which causes the fat globules to float or cream according to Stokes’ equation: where V is the rate of creaming; Y, the radius of fat globules; p1 and p2, the densities of the continuous and dispersed phases, respectively; g, acceleration due to gravity; and rl, viscosity of the system. If creaming is not accompanied by other changes, it is readily reversible by gentle agitation. 2. The interfacial tension between the oil and aqueous phases. Although interfacial tension is reduced by the use of an emulsifier, the interfacial film may be imperfect. When two globules collide, they may adhere (flocculate), e.g. by sharing emulsifier, or they may coalesce due to the Laplace principle which states that the pressure is greater inside small globules than inside large globules and hence there is a tendency for large fat globules (or gas bubbles) to grow at the expense of smaller ones. Taken to the extreme, this will lead to the formation of a continuous mass of fat. Destabilization processes in emulsions are summarized schematically in Figure 3.19. The rate of destabilization is influenced by the fat content, shear rate (motion), liquid: solid fat ratio, inclusion of air and globule size. Previous Page

MILK LIPIDS In 1889, Babcock postulated that creaming of cows milk resulted from in agglutination-type reaction, similar to the agglutination of red blood cells; this hypothesis has been confirmed Creaming is enhanced by adding blood serum or colostrum to milk; the responsible agents are immunog- lobulins(Ig, which are present at high levels in colostrum), especially IgM Because these Igs aggregate and precipitate at low temperature(<37c) and redisperse on warming, they are often referred to as cryoglobulins Aggregation is also dependent on ionic strength and ph. when aggregation of the cryoglobulins occurs in the cold they may precipitate on to the surfaces of large particles, e.g. fat globules, causing them to agglutinate, probably through a reduction in surface(electrokinetic) potential. The ryoprecipitated globulins may also form a network in which the fat globules are entrapped. The clusters can be dispersed by gentle stirring and are completely dispersed on warming to 37c or higher. Creaming is strongly dependent on temperature and does not occur above 37'C (Figure 3.20). The milks of buffalo, sheep and goat do not exhibit flocculation and he milks of some cows exhibit little or none, apparently a genetic trait The rate of creaming and the depth of the cream layer show considerable variation. The concentration of cryoglobulin might be expected to influence the rate of creaming and although colostrum(rich in Ig) creams well ar late lactation milk(deficient in Ig)creams poorly, there is no correlation in mid-lactation milks between Ig concentration and the rate of creaming. An uncharacterized lipoprotein appears to act synergistically with cryoglobulin in promoting clustering. The rate of creaming is increased by increasing the ionic strength and retarded by acidification. High-fat milks, which also tend to have a higher proportion of larger fat globules, cream quickly, probably because the probability of collisions between globules is greater and because large globules tend to form larger aggregates. The depth of the cream layer in high-fat milks is also greater than might be expected, possibly because of greater ' dead space in the interstices of aggregates formed from large globules The rate of creaming and the depth of the cream layer are very markedly influenced by processing operations. Creaming is faster and more complete at low temperatures(< 20"; Figure 3. 20), probably because of the tempera ture-dependent precipitation of the cryoglobulins. Gentle(but not pro- longed) agitation during the initial stages of creaming promotes and enhances cluster formation and creaming, possibly because of an increased probability of collisions. It would be expected that stirring cold milk would lead to the deposition of all the cryoglobulin on to the fat globule surfaces and rapid creaming, without a time lag, would be expected when stirring ceased. However, milk so treated does not cream at all or only slightly after a prolonged lag period. If cold, creamed milk is agitated gently, the clusters are dispersed and do not reform unless the milk is rewarmed to c. 40C and then recooled, i.e. the whole cycle repeated. Violent agitation is detrimental

MILK LIPIDS 107 In 1889, Babcock postulated that creaming of cows’ milk resulted from an agglutination-type reaction, similar to the agglutination of red blood cells; this hypothesis has been confirmed. Creaming is enhanced by adding blood serum or colostrum to milk; the responsible agents are immunoglobulins (Ig, which are present at high levels in colostrum), especially IgM. Because these Igs aggregate and precipitate at low temperature ( c 37°C) and redisperse on warming, they are often referred to as cryoglobulins. Aggregation is also dependent on ionic strength and pH. When aggregation of the cryoglobulins occurs in the cold they may precipitate on to the surfaces of large particles, e.g. fat globules, causing them to agglutinate, probably through a reduction in surface (electrokinetic) potential. The cryoprecipitated globulins may also form a network in which the fat globules are entrapped. The clusters can be dispersed by gentle stirring and are completely dispersed on warming to 37°C or higher. Creaming is strongly dependent on temperature and does not occur above 37°C (Figure 3.20). The milks of buffalo, sheep and goat do not exhibit flocculation and the milks of some cows exhibit little or none, apparently a genetic trait. The rate of creaming and the depth of the cream layer show considerable variation. The concentration of cryoglobulin might be expected to influence the rate of creaming and although colostrum (rich in Ig) creams well and late lactation milk (deficient in Ig) creams poorly, there is no correlation in mid-lactation milks between Ig concentration and the rate of creaming. An uncharacterized lipoprotein appears to act synergistically with cryoglobulin in promoting clustering. The rate of creaming is increased by increasing the ionic strength and retarded by acidification. High-fat milks, which also tend to have a higher proportion of larger fat globules, cream quickly, probably because the probability of collisions between globules is greater and because large globules tend to form larger aggregates. The depth of the cream layer in high-fat milks is also greater than might be expected, possibly because of greater ‘dead space’ in the interstices of aggregates formed from large globules. The rate of creaming and the depth of the cream layer are very markedly influenced by processing operations. Creaming is faster and more complete at low temperatures (c 20°C; Figure 3.20), probably because of the temperature-dependent precipitation of the cryoglobulins. Gentle (but not prolonged) agitation during the initial stages of creaming promotes and enhances cluster formation and creaming, possibly because of an increased probability of collisions. It would be expected that stirring cold milk would lead to the deposition of all the cryoglobulin on to the fat globule surfaces, and rapid creaming, without a time lag, would be expected when stirring ceased. However, milk so treated does not cream at all or only slightly after a prolonged lag period. If cold, creamed milk is agitated gently, the clusters are dispersed and do not reform unless the milk is rewarmed to c. 40°C and then recooled, i.e. the whole cycle repeated. Violent agitation is detrimental

DAIRY CHEMISTRY AND BIOCHEMISTRY to creaming, possibly due to denaturation of the cryoglobulins and/ c alteration to the fat globule surface. If milk is separated at 40C or above the cryoglobulins are present predominantly in the serum, whereas they are in the cream produced at lower temperatures. Agglutination and creaming are impaired or prevented by heating(e.g.70°C×30 min or77c×20s) owing to denaturation of the cryoglobulins; addition of Igs to heated milk restores creaming(except after very severe heat treatment, e. g. 2 min at 95C or equivalent). Homogenization prevents creaming, not only due to the reduction of fat globule size but also to some other factor since a blend of raw cream and homogenized skim milk does not cream well. In fact two types of euglobulin appear to be involved in agglutination, one of which is denatured by heating, the other by homogenization. Thus, a variety of factors which involve temperature changes, agitation or homogenization influence the rate and extent of creaming 3.10 Influence of processing operations on the fat globule membrane As discussed in section 3.8.7, the milk fat globule membrane(mfgm) relatively fragile and susceptible to damage during a range of processing operations; consequently, emulsion stability is reduced by dislodging inter facial material by agitation, homogenization, heat treatment, concentration, drying and freezing. Rearrangement of the membrane increases the suscep- tibility of the fat to hydrolytic rancidity, light-activated flavo oiling-off'of the fat, but reduces susceptibility to metal-catalysed oxidation The influence of the principal dairy processing operations on MFGM and concomitant defects are discussed below 3. 10. I Milk supply: hydrolyte The production of milk on the farm and transportation to the processing lant are potentially major causes of damage to the MFGM. Damage to the membrane may occur at several stages of the milking operation: foaming dt sucked in at teat-cups, agitation due to vertical sections(risers) in milk pipelines, constrictions and/or expansion in pipelines, pump pecially if not operating at full capacity, surface coolers, agitators in bulk tanks and freezing of milk on the walls of bulk tanks. while some oiling-off and perhaps other physical damage to the milk fat emulsion may accrue from such damage, by far the most serious consequence is the development of hydrolytic rancidity. The extent of lipolysis is commonly expressed acid degree value(ADV) of the fat as millimoles of free fatty acids per 100 g fat; ADVs greater than 1 are undesirable and are probably perceptible by taste to most people

108 DAIRY CHEMISTRY AND BIOCHEMISTRY to creaming, possibly due to denaturation of the cryoglobulins and/or alteration to the fat globule surface. If milk is separated at 40°C or above, the cryoglobulins are present predominantly in the serum, whereas they are in the cream produced at lower temperatures. Agglutination and creaming are impaired or prevented by heating (eg 70°C x 30min or 77°C x 20s) owing to denaturation of the cryoglobulins; addition of Igs to heated milk restores creaming (except after very severe heat treatment, e.g. 2 min at 95°C or equivalent). Homogenization prevents creaming, not only due to the reduction of fat globule size but also to some other factor since a blend of raw cream and homogenized skim milk does not cream well. In fact two types of euglobulin appear to be involved in agglutination, one of which is denatured by heating, the other by homogenization. Thus, a variety of factors which involve temperature changes, agitation or homogenization influence the rate and extent of creaming. 3.10 Influence of processing operations on the fat globule membrane As discussed in section 3.8.7, the milk fat globule membrane (MFGM) is relatively fragile and susceptible to damage during a range of processing operations; consequently, emulsion stability is reduced by dislodging interfacial material by agitation, homogenization, heat treatment, concentration, drying and freezing. Rearrangement of the membrane increases the susceptibility of the fat to hydrolytic rancidity, light-activated flavours and ‘oiling-off of the fat, but reduces susceptibility to metal-catalysed oxidation. The influence of the principal dairy processing operations on MFGM and concomitant defects are discussed below. 3. IO. I Milk supply: hydrolytic rancidity The production of milk on the farm and transportation to the processing plant are potentially major causes of damage to the MFGM. Damage to the membrane may occur at several stages of the milking operation: foaming due to air sucked in at teat-cups, agitation due to vertical sections (risers) in milk pipelines, constrictions and/or expansion in pipelines, pumps, especially if not operating at full capacity, surface coolers, agitators in bulk tanks and freezing of milk on the walls of bulk tanks. While some oiling-off and perhaps other physical damage to the milk fat emulsion may accrue from such damage, by far the most serious consequence is the development of hydrolytic rancidity. The extent of lipolysis is commonly expressed as ‘acid degree value’ (ADV) of the fat as millimoles of free fatty acids per 100 g fat; ADVs greater than 1 are undesirable and are probably perceptible by taste to most people

MILK LIPIDS The principal lipase in bovine milk is a lipoprotein lipase(LPL; Chapt 8)which is associated predominantly with the casein micelles and is isolated from its substrate, milk fat, by the MFGM, i.e. the enzyme and its substrate are compartmentalized. However, even slight damage to the membrane permits contact between enzyme and substrate, resulting in hydrolytic rancidity. The enzyme is optimally active at around 37 C and pH 8.5 and is stimulated by divalent cations, e.g. Ca2'*( Ca2+ complex free fatty acids, which are strongly inhibitory ) The initial turnover of milk LPl is c. 3000s, i.e. 3000 fatty acid molecules are liberated per second per mole of enzyme(milk usually contains 1-2 mg lipase", i.e. 10-20 nM)which, if fully active, is sufficient to induce rancidity in about 10s. This never happens in milk due to a variety of factors, e. g. the pH, ionic strength and usually, the temperature are not optimal; the lipase is bound to the casein micelles; the substrate is not readily available; milk probably contains lipase inhibitors, including caseins. The activity of lipase in milk is not correlated with its concentration due to the various inhibitory and adverse facto Machine milking, especially pipe- line milking systems, markedly increases the incidence of hydrolytic rancidity unless adequate precautions are taken The effectors are the clawpiece and the tube taking the milk from the clawpiece to the pipeline; damage at the clawpiece may be minimized by proper regulation of air intake, and low-line milking installations cause less damage than high-line systems but the former are more expensive and less convenient for operators, Larger-diameter pipelines(e.g. 5 cm) reduce the incidence of rancidity but may cause cleaning problems and high milk losses. The receiving jar, pump(diaphragm or centrifugal, provided they are operated properly) and type of bulk tank, including agitator, transportation in bulk tankers or preliminary processing operations(e.g. pumping and refrigerated storage)a the factory, make little if any contribution to hydrolytic rancidity The frequency and severity of lipolysis increases in late lactation, possibly wing to a weak MfGm and the low level of milk produced(which may aggravate agitation); this problem is particularly acute when milk produc- tion is seasonal, e.g. as in Ireland or New Zealand The lipase system can also be activated by cooling freshly drawn milk to 5C, rewarming to 30C and recooling to 5 C. Such a temperature cycle may occur under farm conditions, e.g. addition of a large quantity of warm milk to a small volume of cold milk. It is important that bulk tanks be emptied completely at each collection(this practice is also essential for the mainten- ance of good hygiene). No satisfactory explanation for temperature activa tion is available but changes in the physical state of fat(liquid /solid ratio) have been suggested; damage/alteration of the globule surface and binding of lipoprotein co-factor may also be involved Some cows produce milk which is susceptible to a defect known as pontaneous rancidity'-no activation treatment, other than cooling of the milk, is required; the frequency of such milks may be as high as 30% of the

MILK LIPIDS 109 The principal lipase in bovine milk is a lipoprotein lipase (LPL; Chapter 8) which is associated predominantly with the casein micelles and is isolated from its substrate, milk fat, by the MFGM, i.e. the enzyme and its substrate are compartmentalized. However, even slight damage to the membrane permits contact between enzyme and substrate, resulting in hydrolytic rancidity. The enzyme is optimally active at around 37°C and pH 8.5 and is stimulated by divalent cations, e.g. Ca2+ (CaZ+ complex free fatty acids, which are strongly inhibitory). The initial turnover of milk LPL is c. 3000 s-', i.e. 3000 fatty acid molecules are liberated per second per mole of enzyme (milk usually contains 1-2 mg lipase l-', i.e. 10-20 nM) which, if fully active, is sufficient to induce rancidity in about 10s. This never happens in milk due to a variety of factors, e.g. the pH, ionic strength and, usually, the temperature are not optimal; the lipase is bound to the casein micelles; the substrate is not readily available; milk probably contains lipase inhibitors, including caseins. The activity of lipase in milk is not correlated with its concentration due to the various inhibitory and adverse factors. Machine milking, especially pipe-line milking systems, markedly increases the incidence of hydrolytic rancidity unless adequate precautions are taken. The effectors are the clawpiece and the tube taking the milk from the clawpiece to the pipeline; damage at the clawpiece may be minimized by proper regulation of air intake, and low-line milking installations cause less damage than high-line systems but the former are more expensive and less convenient for operators. Larger-diameter pipelines (e.g. 5 cm) reduce the incidence of rancidity but may cause cleaning problems and high milk losses. The receiving jar, pump (diaphragm or centrifugal, provided they are operated properly) and type of bulk tank, including agitator, transportation in bulk tankers or preliminary processing operations (e.g. pumping and refrigerated storage) at the factory, make little if any contribution to hydrolytic rancidity. The frequency and severity of lipolysis increases in late lactation, possibly owing to a weak MFGM and the low level of milk produced (which may aggravate agitation); this problem is particularly acute when milk production is seasonal, e.g. as in Ireland or New Zealand. The lipase system can also be activated by cooling freshly drawn milk to 5"C, rewarming to 30°C and recooling to 5°C. Such a temperature cycle may occur under farm conditions, e.g. addition of a large quantity of warm milk to a small volume of cold milk. It is important that bulk tanks be emptied completely at each collection (this practice is also essential for the maintenance of good hygiene). No satisfactory explanation for temperature activation is available but changes in the physical state of fat (liquid/solid ratio) have been suggested; damage/alteration of the globule surface and binding of lipoprotein co-factor may also be involved. Some cows produce milk which is susceptible to a defect known as 'spontaneous rancidity' - no activation treatment, other than cooling of the milk, is required; the frequency of such milks may be as high as 30% of the

MILK LIPIDS 113 efficiency of separation increases with temperature, especially in the range 20-40 C In the past, separation was usually performed at 40"C or above but modern separators are very efficient even at low temperatures s discussed in section 3.9.2, cryoglobulins are entirely in the serum phase at temperatures above about 37 C, as a result of which creams prepared at these temperatures have poor natural creaming properties and the skim milk foams copiously due to the presence of cryoglobulins Following separation at low temperatures(below 10-15 C), most of the cryoglobulins remain in the cream phase. Considerable incorporation of air and foaming may occur during separation, especially with older machines, causing damage to the MFGM. The viscosity of cream produced by low-temperature separation is much higher than that produced at higher temperatures, presumably due to the presence of cryoglobulins in the former Centrifugal force is also applied in the clarification and bactofugation of milk. Clarification is used principally to remove somatic cells and physical dirt, while bactofugation, in addition to removing these, also removes 95-99% of the bacterial cells present. One of the principal applications of bactofugation is the removal of clostridial spores from milk intended for Swiss and Dutch-type cheeses, in which they cause late blowing. A large proportion (around 90%) of the bacteria and somatic cells in milk are entrapped in the fat globule clusters during natural creaming and are present in the cream layer; presumably, they become agglutinated by the cryoglobulin Homogenization is widely practised in the manufacture of liquid milk and milk products. The process essentially involves forcing milk through a small orifice(Figure 3. 23)at high pressure(13-20 m ) usually at about )C (at this temperature, the fat is liquid; homogenization is less effective at lower temperatures when the fat is partially solid). The principal effect of homogenization is to reduce the average diameter of the fat globules to below 1 um(the vast majority of the globules in homogenized milk have diameters below 2 um)(Figure 3.24). Reduction is achieved through the combined action of shearing, impingement, distention and cavitation. Fol owing a single passage of milk through a homogenizer, the small fat globules occur in clumps, causing an increase in viscosity; a second-stage homogenization at a lower pressure(e.g. 3.5 MN m )disperses the clumps and reduces the viscosity. Clumping arises from incomplete coverage of the greatly increased emulsion interfacial area during the short passage time through the homogenizer valve, resulting in the sharing of casein micelles by neighbouring globules

MILK LIPIDS 113 efficiency of separation increases with temperature, especially in the range 20-40°C. In the past, separation was usually performed at 40°C or above but modern separators are very efficient even at low temperatures. As discussed in section 3.9.2, cryoglobulins are entirely in the serum phase at temperatures above about 37"C, as a result of which creams prepared at these temperatures have poor natural creaming properties and the skim milk foams copiously due to the presence of cryoglobulins. Following separation at low temperatures (below lO-l5"C), most of the cryoglobulins remain in the cream phase. Considerable incorporation of air and foaming may occur during separation, especially with older machines, causing damage to the MFGM. The viscosity of cream produced by low-temperature separation is much higher than that produced at higher temperatures, presumably due to the presence of cryoglobulins in the former. Centrifugal force is also applied in the clarification and bactofugation of milk. Clarification is used principally to remove somatic cells and physical dirt, while bactofugation, in addition to removing these, also removes 95-99% of the bacterial cells present. One of the principal applications of bactofugation is the removal of clostridial spores from milk intended for Swiss and Dutch-type cheeses, in which they cause late blowing. A large proportion (around goo/,) of the bacteria and somatic cells in milk are entrapped in the fat globule clusters during natural creaming and are present in the cream layer; presumably, they become agglutinated by the cryoglo bulins. 3.10.3 Homogenization Homogenization is widely practised in the manufacture of liquid milk and milk products. The process essentially involves forcing milk through a small orifice (Figure 3.23) at high pressure (13-20 MNmP2), usually at about 40°C (at this temperature, the fat is liquid; homogenization is less effective at lower temperatures when the fat is partially solid). The principal effect of homogenization is to reduce the average diameter of the fat globules to below 1 pm (the vast majority of the globules in homogenized milk have diameters below 2 pm) (Figure 3.24). Reduction is achieved through the combined action of shearing, impingement, distention and cavitation. Following a single passage of milk through a homogenizer, the small fat globules occur in clumps, causing an increase in viscosity; a second-stage homogenization at a lower pressure (e.g. 3.5 MN m-2) disperses the clumps and reduces the viscosity. Clumping arises from incomplete coverage of the greatly increased emulsion interfacial area during the short passage time through the homogenizer valve, resulting in the sharing of casein micelles by neighbouring globules