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
110 DAIRY CHEMISTRY AND BIOCHEMISTRY population. Suggested causes of spontaneous rancidity include: a second lipase located in the membrane rather than on the casein micelles: a weak membrane which does not adequately protect the fat from the normal LPL; and a high level of lipoprotein co-factor which facilitates attachment of the LPL to the fat surface; this appears to be the most probable cause Mixing of normal milk with susceptible milk in a ratio of 4: 1 prevents spontaneous rancidity and therefore the problem is not serious except in small or abnormal herds. The incidence of spontaneous rancidity increases with advancing lactation and with dry feeding Whole milk Cream Skim who Figure 3.21 Flow of cream and skim milk in the space between a pair of discs in a centrifugal ator(a); a stad ck of discs(b) and separator disc showing holes for the channelling of and spacers(caulks)(c).( From Towler. 1994.)
110 DAIRY CHEMISTRY AND BIOCHEMISTRY population. Suggested causes of spontaneous rancidity include: 0 a second lipase located in the membrane rather than on the casein micelles; 0 a weak membrane which does not adequately protect the fat from the normal LPL; and 0 a high level of lipoprotein co-factor which facilitates attachment of the LPL to the fat surface; this appears to be the most probable cause. Mixing of normal milk with susceptible milk in a ratio of 4: 1 prevents spontaneous rancidity and therefore the problem is not serious except in small or abnormal herds. The incidence of spontaneous rancidity increases with advancing lactation and with dry feeding. r”’ / Who‘e mi’k ( b) Figure 3.21 Flow of cream and skim milk in the space between a pair of discs in a centrifugal separator (a); a stack of discs (b); and a separator disc showing holes for the channelling of milk and spacers (caulks) (c). (From Towler, 1994.)
MILK LIPIDS 111 Figure 3.21 (Continued) 3. 10.2 Mechanical separation of milk Gravity creaming is relatively efficient, especially in the cold(a fat content of 0. 1% in the skim phase may be obtained). However, it is slow and inconvenient for industrial-scale operations. Mechanical milk separators were developed independently in the 1880s by alpha and Laval; schematic representations of a modern separator are shown in Figures 3. 21 and 3. 22. In centrifugal separation, g in Stokes'equation is replaced by centrifugal force,a2R, where a is the centrifugal speed in radianss-I(2 radi ans= 360 )and R is the distance (cm)of the particle from the axis of (2R (2xS)2 where S is the bowl speed in r.p.m. Inserting this value for g into Stokes equation and simplifying gives 000244P1-p2)r2S2R n Thus, the rate of separation is influenced by the radius of the fat globules, he radius and speed of the separator, the difference in density of the continuous ar nd dispersed phases and the viscosity of the milk; temperature influences r,(p1-p2)and n
MILK LIPIDS 111 Figure 3.21 (Continued), 3.10.2 Mechanical separation of milk Gravity creaming is relatively efficient, especially in the cold (a fat content of 0.1% in the skim phase may be obtained). However, it is slow and inconvenient for industrial-scale operations. Mechanical milk separators were developed independently in the 1880s by Alpha and Laval; schematic representations of a modern separator are shown in Figures 3.21 and 3.22. In centrifugal separation, g in Stokes' equation is replaced by centrifugal force, wZR, where w is the centrifugal speed in radianss-' (2n radians = 360") and R is the distance (cm) of the particle from the axis of rotation. where S is the bowl speed in r.p.m. Inserting this value for g into Stokes' equation and simplifying gives: O.O0244(p - pz)rZS2R rl Thus, the rate of separation is influenced by the radius of the fat globules, the radius and speed of the separator, the difference in density of the continuous and dispersed phases and the viscosity of the milk; temperature influences r, (pi - p2) and q. V=
DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 3. 22 Cutaway diagram of a modern milk separator( from Towler. 1994) Fat globules of less than 2 um diameter are incompletely removed by cream separators and since the average size of fat globules decreases with dvancing lactation(Figure 3. 15), the efficiency of separation decreases oncomitantly. The percentage fat in cream is regulated by manipulating the ratio of cream to skim-milk streams from the separator, which in effect regulates back-pressure. With any particular separator operating under more or less fixed conditions, temperature is the most important variable affecting the efficiency of separation via its effects on r, n and(P1-p2). The
112 DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 3.22 Cutaway diagram of a modern milk separator (from Towler, 1994). Fat globules of less than 2pm diameter are incompletely removed by cream separators and since the average size of fat globules decreases with advancing lactation (Figure 3.19, the efficiency of separation decreases concomitantly. The percentage fat in cream is regulated by manipulating the ratio of cream to skim-milk streams from the separator, which in effect regulates back-pressure. With any particular separator operating under more or less fixed conditions, temperature is the most important variable affecting the efficiency of separation via its effects on r, q and (pl - pz). 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