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 105 apld creaming e flocculation MILK slow creaming disruption ●。 Before creaming re 3. 19 Schematic representation of different forms of emulsion tion(modified Mulder and walstra, 1974)
MILK LIPIDS 105 coalescence I rapid creaming * flocculation I slow creaming * disruption I Before creaming After creaming Figure 3.19 Schematic representation of different forms of emulsion destabilization (modified from Mulder and Walstra, 1974)
106 DAIRY CHEMISTRY AND BIOCHEMISTRY 3. 9.2 The creaming process in milk a cream layer may be evident in milk within 20 min after milking appearance of a cream layer, if formed as a result of the rise of indi globules of 4 um diameter according to Stokes'equation, would take approximately 50 h. The much more rapid rate of creaming in milk is caused by clustering of globules to form approximate spheres, ranging in diameter from 10 to 800 um. As milk is drawn from the cow, the fat exists as individual globules and the initial rate of rise is proportional to the radius (r2) of the individual globules Cluster formation is promoted by the disparity in the size of the fat globules in milk. Initially, the larger globules rise several times faster than the smaller ones and consequently overtake and collide with the slower moving small globules, forming clusters which rise at an increased rate, pick up more globules and continue to rise at a rate comme increased radius. The creaming of clusters only approximates to Stokes quation since they are irregular in geometry and contain considerable occluded serum and therefore Ap is variable
106 DAIRY CHEMISTRY AND BIOCHEMISTRY 3.9.2 A cream layer may be evident in milk within 20min after milking. The appearance of a cream layer, if formed as a result of the rise of individual globules of 4 pm diameter according to Stokes' equation, would take approximately 50 h. The much more rapid rate of creaming in milk is caused by clustering of globules to form approximate spheres, ranging in diameter from 10 to 800pm. As milk is drawn from the cow, the fat exists as individual globules and the initial rate of rise is proportional to the radius (rJ of the individual globules. Cluster formation is promoted by the disparity in the size of the fat globules in milk. Initially, the larger globules rise several times faster than the smaller ones and consequently overtake and collide with the slowermoving small globules, forming clusters which rise at an increased rate, pick up more globules and continue to rise at a rate commensurate with the increased radius. The creaming of clusters only approximates to Stokes' equation since they are irregular in geometry and contain considerable occluded serum and therefore Ap is variable. The creaming process in milk 30 t 40 0 10 20 37 Temperature ("C) Figure 3.20 Effect of temperature on the volume of cream formed after 2 h (modified from Mulder and Walstra, 1974)
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