《乳品生物化学》（英文版） 11 Physical properties of milk.pdf_大学文库

DAIRY CHEMISTRY AND BIOCHEMISTRY 11. 1 lonic strength The ionic strength, I of a solution is defined where ci is the molar concentration of the ion of type i and z; is its charge The ionic strength of milk is c. 0.08 M 11.2 Density The density (e)of a substance is its mass per unit volume, while its specific gravity (SG)or relative density is the ratio of the density of the substance to that of water(pw )at a specified temperature P= m SG=p/ (11.3) (114) The thermal expansion coefficient governs the influence of temperature density and therefore it is necessary to specify temperature when discussing density or specific gravity. The density of milk is of consequence since fluid milk is normally retailed by volume rather than by mass Measurement of the density of milk using a hydrometer (lactometer) has also beer estimate its total solids content The density of bulk milk (4% fat and 8.95% solids-not-fat) at 20C is approximately 1030 kg m 3and its specific gravity is 1.0321. Milk fat has density of about 902 kg m at 40C. The density of a given milk sample is infuenced by its storage history since it is somewhat dependent on the liquid to solid fat ratio and the degree of hydration of proteins. To minimize effects of thermal history on its density, milk is usually prewarmed to 40-45C to liquify the milk fat and then cooled to the assay temperature ( often20°C The density and specific gravity of milk vary somewhat with breed. M from Ayrshire cows has a mean specific gravity of 1.0317 while that of Jersey and Holstein milks is 1.0330. Density varies with the composition of the milk and its measurement has been used to estimate the total solids content of milk. The density of a multicomponent mixture(like milk) is related to the density of its components by 1/p=X(m/p3) where mx is the mass fraction of component x, and p, its apparent density in the mixture. This apparent density is not normally the same as the true density of the substance since a contraction usually occurs when two components are mixed

438 DAIRY CHEMISTRY AND BIOCHEMISTRY 11.1 Ionic strength The ionic strength, I, of a solution is defined as: (11.1) 12 I = zccizi where ci is the molar concentration of the ion of type i and zi is its charge. The ionic strength of milk is c. 0.08 M. 11.2 Density The density (p) of a substance is its mass per unit volume, while its specific gravity (SG) or relative density is the ratio of the density of the substance to that of water (p,) at a specified temperature: p = m/V (11.2) SG = P/Pw (11.3) P = SGPW (11.4) The thermal expansion coefficient governs the mfluence of temperature on density and therefore it is necessary to specify temperature when discussing density or specific gravity. The density of milk is of consequence since fluid milk is normally retailed by volume rather than by mass. Measurement of the density of milk using a hydrometer (lactometer) has also been used to estimate its total solids content. The density of bulk milk (4% fat and 8.95% solids-not-fat) at 20°C is approximately 1030kgm-3 and its specific gravity is 1.0321. Milk fat has a density of about 902kgm-3 at 40°C. The density of a given milk sample is influenced by its storage history since it is somewhat dependent on the liquid to solid fat ratio and the degree of hydration of proteins. To minimize effects of thermal history on its density, milk is usually prewarmed to 40-45°C to liquify the milk fat and then cooled to the assay temperature (often 20°C). The density and specific gravity of milk vary somewhat with breed. Milk from Ayrshire cows has a mean specific gravity of 1.0317 while that of Jersey and Holstein milks is 1.0330. Density varies with the composition of the milk and its measurement has been used to estimate the total solids content of milk. The density of a multicomponent mixture (like milk) is related to the density of its components by: 1lP = C(mx/px> (11.5) where m, is the mass fraction of component x, and p, its apparent density in the mixture. This apparent density is not normally the same as the true density of the substance since a contraction usually occurs when two components are mixed

PHYSICAL PROPERTIES OF MILK Equations have been developed to estimate the total solids content of milk based on fat and specific gravity(usually estimated using lactometer). Such equations are empirical and sufer from a number of drawbacks: for further discussion see Jenness and Patton(1959). The principal problem is the fact that the coefficient of expansion of milk fat is high and it contracts slowly on cooling and therefore the density of milk fat Chapter 3)is not constant. Variations in the composition of milk fat and in the proportions of other milk constitiuents have less influence on these equations than the physical state of the fat In addition to lactometry (determination of the extent to which a hydrometer sinks), the density of milk can be measured by pycnometry (determination of the mass of a given volume of milk), by hydrostatic weighing of an immersed bulb(e.g. Westphal balance), by dilatometry (measurement of the volume of a known mass of milk) or by measuring the distance that a drop of milk falls through a density gradient column 11.3 Redox properties of milk Oxidation-reduction(redox) reactions involve the transfer of an electron from an electron donor(reducing agent)to an electron acceptor(oxidizing agent). The species that loses electrons is said to be oxidized while that which accepts electrons is reduced. Since there can be no net transfer of electrons to or from a system, redox reactions must be coupled and the oxidation reaction occurs simultaneously with a reduction reaction The tendency of a system to accept or donate electrons is measured using stem into this electrode, which is thus a half-cell. the pt electrode is connected via a potentiometer to another half-cell of known potential(usually,a saturated calomel electrode). All potentials are referred to the hydrogen half-cell IH2#H++e which by convention is assigned a potential of zero when an inert electrode is placed in a solution of unit activity with respect to h*(i.e. ph=0) in equilibrium with H, gas at a pressure of 1.013 x 105 Pa(1 atm). The redox potential of a solution, Eh, is the potential of the half-cell at the inert electrode and is expressed as volts. E, depends not only on the substances present in the half-cell but also on the concentrations of their oxidized and reduced forms. The relationship between E and the concentrations of the oxidized and reduced forms of the compound is described by the Nernst Eb=E。- RT/nF In a/a where E, is the standard redox potential (i.e. potential when reactant and

PHYSICAL PROPERTIES OF MILK 439 Equations have been developed to estimate the total solids content of milk based on % fat and specific gravity (usually estimated using a lactometer). Such equations are empirical and suffer from a number of drawbacks; for further discussion see Jenness and Patton (1959). The principal problem is the fact that the coefficient of expansion of milk fat is high and it contracts slowly on cooling and therefore the density of milk fat (Chapter 3) is not constant. Variations in the composition of milk fat and in the proportions of other milk constitiuents have less influence on these equations than the physical state of the fat. In addition to lactometry (determination of the extent to which a hydrometer sinks), the density of milk can be measured by pycnometry (determination of the mass of a given volume of milk), by hydrostatic weighing of an immersed bulb (e.g. Westphal balance), by dialatometry (measurement of the volume of a known mass of milk) or by measuring the distance that a drop of milk falls through a density gradient column. 11.3 Redox properties of milk Oxidation-reduction (redox) reactions involve the transfer of an electron from an electron donor (reducing agent) to an electron acceptor (oxidizing agent). The species that loses electrons is said to be oxidized while that which accepts electrons is reduced. Since there can be no net transfer of electrons to or from a system, redox reactions must be coupled and the oxidation reaction occurs simultaneously with a reduction reaction. The tendency of a system to accept or donate electrons is measured using an inert electrode (typically platinum). Electrons can pass from the system into this electrode, which is thus a half-cell. The Pt electrode is connected via a potentiomenter to another half-cell of known potential (usually, a saturated calomel electrode). All potentials are referred to the hydrogen half-cell: +H, P H+ + e- (11.6) which by convention is assigned a potential of zero when an inert electrode is placed in a solution of unit activity with respect to H+ (i.e. pH = 0) in equilibrium with H, gas at a pressure of 1.013 x lo5 Pa (1 atm). The redox potential of a solution, Eh, is the potential of the half-cell at the inert electrode and is expressed as volts. E, depends not only on the substances present in the half-cell but also on the concentrations of their oxidized and reduced forms. The relationship between E, and the concentrations of the oxidized and reduced forms of the compound is described by the Nernst equation: E, = E, - RT/nF In ared/aox (11.7) where E, is the standard redox potential (i.e. potential when reactant and

DAIRY CHEMISTRY AND BIOCHEMISTRY product are at unit activity ) n is the number of electrons transferred per molecule, R is the universal gas constant(8.314JK-Imol- ) T is tempera- ture(in Kelvin), F is the Faraday constant(96.5kJV-Imol-)and ared and aox are activities of the reduced and oxidized forms, respectively For dilute solutions, it is normal to approximate activity by molar concentration Equation 11. 7 can be simplified, assuming a temperature of 25.C, a transfer of one electron and that activity a concentration Eh= Eo+0.059 log [Ox]/[Red] Thus, E becomes more positive as the concentration of the oxidized form E=Eoto.059 log [Ox]/[Red]-0059 pH (11.9) The Eh of milk is usually in the range +0. 25 to +0.35V at 25C, at pH 6.6 to 6.7 and in equilibrium with air(Singh, McCarthy and lucey The infuence of ph on the redox potential of a number of systems is shown The concentration of dissolved oxygen is the principal factor affecting the redox potential of milk. Milk is essentially free of O2 when secreted but in equilibrium with air, its O, content is about 0.3 mM. The redox potential of anaerobically drawn milk or milk which has been depleted of dissolved oxygen by microbial growth or by displacement of O2 by other gases is more negative than that of milk containing dissolved O2 The concentration of ascorbic acid in milk (11. 2-17. 2 mgl")is sufficient to infuence its redox potential In freshly drawn milk, all ascorbic acid is the reduced form but can be oxidized reversibly to dehydroascorbate, which e present as a hydrated hemiketal in aqueous systems. Hydrolysis of the tone ring of dehydroascorbate, which results in the formation of 2, 3- diketogulonic acid, is irreversible( Figure 11.2) The oxidation of ascorbate to dehydroascorbate is influenced by O2 partial pressure, pH and temperature and is catalysed by metal (particularly Cu2+, but also Fe 3+). The ascorbate/dehydroascorbate syster in milk stabilizes the redox potential of oxygen-free milk at c 0.0 V and that of oxygen-containing milk at +0.20 to +0.30 V(Sherbon, 1988). Riboflavin can also be oxidized reversibly but its concentration in milk(c. 4uM)is thought to be too low to have a significant influence on redox potenial. The lactate-pyruvate system(which is not reversible unless enzyme-catalysed )is thought not to be significant in influencing the redox potential of milk since it, too, is present at very low concentations. at the concentrations at which they occur in milk, low molecular mass thiols(e. g. free cysteine) have an insignificant influence on the redox potential of milk. Thiol-disulphide interactions between cysteine residues of proteins influence the redox properties of heated milks in which the proteins are denatured. The free

440 DAIRY CHEMISTRY AND BIOCHEMISTRY product are at unit activity), n is the number of electrons transferred per molecule, R is the universal gas constant (8.314JK-'mol-'), T is temperature (in Kelvin), F is the Faraday constant (96.5 kJ V- ' mol-') and ured and uox are activities of the reduced and oxidized forms, respectively. For dilute solutions, it is normal to approximate activity by molar concentration. Equation 11.7 can be simplified, assuming a temperature of 25"C, a transfer of one electron and that activity E, = E, + 0.059 log [Ox]/[Red]. (11.8) Thus, E, becomes more positive as the concentration of the oxidized form of the compound increases. E, is influenced by pH since pH affects the standard potential of a number of half-cells. The above equation becomes: E, = E, + 0.059 log [Ox]/[Red] - 0.059 pH. (11.9) The E, of milk is usually in the range + 0.25 to + 0.35 V at 25"C, at pH 6.6 to 6.7 and in equilibrium with air (Singh, McCarthy and Lucey, 1997). The influence of pH on the redox potential of a number of systems is shown in Figure 11.1. The concentration of dissolved oxygen is the principal factor affecting the redox potential of milk. Milk is essentially free of 0, when secreted but in equilibrium with air, its 0, content is about 0.3 mM. The redox potential of anaerobically drawn milk or milk which has been depleted of dissolved oxygen by microbial growth or by displacement of 0, by other gases is more negative than that of milk containing dissolved 0,. The concentration of ascorbic acid in milk (1 1.2- 17.2 mgl- ') is sufficient to influence its redox potential. In freshly drawn milk, all ascorbic acid is in the reduced form but can be oxidized reversibly to dehydroascorbate, which is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the lactone ring of dehydroascorbate, which results in the formation of 2,3- diketogulonic acid, is irreversible (Figure 11.2). The oxidation of ascorbate to dehydroascorbate is influenced by 0, partial pressure, pH and temperature and is catalysed by metal ions (particularly Cu2 +, but also Fe3 +). The ascorbate/dehydroascorbate system in milk stabilizes the redox potential of oxygen-free milk at c. 0.0 V and that of oxygen-containing milk at + 0.20 to + 0.30 V (Sherbon, 1988). Riboflavin can also be oxidized reversibly but its concentration in milk (c. 4pM) is thought to be too low to have a significant influence on redox potenial. The lactate-pyruvate system (which is not reversible unless enzyme-catalysed) is thought not to be significant in influencing the redox potential of milk since it, too, is present at very low concentations. At the concentrations at which they occur in milk, low molecular mass thiols (e.g. free cysteine) have an insignificant influence on the redox potential of milk. Thiol-disulphide interactions between cysteine residues of proteins influence the redox properties of heated milks in which the proteins are denatured. The free concentration:

DAIRY CHEMISTRY AND BIOCHEMISTRY mately 700kPa at 20C), all of which are interrelated. Since the osmotic pressure of milk remains essentially constant(because it is regulated by that of the cow's blood), the freezing point is also relatively constant The freezing point of an aqueous solution is governed by the concentra- tion of solutes in the solution. The relationship between the freezing point of a simple aqueous solution and concentration of solute is described by a relation based on Raoult s law: T=Ke (11.10) where T is the difference between the freezing point of the solution and that of the solvent, K is the molal depression constant( 1.86C for water) and m is the molal concentration of solute. However, this equation is valid only for dilute solutions containing undissociated solutes. Raoult's law is thus limited to approximating the freezing point of milk The freezing point of bovine milk is usually in the range -0.512 to 0.550C, with a mean value close to -0.522C( Sherbon, 1988)or 0.540C (Jenness and Patton, 1959). Despite variations in the conc tions of individual solutes, the freezing point depression of milk is quite constant since it is proportional to the osmotic pressure of milk(approxi mately 700kPa at 20 C), which is regulated by that of the cows blood. The freezing point of milk is more closely related to the osmotic pressure of mammary venous blood than to that of blood from the jugular vein. Owing to their large particle or molecular mass, fat globules, casein nicelles and whey proteins do not have a significant effect on the freezing point of milk, to which lactose makes the greatest contribution. The freezing nt depression in milk due to lactose alone has been calculated to be 0.296.C. Assuming a mean freezing point depression of 0.522 C, all other constituents in milk depress the freezing point by only 0. 226C. Chloride is also an important contributor to the colligative properties of milk. Assum ing a CI concentration of 0.032 M and that Cl- is accompanied by a monovalent cation (i.e. Na or K), the freezing point depression caused by CI" and its associated cation is 0. 119C. Therefore, lactose, chloride and its accompanying cations together account for about 80% of the freezing point depression in milk. Since the total osmotic pressure of milk is regulated by that of the cows blood there is a strong inverse correlation between lactose and chloride concentrations( Chapter 5) Natural variation in the osmotic pressure of milk(and hence freezing point)is limited by the physiology of the mammary gland. variations in the freezing point of milk have been attributed to seasonality, feed, stage of lactation, water intake, breed of cow, heat stress and time of day. These factors are often interrelated but have relatively little infuence on the freezing point of milk. Likewise, unit operations in dairy processing which do not influence the net number of osmotically active molecules/ ions in solution do not influence the freezing point, Cooling or heating milk causes

444 DAIRY CHEMISTRY AND BIOCHEMISTRY mately 700 kPa at 20"C), all of which are interrelated. Since the osmotic pressure of milk remains essentially constant (because it is regulated by that of the cow's blood), the freezing point is also relatively constant. The freezing point of an aqueous solution is governed by the concentration of solutes in the solution. The relationship between the freezing point of a simple aqueous solution and concentration of solute is described by a relation based on Raoult's law: Tf = K,m (11.10) where is the difference between the freezing point of the solution and that of the solvent, K, is the molal depression constant (136°C for water) and m is the molal concentration of solute. However, this equation is valid only for dilute solutions containing undissociated solutes. Raoult's law is thus limited to approximating the freezing point of milk. The freezing point of bovine milk is usually in the range -0.512 to -O.55O0C, with a mean value close to -02~22°C (Sherbon, 1988) or - 0.540"C (Jenness and Patton, 1959). Despite variations in the concentrations of individual solutes, the freezing point depression of milk is quite constant since it is proportional to the osmotic pressure of milk (approximately 700 kPa at 20"C), which is regulated by that of the cow's blood. The freezing point of milk is more closely related to the osmotic pressure of mammary venous blood than to that of blood from the jugular vein. Owing to their large particle or molecular mass, fat globules, casein micelles and whey proteins do not have a significant effect on the freezing point of milk, to which lactose makes the greatest contribution. The freezing point depression in milk due to lactose alone has been calculated to be 0.296"C. Assuming a mean freezing point depression of 0.522"C, all other constituents in milk depress the freezing point by only 0.226"C. Chloride is also an important contributor to the colligative properties of milk. Assuming a C1- concentration of 0.032M and that C1- is accompanied by a monovalent cation (i.e. Na' or K'), the freezing point depression caused by C1- and its associated cation is 0.119"C. Therefore, lactose, chloride and its accompanying cations together account for about 80% of the freezing point depression in milk. Since the total osmotic pressure of milk is regulated by that of the cow's blood, there is a strong inverse correlation between lactose and chloride concentrations (Chapter 5). Natural variation in the osmotic pressure of milk (and hence freezing point) is limited by the physiology of the mammary gland. Variations in the freezing point of milk have been attributed to seasonality, feed, stage of lactation, water intake, breed of cow, heat stress and time of day. These factors are often interrelated but have relatively little influence on the freezing point of milk. Likewise, unit operations in dairy processing which do not influence the net number of osmotically active molecules/ions in solution do not influence the freezing point. Cooling or heating milk causes

PHYSICAL PROPERTIES OF MILK transfer of salts to or from the colloidal state. However evidence for an effect of cooling or moderate heating(e.g. HTST pasteurization or minimum UhT processing)on the freezing point of milk is contradictory, perhaps ince such changes are slowly reversible over time. Direct UHT treatment involves the addition of water(through condensed steam). This additional water should be removed during fash cooling, which also removes gases, e.g. CO2, from the milk, causing a small increase in freezing point. Vacuum treatment of milk, i.e. vacreation(to remove taints), has been shown to increase its freezing point, presumably by degassing. However, if vacuum treatment is severe enough to cause a significant loss of water, the freezing CO2. Fermentation of milk has a large effect on its freezing point sina point will be reduced, thus compensating fully or partially for the loss fermentation of 1 mol lactose results in the formation of 4 mol lactic acid Likewise, fermentation of citrate influences the freezing point of milk Accurate measurement of the freezing point depression in milk requires great care. The principle used is to supercool the milk sample(by 1.0 to 1. C), to induce crystallization of ice, after which the temperature increases rapidly to the freezing point of the sample(Figure 11.4). For water, the temperature at the freezing point will remain constant until all the latent heat of fusion has been removed (i.e. until all the water is frozen ). However, for milk the temperature is stable at this maximum only momentarily and falls rapidly because ice crystallization causes concentration of solutes which eads to a further depression of freezing point. The observed freezing point of milk(maximum temperature after initiation of crystallization) is not the same as its true freezing point since some ice crystallization will have occurred before the maximum temperature is reached. Correction factors have been suggested to account for this but, in practice, it is usual to report T(°C) Observed freez point of milk sample 0.522 -1.5 Induction of crystallization Figure 11.4 Temperature-time curve for the freezing of milk

PHYSICAL PROPERTIES OF MILK 445 transfer of sdts to or from the colloidal state. However, evidence for an effect of cooling or moderate heating (e.g. HTST pasteurization or minimum UHT processing) on the freezing point of milk is contradictory, perhaps since such changes are slowly reversible over time. Direct UHT treatment involves the addition of water (through condensed steam). This additional water should be removed during flash cooling, which also removes gases, e.g. CO,, from the milk, causing a small increase in freezing point. Vacuum treatment of milk, i.e. vacreation (to remove taints), has been shown to increase its freezing point, presumably by degassing. However, if vacuum treatment is severe enough to cause a significant loss of water, the freezing point will be reduced, thus compensating fully or partially for the loss of CO,. Fermentation of milk has a large effect on its freezing point since fermentation of 1 mol lactose results in the formation of 4 mol lactic acid. Likewise, fermentation of citrate influences the freezing point of milk. Accurate measurement of the freezing point depression in milk requires great care. The principle used is to supercool the milk sample (by 1.0 to 1.2"C), to induce crystallization of ice, after which the temperature increases rapidly to the freezing point of the sample (Figure 11.4). For water, the temperature at the freezing point will remain constant until all the latent heat of fusion has been removed (i.e. until all the water is frozen). However, for milk the temperature is stable at this maximum only momentarily and falls rapidly because ice crystallization causes concentration of solutes which leads to a further depression of freezing point. The observed freezing point of milk (maximum temperature after initiation of crystallization) is not the same as its true freezing point since some ice crystallization will have occurred before the maximum temperature is reached. Correction factors have been suggested to account for this but, in practice, it is usual to report M v) -1.5 -. Induction of crystallization Time Figure 11.4 Temperature-time curve for the freezing of milk