11 Physical properties of milk Milk is a dilute emulsion consisting of an oil /fat dispersed phase and an aqueous colloidal continuous phase. The physical properties of milk are imilar to those of water but are modified by the presence of various solutes (proteins, lactose and salts)in the continuous phase and by the degree of dispersion of the emulsified and colloidal components Data on the physical properties of milk are important since such parameters can infiuence the design and operation of dairy processing equipment (e.g. thermal conductivity or viscosity) or can be used to determine the concentration of specific components in milk(e.g. use of the elevation in freezing point to estimate added water or specific gravity to stimate solids-not-fat), or to assess the extent of biochemical changes in the milk during processing(e.g acidification by starter or the development of a rennet coagulum). Some important physical properties of milk are sum marized in table 11.1 Table 11.1 Some physical properties of milk (Walstra and Jenness, 1984: Sherbon, 1988; Singh, Osmotic pressure ~100.15°C 0.522C(approx ex 1.3440-1.3485 ctive index Density(20 C) gravity(20°C) 0.0050ohm"1cm 08M 27 mPa s 0.559W Thermal diffusivity(15-20C) 125×10个 331kK table acidity .3-2.0 meq oH per 100 ml (0.14-0.16% as lactic acid) Coefficient of cubic expansion(273-333 K) 0.0008m3m-3K Redox potential (25.C, pH 6.6, in equilibrium with air) +0.25to+0.35v
11 Physical properties of milk Milk is a dilute emulsion consisting of an oil/fat dispersed phase and an aqueous colloidal continuous phase. The physical properties of milk are similar to those of water but are modified by the presence of various solutes (proteins, lactose and salts) in the continuous phase and by the degree of dispersion of the emulsified and colloidal components. Data on the physical properties of milk are important since such parameters can influence the design and operation of dairy processing equipment (e.g. thermal conductivity or viscosity) or can be used to determine the concentration of specific components in milk (e.g. use of the elevation in freezing point to estimate added water or specific gravity to estimate solids-not-fat), or to assess the extent of biochemical changes in the milk during processing (e.g. acidification by starter or the development of a rennet coagulum). Some important physical properties of milk are summarized in Table 11.1. Table 11.1 Some physical properties of milk (Walstra and Jenness, 1984; Sherbon, 1988; Singh, McCarthy and Lucey, 1997) Osmotic pressure - 700 kPa a, -0.993 Boiling point - 100.15"C Freezing point -0.522"C (approx.) Refractive index, np 1.3440-1.3485 Specific refractive index -0.2075 Density (20°C) Specific gravity (20°C) .. 1.0321 Specific conductance -0.OO50 ohm-' cm-' Ionic strength -0.08 M Surface tension (20°C) Coefficient of viscosity Thermal conductivity (2.9% fat) Thermal diffusivity (15-20°C) Specific heat pH (at 25°C) - 6.6 Titratable acidity Coefficient of cubic expansion (273-333 K) Redox potential (25"C, pH 6.6, in equilibrium with air) - 1030 kg m-3 -52 N m-' 2.127 mPa s -0.559 W m-' K-' - 1.25 x lo-' mz s-' -3.931 kJ kg-' K-' 1.3-2.0 meq OH- per 100 mi (0.14-0.16% as lactic acid) 0.0008 m3 m-3 K-' +0.25 to +0.35V
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:
PHYSICAL PROPERTIES OF MILK 040 ethylene npopheno 十02 Ascor bat● Riboflay 0.20 H, Electrode 0.30 0.40 H Figure 11.1 Effect of pH on the oxidation-reduction potential of various systems(from aldehyde group of lactose can be oxidized to a carboxylic acid(lactobionic acid)at alkaline pH but this system contributes little to the redox properties of milk at pH 6.6 The En of milk is influenced by exposure to light and by a number processing operations, including those which cause changes in the concen tration of O2 in the milk. Addition of metal ions(particularly Cu2+)also infuences the redox potential Heating of milk causes a decrease in its e
PHYSICAL PROPERTIES OF MILK 44 1 PH Figure 11.1 Effect of pH on the oxidation-reduction potential of various systems (from Sherbon, 1988). aldehyde group of lactose can be oxidized to a carboxylic acid (lactobionic acid) at alkaline pH but this system contributes little to the redox properties of milk at pH 6.6. The E, of milk is influenced by exposure to light and by a number of processing operations, including those which cause changes in the concentration of 0, in the milk. Addition of metal ions (particularly CuZf) also influences the redox potential. Heating of milk causes a decrease in its E