Physical Properties of Milk
We will cover the following physical properties. More information can be found in Walstra's text.
- Freezing Point
- Acid-base Equilibria
- Optical Properties
The density of milk and milk products is used for the following;
- to convert volume into mass and vice versa
- to estimate the solids content
- to calculate other physical properties (e.g. kinematic viscosity)
Density, the mass of a certain quantity of material divided by its volume, is dependant on the following:
- temperature at the time of measurement
- temperature history of the material
- composition of the material (especially the fat content)
- inclusion of air (a complication with more viscous products)
With all of this in mind, the density of milk varies within the range of 1027 to 1033 kg /m3 at 20° C.
The following table gives the density of various fluid dairy products as a function of fat and solids-not-fat (SNF) composition:
Product Density (kg/L) at:
Product Fat (%) SNF (%) 4.4°C 10°C 20°C 38.9°C
Producer milk 4.00 8.95 1.035 1.033 1.030 1.023
Homogenized milk 3.6 8.6 1.033 1.032 1.029 1.022
Skim milk, pkg 0.02 8.9 1.036 1.035 1.033 1.026
Fortified skim 0.02 10.15 1.041 1.040 1.038 1.031
Half and half 12.25 7.75 1.027 1.025 1.020 1.010
Half and half, fort. 11.30 8.9 1.031 1.030 1.024 1.014
Light cream 20.00 7.2 1.021 1.018 1.012 1.000
Heavy cream 36.60 5.55 1.008 1.005 0.994 0.978
Viscosity of milk and milk products is important in determining the following:
- the rate of creaming
- rates of mass and heat transfer
- the flow conditions in dairy processes
Milk and skim milk, excepting cooled raw milk, exhibit Newtonian behavior, in which the viscosity is independent of the rate of shear. The viscosity of these products depends on the following:
- cooler temperatures increase viscosity due to the increased voluminosity of casein micelles
- temperatures above 65° C increase viscosity due to the denaturation of whey proteins
- pH: an increase or decrease in pH of milk also causes an increase in casein micelle voluminosity
Cooled raw milk and cream exhibit non-Newtonian behavior in which the viscosity is dependant on the shear rate. Agitation may cause partial coalescence of the fat globules (partial churning) which increases viscocity. Fat globules that have under gone cold agglutination, may be dispersed due to agitation, causing a decrease in viscosity.
Freezing point depression is a colligative property which is determined by the molarity of solutes rather than by the percentage by weight or volume. In the dairy industry, freezing point of milk is mainly used to determine added water but it can also been used to determine lactose content in milk, estimate whey powder contents in skim milk powder, and to determine water activity of cheese. The freezing point of milk is usually in the range of -0.512 to -0.550° C with an average of about -0.522° C.
Correct interpretation of freezing point data with respect to added water depends on a good understanding of the factors affecting freezing point depression. With respect to interpretation of freezing points for added water determination, the most significant variables are the nutritional status of the herd and the access to water. Under feeding causes increased freezing points. Large temporary increases in freezing point occur after consumption of large amounts of water because milk is iso-osmotic with blood. The primary sources of non-intentional added water in milk are residual rinse water and condensation in the milking system.
Both titratable acidity and pH are used to measure milk acidity. The pH of milk at 25° C normally varies within a relatively narrow range of 6.5 to 6.7. The normal range for titratable acidity of herd milks is 13 to 20 mmol/L. Because of the large inherent variation, the measure of titratable acidity has little practical value except to measure changes in acidity (eg., during lactic fermentation) and even for this purpose, pH is a better measurement.
There are many components in milk which provide a buffering action. The major buffering groups of milk are caseins and phosphate.
Optical properties provide the basis for many rapid, indirect methods of analysis such as proximate analysis by infrared absorbency or light scattering. Optical properties also determine the appearance of milk and milk products. Light scattering by fat globules and casein micelles causes milk to appear turbid and opaque. Light scattering occurs when the wave length of light is near the same magnitude as the particle. Thus, smaller particles scatter light of shorter wavelengths. Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more than the red. The carotenoid precursor of vitamin A, ß -carotene, contained in milk fat, is responsible for the 'creamy' colour of milk. Riboflavin imparts a greenish colour to whey.
Refractive index (RI) is normally determined at 20° C with the D line of the sodium spectrum. The refractive index of milk is 1.3440 to 1.3485 and can be used to estimate total solids.