Physically, milk is a dilute emulsion, colloidal dispersion and true solution.
Acidity: Fresh milk has a pH of about 6.5-6.7 at 250C. As milk stands exposed to air, its acidity decreases slightly because of the loss of carbon dioxide. Raw milk, which normally contains some lactic acid producing bacteria gradually increase in acidity on storage. Pasteurisation destroys lactic acid bacteria.
Viscosity: Factors affecting viscosity of milk are concentration of protein and fat, temperature of milk, age of milk etc. Conditions and treatments that affect the stability of casein are important in the viscosity of milk such as acidity, salt balance, heat treatment and the action of various enzymes and bacteria. Viscosity depends on the amount of fat, size of the fat globules and the extent of clustering of the globules. Homogenisation and ageing increase the viscosity.
Freezing point: The freezing point of milk is - 0.550C. The freezing point is affected by the soluble constituents, lactose and ash which are constant. This fact makes it possible to determine whether or not milk is diluted. Addition of 1 % of water to milk decreases freezing point by -0.00550C.
Boiling point: Milk boils at 100.20C. At this temperature all organisms are destroyed. The loss of nutritive value of boiled milk is more than compensated by the avoidance of milk borne diseases.
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