Milk is a very complex fluid. It contains several hundred molecular species, mostly at trace levels.
Two physical phenomena are primarily responsible for the visual appearance of milk. First, milk is a protein-stabilized emulsion of fat in a continuous aqueous solution.
Secondly, milk is a suspension of insoluble colloidal mineral particles.
Milk is a white or yellow-white, opaque liquid. The color is influenced by scattering and absorption of light by milk fat globules and protein micelles. Therefore, skim milk also retains its white color.
A yellowish, i.e., yellow-green, color is derived from carotene (ingested primarily during pasture grazing) present in the fat phase and from riboflavin present in the aqueous phase.
The specific density of milk decreases with increasing fat content, and increases with increasing amounts of protein, milk sugar and salts.
The specific density of cow’s milk ranges from 1.029 to 1.039 (15 degree 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 in 1.0330.
Defatted (skim) milk has a higher specific density than whole milk. The freezing point of milk is -0.53 to -0.55 degree C. This rather constant value is a suitable test for detection of watering of milk.
Physical properties of liquid milk are measured by traditional methods that are included in the control of milk at receptions. Some of these properties are routinely controlled because they can give indirect information about alterations or adulterations of milk.
Physical properties of milk
