A heterogeneous liquid secreted by the mammary gland and composed (for cows' milk) of approximately 87% water, 3.8% emulsified particles of fat and fatty acids, 3% casein, 5% sugar (lactose), serum proteins, calcium, phosphorus, potassium, iron, magnesium, copper, and several vitamins. The fat particles are from 6 to 10 micrometers in diameter, much larger than colloidal size; they are coated with an adsorbed layer of protein (protective colloid), that maintains the emulsion. The casein is closely associated with the calcium, forming micelles of calcium caseinate that are of colloidal dimensions. The white color of milk is largely due to light scattering by these particles, and to some extent by the fat particles, rather than to the presence of a pigment. The casein complex coagulates (1) when high temperature or bacteria convert the lactose to lactic acid, as in souring, or (2) when acid or certain enzymes (rennet) are intentionally added. The serum proteins lactalbumin and lactoglobulin are also colloidally dispersed. The lactose and mineral salts are in true molecular solution. Thus milk is a complex system exhibiting several levels of dispersion, from the molecular through colloidal and into the microscopic size range. Important processes applied to milk on an industrial scale include pasteurization, homogenizaton, coagulation, dehydration, and condensation. The milks of animals other than cows show considerable variations in composition, especially fat and protein content.See Colloid Chemistry; Emulsion; Casein.
