Milk and Its Composition
Milk and Its Composition is tested through constituent percentages, the physical state of fat and casein, and the factors that shift normal values. Treat milk as an emulsion of fat globules in a continuous aqueous phase containing lactose, proteins, minerals and salts.
Core composition
- Water about 87%; total solids 12-13%; solids-not-fat (SNF) 8.5-9.0%.
- Fat 3-5%, lactose 4.6-5.0%, total protein 3.2-3.5%. Casein is about 80% of protein; whey proteins about 20%.
- Minerals/ash about 0.7-0.8%, mainly calcium, phosphate, potassium and chloride.
- Casein exists as calcium phosphate-stabilised micelles; fat is dispersed as globules with a membrane.
Factors that change composition
Breed, species, lactation stage, feed, season, milking interval and health alter fat and SNF. Colostrum is not normal milk: it is high in protein, immunoglobulins and ash but low in lactose, and it is heat-unstable. Mastitic milk has higher chloride, higher pH and lower casein.
Worked example
A tank contains 1,000 kg milk with 4.2% fat and 8.7% SNF. Fat = 1,000 × 0.042 = 42 kg. SNF = 1,000 × 0.087 = 87 kg. Total solids = 42 + 87 = 129 kg, or 12.9%. Water = 1,000 − 129 = 871 kg. The calculation uses mass percentages, not volume percentages.
Traps
Do not add fat and SNF percentages to get water directly without converting to mass. Do not treat colostrum as saleable normal milk. Milk solids includes fat, while SNF excludes it. Breed averages are not fixed legal standards for every sample.