Milk: composition, physical and chemical properties
What this domain tests
This section tests how well you know the gross composition of milk and the physical and chemical reasons behind routine dairy measurements. Expect questions on fat, SNF, density, freezing point, acidity and pH, and on why buffalo and cow milk differ.
Core facts
- Milk is about 87% water; cow milk commonly contains 3.5–4.5% fat and 8.5–9.0% SNF, while buffalo milk is higher in fat, often 6–7%, and in SNF.
- Fat is an emulsion of globules; casein is present as colloidal micelles; lactose and salts are in true solution.
- Fresh milk pH is about 6.6–6.8; titratable acidity is about 0.14–0.16% lactic acid.
- Density at 20°C is about 1.028–1.032 g/mL; freezing point is about −0.53 to −0.55°C.
- Heating coagulates whey proteins and alters casein; it also reduces cream layer formation.
Worked example
A 100 kg lot contains 4.0% fat and 8.6% SNF. Fat = 0.040 × 100 = 4.0 kg. SNF = 0.086 × 100 = 8.6 kg. Total solids = 12.6 kg, so water = 100 − 12.6 = 87.4 kg. This mass-balance approach is the basis for standardization and pricing calculations.
Traps
Do not treat pH and titratable acidity as the same number: pH measures hydrogen-ion activity, while titration measures total acid. Density falls as fat rises and changes with temperature, so a lactometer reading alone cannot prove adulteration. Colostrum is not normal milk: it has much higher solids and protein.