Physico-Chemical Properties
Physico-Chemical Properties covers density, freezing point, pH, titratable acidity, viscosity, surface tension and colligative behaviour. The exam uses these properties to judge milk quality and detect added water or developed acidity.
Core values and meaning
- Density of normal mixed milk is about 1.028-1.032 g/mL at 20 C.
- Freezing point is about −0.53 to −0.56 C; boiling point is only slightly above 100 C.
- pH is usually 6.6-6.8; fresh titratable acidity is about 0.14-0.16% lactic acid.
- Density rises with SNF and falls with fat. Freezing point is depressed by dissolved lactose and salts, not by fat.
Worked example
A sample freezes at −0.40 C. Normal freezing point depression is taken as 0.53 C. Added water % = (0.53 − 0.40) / 0.53 × 100 = 24.5%. Reasoning: added water dilutes lactose and salts, so the freezing point moves closer to 0 C. A density reading alone cannot confirm watering because fat and SNF both affect density.
Traps
Do not confuse pH with titratable acidity: pH measures hydrogen ion activity, while titration measures total acid. Do not read a lactometer without temperature correction. Do not reject milk on density alone; high-fat milk can have low density while high-SNF milk can have high density. Report freezing point as a temperature, not as a percentage.