Milk and its processing
Milk is the secretion of the mammary glands. Approximate composition of cow milk: water about 87%, fat about 4%, protein about 3.3% (mainly casein), lactose (milk sugar) about 4.8%, minerals about 0.7%. Buffalo milk has more fat (around 7%) and total solids, and is whiter because buffaloes convert carotene to vitamin A; cow milk is slightly yellowish due to carotene. The non-fat part is called solids-not-fat (SNF).
Quality tests: lactometer measures specific gravity (cow milk about 1.028–1.032); adding water lowers it. Fat is measured by the Gerber method.
Processing steps: clean collection → chilling to about 4 °C at village or bulk coolers → transport → filtration/clarification → standardisation (adjusting fat) → pasteurisation → homogenisation (breaking fat globules so cream does not rise) → packaging → cold storage and distribution.
Pasteurisation heats milk to kill disease-causing germs without much change in taste:
- LTLT (holder) method: 63 °C for 30 minutes.
- HTST method: 72 °C for 15 seconds.
- UHT: about 135–150 °C for a few seconds; packed aseptically, keeps without refrigeration for months.
Sterilisation at higher temperature for longer kills all organisms.
Milk products:
- Cream: fat-rich layer separated by a cream separator.
- Butter: made by churning cream; about 80% fat.
- Ghee: clarified butter fat made by heating butter or cream; nearly all fat.
- Curd (dahi): milk fermented by lactic acid bacteria.
- Paneer: made by heating milk and coagulating with an acid (citric acid or lemon juice), then pressing.
- Khoa: milk concentrated by open-pan boiling; base for many sweets.
- Cheese: coagulated with rennet and ripened; also ice cream, condensed milk and milk powder (spray drying).
Marketing: cooperatives (Amul pattern) collect milk at village centres, pay by fat and SNF, process and sell under brand names, reducing middlemen.
Key points
- Main milk protein: casein; milk sugar: lactose.
- Lactometer → specific gravity; Gerber → fat.
- LTLT 63 °C/30 min; HTST 72 °C/15 s.
- Homogenisation prevents cream separation.
- Paneer: acid + heat; cheese: rennet.
Exam tips
Expect temperature–time pairs, instrument names and product definitions.
Common mistakes
- Thinking pasteurisation sterilises milk; it kills harmful germs, not all microbes.
- Mixing up homogenisation and pasteurisation.