Definition of Physico-Chemical Properties of Milk
Physico-chemical properties of milk refer to the physical and chemical characteristics that define the composition, behaviour, and quality of milk under various conditions. These properties determine how milk can be stored, processed, and converted into various dairy products like butter, cheese, paneer, curd, and milk powder.
Physico-Chemical Properties of Milk
The main physico-chemical properties of milk include:
1. Appearance and Colour
- Fresh milk is an opaque, white or slightly yellowish liquid.
- The white colour is due to casein micelles and fat globules that scatter light.
- Yellowish tint comes from carotene, especially in cow milk.
2. Odour and Taste
- Normal milk has a slightly sweet and pleasant aroma.
- The sweetness is due to lactose.
- Spoiled milk develops a sour or off-odour due to microbial action.
3. Specific Gravity
- Milk is heavier than water, with a specific gravity ranging from 1.028 to 1.035 at 15°C.
- This helps in detecting adulteration — addition of water reduces the specific gravity.
4. pH (Acidity)
- Fresh milk has a pH around 6.6 to 6.8, which is slightly acidic.
- As milk spoils, lactic acid bacteria convert lactose into lactic acid, lowering the pH.
- Monitoring pH helps determine milk freshness and suitability for processing.
5. Buffering Capacity
- Milk has the ability to resist changes in pH when acids or bases are added.
- Important during fermentation (like curdling), cheese production, and pasteurisation.
6. Freezing Point
- The average freezing point of milk is around -0.54°C.
- Adding water raises the freezing point — so it is used to detect adulteration.
7. Boiling Point
- Slightly higher than water, around 100.17°C, due to dissolved salts and proteins.
8. Surface Tension
- Milk has a lower surface tension than water due to the presence of fat and protein.
- This affects the formation of cream layer and foam during boiling or agitation.
9. Viscosity
- Milk is more viscous than water due to fat, proteins, and other solids.
- Viscosity affects the flow of milk through pipes, pumps, and spray dryers.
10. Refractive Index
- Used to determine solids content in milk.
- Helps in assessing milk concentration and purity.
11. Electrical Conductivity
- Depends on the amount of minerals (especially sodium and potassium).
- Useful in detecting mastitis in animals (where conductivity increases).
12. Emulsion Properties
- Milk is a natural oil-in-water emulsion — fat globules are suspended in a water-based solution.
- Stability of this emulsion affects cream separation and butter making.
13. Colloidal Nature
- Proteins like casein are present in colloidal form, which influences coagulation and cheese-making processes.
How This Knowledge Helps in Effective Quality Control and Processing
Understanding physico-chemical properties is essential at every stage of the dairy value chain, from collection and storage to processing and product manufacturing.
1. Quality Control at Procurement Stage
- Freezing point, specific gravity, and pH are used to detect adulteration or spoilage.
- Ensures that only pure and fresh milk is accepted at dairy plants.
2. Standardisation of Milk
- Before making products like toned milk, double toned milk, or standardised milk, knowledge of fat and SNF content is essential.
- Helps in adding or removing cream or skimmed milk to meet FSSAI standards.
3. Efficient Pasteurisation
- Understanding pH and buffering capacity helps set the temperature and time for pasteurisation.
- Prevents curdling and preserves nutritional value.
4. Cream and Butter Production
- Surface tension, viscosity, and emulsion properties affect the separation of cream and churning of butter.
- Helps design the right equipment and process conditions.
5. Cheese and Paneer Making
- Coagulation of casein proteins depends on pH, temperature, and colloidal nature of milk.
- Exact control over these parameters ensures better yield and texture of paneer or cheese.
6. Fermented Products (Curd, Lassi, etc.)
- Knowing the acidity and buffering capacity helps in controlling fermentation.
- Results in uniform taste, texture, and shelf life.
7. Milk Powder Production
- Boiling point, viscosity, and solid content affect the drying process.
- Spray drying efficiency and powder quality depend on these properties.
8. Packaging and Storage
- pH and microbial activity guide the selection of packaging material (like tetra pack, polypack).
- Helps in deciding the storage temperature and shelf life.
9. Detection of Spoilage or Contamination
- Rapid tests based on pH, electrical conductivity, and odour help in identifying mastitis milk, bacterial contamination, or old milk.
10. Pricing and Payment to Farmers
- Milk pricing systems like the two-axis pricing are based on fat and SNF, which are part of physico-chemical properties.
- Ensures fair compensation based on milk quality.
Conclusion
Understanding the physico-chemical properties of milk is fundamental for ensuring quality, safety, and efficiency in the dairy industry. These properties guide everything from milk testing and storage to the manufacture of value-added products like butter, paneer, cheese, and milk powder. In the Indian context, where dairy supports the livelihood of millions of rural households, this knowledge enables better income, nutrition, and food security.