Definition: Physico-Chemical Properties of Milk
The physico-chemical properties of milk refer to the physical and chemical characteristics of milk that define its composition, appearance, texture, behavior during processing, and nutritional value. These properties include density, viscosity, pH, acidity, freezing point, specific gravity, refractive index, heat stability, and chemical composition such as fat, protein, lactose, minerals, and water content.
Milk is a natural emulsion, mainly of fat globules suspended in an aqueous solution of proteins, lactose, and minerals. Understanding its physico-chemical properties is crucial for maintaining quality, safety, and consistency in dairy processing.
Important Physico-Chemical Properties of Milk
1. Composition of Milk
Milk is a complex mixture. The average composition of cow’s milk is:
- Water: 87.0%
- Fat: 3.5% to 4.5%
- Protein: 3.0% to 3.5% (mainly casein and whey proteins)
- Lactose (milk sugar): 4.8% to 5.0%
- Minerals (Ash): 0.7%
The composition varies based on species (cow, buffalo, goat), breed, stage of lactation, feed, and health of the animal.
2. Density and Specific Gravity
- Specific Gravity of milk ranges from 1.028 to 1.035 at 15°C.
- Fat is lighter than water, while solids-not-fat (SNF) are heavier. Hence, skim milk has higher specific gravity than whole milk.
Application: Helps in detecting adulteration (e.g., water addition) and measuring solids.
3. pH and Acidity
- Fresh milk has a pH of about 6.6 to 6.8 (slightly acidic).
- Titratable acidity of cow milk is around 0.13% to 0.17% lactic acid.
Application: pH is critical for:
- Monitoring milk freshness and spoilage.
- Coagulation of casein during curd or cheese production.
- Heat stability during sterilization or UHT treatment.
4. Freezing Point
- Average freezing point of milk is –0.53°C to –0.55°C.
- This is used to detect adulteration with water, as added water raises the freezing point.
5. Refractive Index
- It is the measure of how light bends through milk. Usually applied for milk fat measurement.
- Helps in quality checking and standardization.
6. Viscosity
- Milk viscosity varies with fat content and temperature.
- Whole milk is more viscous than skim milk.
- Viscosity influences processing operations like pumping, homogenization, and evaporation.
7. Surface Tension
- Surface tension of milk is around 0.050 N/m at 20°C.
- Influences foam formation and stability in milk beverages.
8. Heat Stability
- Heat stability of milk depends on protein and mineral balance.
- Important for UHT processing, condensed milk, and sterilization.
9. Optical Properties
- Milk appears white and opaque due to the scattering of light by fat globules and casein micelles.
- Important in consumer appeal and marketing.
10. Buffers in Milk
- Milk contains natural buffer systems (mainly phosphates and proteins) that resist sudden changes in pH.
Application: Helps in fermentation and in maintaining stability during processing.
How Knowledge of Physico-Chemical Properties Helps in Quality Control and Dairy Processing
Understanding the physico-chemical properties of milk is essential for maintaining quality, designing processing methods, and ensuring food safety.
A. In Quality Control
1. Detection of Adulteration
- Measuring specific gravity, freezing point, and fat content helps identify if water or other substances have been added.
- Example: Water lowers specific gravity and raises the freezing point of milk.
2. Freshness and Spoilage Check
- pH and acidity levels indicate microbial activity.
- Rising acidity or dropping pH signals souring or spoilage.
3. Standardization of Milk
- Milk is standardized to specific fat and SNF levels before sale.
- Requires knowledge of composition and dilution/concentration ratios.
4. Grading of Milk
- Based on fat, protein, and SNF, milk is graded into different qualities for payment and processing suitability.
B. In Dairy Processing
1. Pasteurization and Sterilization
- Heat treatment is designed based on heat stability of proteins and pH.
- Poor understanding may lead to coagulation or curdling.
2. Fermented Products (Curd, Yogurt, Lassi)
- pH and acidity control are critical to ensure proper texture and taste.
- Knowledge of buffer capacity helps control fermentation speed.
3. Cheese and Paneer Production
- Coagulation depends on casein properties and pH.
- Understanding milk’s protein structure allows better yield and texture control.
4. Butter and Ghee Production
- Requires churning fat globules, so knowledge of emulsion properties and fat content is essential.
- Melting point and moisture levels affect ghee quality.
5. Milk Powder and Condensed Milk
- Knowledge of solids content, heat stability, and water activity is important during drying and evaporation.
6. Ice Cream and Dairy Desserts
- Requires control over viscosity, freezing point, emulsification, and fat stability for smooth texture and taste.
C. Equipment and Packaging Design
- Viscosity, density, and surface tension are essential parameters in designing pipes, pumps, homogenizers, and filling machines.
- Helps in minimizing foaming, sticking, or clogging in machinery.
Conclusion
The physico-chemical properties of milk are the foundation of quality control and processing in the dairy industry. Whether it’s milk collection, storage, testing, processing, or product formulation, an in-depth understanding of these properties ensures that milk remains safe, nutritious, and suitable for a wide range of dairy products.
In the Indian dairy sector, where millions depend on milk for livelihood and nutrition, leveraging scientific knowledge of milk properties is essential to deliver high-quality, hygienic, and value-added products to consumers.