Physico-Chemical Properties of Milk

By Skumar -

 

Definition and Composition of Milk

Physico-Chemical Properties of Milk


Introduction

Milk is one of the most complete natural foods and plays a crucial role in human nutrition. It is the primary secretion of the mammary gland of mammals and forms the foundation of the dairy industry. Understanding the composition and physico-chemical properties of milk is essential for milk processing, quality control, product development, and competitive examinations as per ICAR norms.

Definition of Milk

According to ICAR / Prevention of Food Adulteration (PFA) standards:

Milk is the normal, clean, fresh secretion obtained by complete milking of healthy milch animals, excluding colostrum, without either addition to it or extraction from it.

This definition is universally accepted in dairy science and competitive examinations.

General Composition of Milk

Milk is a complex biological fluid consisting of water, fat, proteins, lactose, minerals, vitamins, and enzymes.

Average Composition of Cow Milk (ICAR Standard)

Component                                                      Percentage (%)
Water87.0
Fat3.5–4.0
Protein3.2–3.4
Lactose4.8–5.0
Minerals (Ash)0.7
Total Solids12.5–13.0
Solids-Not-Fat (SNF)8.5–9.0

(Buffalo milk contains higher fat and total solids than cow milk)

Major Constituents of Milk

1. Water

  • Acts as a solvent and dispersion medium

  • Determines milk volume and fluidity

  • Present in free and bound form

2. Milk Fat

  • Present as fat globules in emulsion form

  • Main energy-giving component

  • Responsible for:

    • Flavor

    • Creaminess

    • Butter and ghee yield

  • Rich in fat-soluble vitamins (A, D, E, K)

3. Milk Proteins

Total protein content: ~3.2–3.4%

Types:

  • Casein (≈ 80%)

    • Exists as colloidal particles (micelles)

    • Important for cheese and curd formation

  • Whey proteins (≈ 20%)

    • Lactalbumin

    • Lactoglobulin

    • Highly digestible

4. Lactose

  • Milk sugar (disaccharide)

  • Enhances calcium absorption

  • Responsible for mild sweetness

  • Important in fermentation (curd, yoghurt)

5. Minerals (Ash)

  • Calcium, phosphorus, potassium, sodium, magnesium

  • Important for:

    • Bone development

    • Buffering capacity of milk

6. Vitamins

  • Fat soluble: A, D, E, K

  • Water soluble: B-complex, Vitamin C

  • Milk is a good source of riboflavin (B₂)

Physico-Chemical Properties of Milk

Physico-chemical properties are essential for milk quality evaluation, processing, and preservation.

1. Colour

  • Natural colour: White to pale yellow

  • Due to:

    • Casein particles

    • Calcium phosphate

    • Carotene (yellowish tint in cow milk)

2. Flavour and Taste

  • Slightly sweet due to lactose

  • Pleasant and bland in fresh milk

  • Off-flavours indicate spoilage or adulteration

3. Density / Specific Gravity

  • Normal range: 1.028 – 1.034

  • Depends on:

    • Fat content

    • SNF content

  • Used for detecting adulteration with water

4. pH Value

  • Fresh milk pH: 6.6 – 6.8

  • Slightly acidic in nature

  • pH decreases during souring due to lactic acid formation

5. Acidity

  • Natural acidity: 0.13–0.17% lactic acid

  • Due to proteins, phosphates, and citrates

  • Increase in acidity indicates microbial activity

6. Boiling Point

  • Slightly higher than water

  • ~100.17°C

  • Increased due to dissolved solids

7. Freezing Point

  • Average: –0.55°C

  • Constant for genuine milk

  • Used to detect water adulteration

8. Electrical Conductivity

  • Depends on mineral content

  • Increased conductivity indicates mastitis

9. Surface Tension

  • Lower than water

  • Affects foaming and creaming properties

10. Buffering Capacity

  • Milk resists change in pH

  • Due to proteins, phosphates, and citrates

  • Important in heat processing and fermentation

Importance of Studying Physico-Chemical Properties

  • Quality control in dairy industry

  • Detection of adulteration

  • Standardization of milk products

  • Processing efficiency

  • Competitive exam questions (ICAR pattern)

Conclusion

Milk is a complex biological fluid with unique physico-chemical properties and balanced composition. Knowledge of the definition, composition, and physico-chemical characteristics of milk, as described in ICAR-recommended dairy science textbooks, is fundamental for dairy technology, food science, and agricultural competitive examinations. These properties govern milk quality, processing behavior, and consumer acceptability.

ICAR-Style Answer Explanations 

1. Definition of milk

Explanation: ICAR/PFA defines milk as the normal secretion obtained by complete milking of healthy animals excluding colostrum, with no addition or removal. Colostrum is excluded due to abnormal composition.

2. Authority for milk definition

Explanation: In India, milk standards are defined under PFA/FSSAI, followed by ICAR in academic and competitive exams.

3. Major constituent of milk

Explanation: Water (~87%) acts as the solvent and dispersion medium for all milk constituents.

4. Water content in cow milk

Explanation: Average cow milk contains ~87% water, varying slightly with breed, feed, and lactation stage.

5. Total solids

Explanation: Total solids = Fat + SNF ≈ 12.5–13% in cow milk (higher in buffalo milk).

6. SNF content

Explanation: SNF includes protein, lactose, minerals → ~8.5–9.0% in cow milk.

7. Physical nature of fat

Explanation: Milk fat exists as oil-in-water emulsion, stabilized by phospholipid-protein membrane.

8. Energy-giving component

Explanation: Fat supplies 9 kcal/g, highest among milk constituents.

9. Casein proportion

Explanation: Casein accounts for ~80% of total milk proteins; whey proteins are ~20%.

10. Whey proteins

Explanation: Whey proteins are highly digestible, heat-sensitive, and nutritionally superior.

11. Milk carbohydrate

Explanation: Lactose is the natural sugar of milk, absent in other foods.

12. Nature of lactose

Explanation: Lactose is a disaccharide (glucose + galactose).

13. Ash content

Explanation: Minerals in milk are measured as ash after combustion.

14. Buffering minerals

Explanation: Calcium & phosphorus (as phosphates and citrates) maintain milk buffering capacity.

15. Yellow colour of cow milk

Explanation: Cow milk contains carotene, giving yellowish tint; buffalo milk lacks carotene.

16. White colour of milk

Explanation: White colour results from light scattering by casein micelles and calcium phosphate.

17. Normal pH

Explanation: Fresh milk pH = 6.6–6.8, slightly acidic due to natural buffering salts.

18. Nature of milk

Explanation: Milk is slightly acidic, not neutral.

19. Natural acidity

Explanation: Natural acidity is due to casein, phosphates, citrates, not lactic acid initially.

20. Normal acidity value

Explanation: Expressed as lactic acid equivalent = 0.13–0.17%.

21. Specific gravity range

Explanation: Normal milk specific gravity = 1.028–1.034, influenced by fat & SNF.

22. Effect of fat on density

Explanation: Fat is lighter than water → increased fat lowers specific gravity.

23. Freezing point

Explanation: Genuine milk freezes at –0.55°C, a constant physico-chemical property.

24. Purpose of freezing point test

Explanation: Addition of water raises freezing point → used to detect adulteration.

25. Boiling point

Explanation: Dissolved solids elevate boiling point → milk boils slightly above water.

26. Exact boiling point

Explanation: Average boiling point ≈ 100.17°C.

27. Electrical conductivity

Explanation: Depends on ionic salts (Na⁺, K⁺, Cl⁻).

28. Mastitis indicator

Explanation: Mastitis increases Na⁺ and Cl⁻ → higher electrical conductivity.

29. Buffering components

Explanation: Proteins + phosphates + citrates resist pH change during heating/fermentation.

30. Role of buffering capacity

Explanation: Prevents sudden pH change → crucial in dairy processing.

31. Surface tension

Explanation: Milk surface tension < water due to fat globules & proteins.

32. Functional importance

Explanation: Lower surface tension aids foaming, creaming, emulsification.

33. Vitamin richness

Explanation: Milk is rich in riboflavin (B₂) → greenish-yellow whey colour.

34. Fat-soluble vitamins

Explanation: Milk fat carries A, D, E, K.

35. Highest fat milk

Explanation: Buffalo milk contains higher fat (6–8%) and TS.

36. Form of casein

Explanation: Casein exists as micelles, stabilized by κ-casein.

37. Adulteration detection

Explanation: Freezing point is most reliable constant for detecting water addition.

38. Souring effect

Explanation: Lactic acid formation → pH decreases.

39. Role of lactose

Explanation: Lactose provides mild sweetness and substrate for lactic fermentation.

40. Lowest constituent

Explanation: Minerals are present in smallest quantity (~0.7%).

41. Nature of milk

Explanation: Milk is a complex biological fluid, not a simple solution.

42. Mastitis-affected property

Explanation: Mastitis alters ionic balance → electrical conductivity rises.

43. SNF exclusion

Explanation: SNF = all solids except fat → fat is excluded.

44. Density measurement

Explanation: Lactometer measures specific gravity of milk.

45. Resistance to coagulation

Explanation: Buffering system delays pH drop and coagulation.

46. Boiling point elevation

Explanation: Dissolved solids cause colligative property effect.

47. Colour variation

Explanation: Depends on fat content, pigments, breed, feed.

48. Protein classification

Explanation: Milk proteins = casein + whey proteins.

49. Most constant property

Explanation: Freezing point is least affected by fat, season, breed.

50. Importance of study

Explanation: Physico-chemical properties guide quality control, processing, adulteration detection, core ICAR area.

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