Milk is nutritionally one of the best food items. This high nutritional value of milk is quite useful to mammals because milk remains the only food for the young mammal for a long period. The chief essential dietary ingredients which are less or absent in milk are the two minerals, namely iron and copper, both needed for red blood cell formation.

Milk is also considered a complete food for infants because it contains nearly all essential nutrients in balanced proportions, although it is not fully sufficient for adult nutritional requirements unless supplemented.

   Physical Characteristics:

Milk is a liquid, white to yellowish-white in color due to substances such as carotene and xanthophylls dissolved in milk fat. Its specific gravity ranges from 1.026 to 1.036. Milk is slightly acidic with a pH between 6.6 and 6.9. Fresh milk does not coagulate on boiling, but a surface film is produced that contains denatured whey proteins (mainly lactalbumin and lactoglobulin) and calcium salts.

Milk is a colloidal system, mainly an oil-in-water emulsion combined with protein micelles suspended in aqueous phase. Because of this structure, milk shows both colloidal and true solution properties.

   Chemical Constituents of Milk

The exact composition of milk varies with the breed of cattle, the feed used, and the period of lactation. Milk contains proteins, lipids, carbohydrates, vitamins, enzymes, minerals, and many biologically active organic constituents.

In addition to nutrients, milk also contains bioactive compounds such as growth factors, hormones, antimicrobial agents, and immune-related proteins.

   Milk Proteins

These are casein, lactalbumin, and lactoglobulin.

  Casein

Casein is the characteristic and most abundant protein of milk. It occurs as its calcium salt, i.e., calcium caseinate. It is a phosphoprotein, and its isoelectric pH is 4.6. Casein is not a single protein but consists of several types (α, β, and κ caseins) which differ mainly in their degree of phosphorylation.

Casein exists in milk as casein micelles, which are complex spherical structures containing casein molecules bound with calcium phosphate (colloidal calcium phosphate). This micellar structure is responsible for the stability of milk.

Casein is relatively low in methionine and tryptophan but rich in essential amino acids like lysine.

Casein can be precipitated by adding acid to milk:

  Ca-caseinate + 2HCl -------------------> Casein + CaCl₂

The proteolytic enzymes rennin (chymosin), pepsin, and chymotrypsin can produce curdling of milk, which takes place by hydrolysis of κ-casein, destabilizing the micelle structure.

  Casein --------------> Paracasein (partial hydrolysis)

Paracasein then combines with Ca²⁺ ions to form calcium paracaseinate, which forms curds.

  Paracasein + Ca²⁺ --------------------> Calcium paracaseinate

Cow’s milk has relatively more casein than human milk and therefore forms firmer curds, which are slower to digest.

  Lactalbumin (Whey Proteins)

Lactalbumin is not a single protein but includes several whey proteins, mainly α-lactalbumin, serum albumin, and minor protein fractions. These proteins remain in solution after casein precipitation and are highly digestible.

α-lactalbumin plays an important role in lactose synthesis in the mammary gland.

  Lactoglobulin

Lactoglobulin is the least abundant milk protein fraction. It is absent in human milk but present in cow’s milk. It also contains immunoglobulins (antibodies), especially in colostrum, which provide passive immunity to newborns.

  Additional Milk Proteins

Milk also contains:

• Immunoglobulins (IgA, IgG, IgM) → immune protection
• Lactoferrin → binds iron and inhibits bacterial growth
• Lysozyme → antibacterial enzyme
• Growth factors → support intestinal and tissue development
• Serum albumin → minor plasma-derived protein

   Milk Lipids

Milk is an oil-in-water emulsion stabilized by phospholipids and membrane proteins forming the milk fat globule membrane (MFGM).

Cow’s milk triglycerides contain fatty acids ranging in chain length from 4C to 20C. In human milk, there is a higher proportion of long-chain polyunsaturated fatty acids important for brain development.

The most abundant fatty acid in milk fat is palmitic acid, while oleic acid is also present in significant amounts.

Milk also contains essential fatty acids such as linoleic acid and small amounts of conjugated linoleic acid (CLA), which has been studied for metabolic effects.

Cow’s milk has about 11 mg cholesterol per 100 ml, most of which is in free form.

   Milk Carbohydrates

Lactose is the chief sugar of milk. Two isomeric forms, α-lactose and β-lactose, exist in equilibrium.

Lactose is synthesized in the mammary gland from glucose and galactose and is essential for maintaining osmotic balance of milk secretion.

If lactose is fermented by bacteria, milk becomes sour due to lactic acid formation:

  Lactose → Glucose + Galactose → Lactic acid

(Some bacterial or yeast contamination may also produce ethanol and CO₂, but this is not typical of normal lactic acid fermentation in milk.)

A factor named bifidus factor occurs in human milk, now known to consist mainly of human milk oligosaccharides (HMOs). These selectively promote growth of beneficial gut bacteria such as Bifidobacterium.

Lactose also enhances intestinal absorption of calcium and phosphorus, particularly in infants.

   Vitamins in Milk

Milk is a good source of vitamin A and its precursors. The vitamin D content of natural milk is very small, but commercially available milk is often fortified.

Ascorbic acid (vitamin C) is present in low amounts and is heat sensitive.

Milk contains most B vitamins in variable amounts:

• Rich in: riboflavin (B2), vitamin B12, pantothenic acid
• Moderate: niacin
• Low: folate, thiamine (B1), vitamin B6

   Enzymes in Milk

Milk contains many enzymes, including:

• Alkaline phosphatase (important for pasteurization testing)
• Catalase
• Lipase
• Xanthine oxidase
• Amylase
• Lactoperoxidase (important antimicrobial enzyme system)

The presence or absence of alkaline phosphatase activity is used as an indicator of proper pasteurization.

   Milk Ash (Mineral Content)

Milk is rich in calcium and phosphorus in an optimal ratio for bone development.

Other minerals include potassium, sodium, magnesium, zinc, and trace elements.

Iron and copper are present only in negligible amounts, which is why infants rely on prenatal iron stores or supplementation.

Calcium in milk is highly bioavailable due to its association with casein and presence of lactose.

   Comparison of Human and Cow’s Milk

Milk Component	Human Milk	Cow's Milk
Proteins %	1.4	3.2–4.0
Lactose %	7.0	4.8–5.0
Fat %	4.0	3.5–4.0
Ash %	0.25	0.70–0.75

Human milk contains lower protein but higher lactose, making it more suitable for infant kidney load and brain energy needs.

The caloric value of human and cow’s milk is approximately 67 and 65 kcal/100 g respectively.

   Colostrum

The milk secreted during the first few days after parturition is called colostrum. It is more viscous than mature milk and slightly alkaline in reaction.

Colostrum contains:

• Very high protein content
• High immunoglobulins (especially IgA)
• High lactoferrin and growth factors
• Low lactose and fat compared to mature milk

A characteristic property is that it coagulates on heating due to high serum protein content.
Colostrum plays a critical role in providing passive immunity to the newborn and supports maturation of the gastrointestinal tract.

Secretion of colostrum gradually decreases and is replaced by mature milk within the first few days after parturition.

   Additional Important Concepts

  1. Milk as a Functional Food

Milk is not only a nutrient source but also a functional food, providing:

• immune protection
• gut microbiome regulation
• growth stimulation
• antimicrobial defense

  2. Pasteurization and Processing Effects

Heat treatment affects milk by:

• destroying pathogenic microorganisms
• inactivating enzymes (like phosphatase)
• reducing vitamin C and some B vitamins slightly
• improving safety and shelf life

Homogenization prevents cream separation by reducing fat globule size.

  3. Milk Microbiology

Raw milk contains:

• lactic acid bacteria
• possible pathogens (if contaminated)

Controlled fermentation produces products like yogurt and cheese.

  4. Hormones and Bioactive Molecules in Milk

Milk may contain trace amounts of:

• insulin-like growth factors (IGF)
• leptin
• cortisol
• prolactin

These are more significant in early life nutrition and gut development.