Other Constituents of Milk

Fresh milk contains several substances beyond proteins, fats, lactose, vitamins, and minerals. These components, often present in smaller quantities, contribute to milk’s flavor, functionality, nutritional value, and biological roles. Below is a list of these substances with brief discussions, focusing primarily on cow milk (the most studied), with notes on variability where applicable.

Other Substances in Fresh Milk

1. Water

• Concentration: ~87-88% in cow milk (varies by species, e.g., ~81-83% in sheep milk, ~89-90% in horse milk).

• Role: The primary component, serving as a solvent for all other constituents (e.g., lactose, minerals, water-soluble vitamins).

• Significance: Provides hydration; its high content makes milk a fluid food suited for neonates.

• Variability: Lower in high-fat milks (e.g., buffalo, sheep); higher in low-solid milks (e.g., horse).

2. Enzymes

• Examples:

  • Lipase: Breaks down triglycerides into fatty acids and glycerol.

  • Lactoperoxidase: Antibacterial, uses hydrogen peroxide to oxidize pathogens.

  • Alkaline Phosphatase: Indicator of pasteurization (inactivated by heat).

  • Proteases: Degrade proteins (e.g., plasmin).

• Concentration: Varies (e.g., lactoperoxidase ~0.01-0.03 mg/100 mL).

• Role: Natural catalysts; some protect milk (antibacterial), others affect storage/processing (lipolysis, proteolysis).

• Significance: Contribute to flavor (e.g., rancidity from lipase) and shelf life; pasteurization inactivates most.

3. Gases

• Examples: Oxygen (O₂), carbon dioxide (CO₂), nitrogen (N₂).

• Concentration: ~5-10 mg/L (dissolved from air during milking).

• Role: Present in small amounts; affect milk’s oxidative stability.

• Significance:

  • O₂ can oxidize fats/vitamins (e.g., vitamin C), reducing quality.

  • CO₂ naturally present or added in some processes to extend shelf life.

• Variability: Higher in freshly milked, aerated milk; decreases with storage.

4. Organic Acids

• Examples:

  • Citric Acid: ~150-200 mg/100 mL.

  • Lactic Acid: Trace in fresh milk, increases with bacterial activity.

  • Orotic Acid: ~5-10 mg/100 mL (unique to ruminant milk).

• Role:

  • Citric acid complexes with calcium, aiding mineral solubility.

  • Lactic acid forms post-milking if fermentation begins.

  • Orotic acid is a precursor to pyrimidine nucleotides.

• Significance: Influence flavor (mild acidity), mineral bioavailability, and microbial growth.

5. Nitrogenous Compounds (Non-Protein Nitrogen, NPN)

• Examples: Urea, ammonia, amino acids, peptides, creatine.

• Concentration: ~0.02-0.05% (~25-50 mg N/100 mL), ~5-7% of total nitrogen.

• Role: Metabolic byproducts from protein breakdown or blood.

• Significance:

  • Urea reflects animal protein metabolism (higher with high-protein diets).

  • Free amino acids contribute to flavor and nutrition.

• Variability: Higher in colostrum; varies with diet and species.

6. Hormones and Growth Factors

• Examples:

  • Insulin-like Growth Factor (IGF-1): Trace amounts.

  • Prolactin, Cortisol, Estrogens: Microgram levels.

• Concentration: Very low (e.g., IGF-1 ~1-10 ng/mL).

• Role: Naturally present from mammary gland secretion; support neonatal growth.

• Significance: Bioactive in raw milk; mostly degraded by pasteurization or digestion.

7. Immune Factors

• Examples:

  • Immunoglobulins (IgG, IgA, IgM): Part of whey proteins but distinct in function.

  • Lactoferrin: Antibacterial iron-binding protein.

  • Lysozyme: Antibacterial enzyme.

• Concentration: High in colostrum (e.g., IgG ~1-10 g/100 mL), lower in mature milk (e.g., lactoferrin ~0.01-0.1 mg/100 mL).

• Role: Provide passive immunity to newborns.

• Significance: More prominent in human and horse milk; reduced by heat processing.

8. Flavor Compounds

• Examples: Acetone, acetaldehyde, dimethyl sulfide, short-chain fatty acids (e.g., butyric acid).

• Concentration: Trace (parts per million or billion).

• Role: Derived from milk fat breakdown, microbial activity, or feed (e.g., silage).

• Significance: Contribute to milk’s characteristic taste and aroma; off-flavors (e.g., rancid, metallic) arise from oxidation or contamination.

9. Pigments

• Examples:

  • Riboflavin: Gives a slight yellow-green tint to whey.

  • β-Carotene: Yellow-orange in milk fat (from forage).

• Concentration: Riboflavin ~0.15-0.18 mg/100 mL; β-carotene ~0.01-0.02 mg/100 mL.

• Role: Natural colorants; β-carotene is a vitamin A precursor.

• Significance: More pronounced in pasture-fed cow milk (creamier color); absent in skim milk.

10. Cells

• Examples: Somatic cells (e.g., leukocytes, epithelial cells).

• Concentration: ~50,000-200,000 cells/mL in healthy milk; higher in mastitis.

• Role: Reflect mammary gland health; leukocytes combat infection.

• Significance: High counts indicate poor milk quality; filtered out in processing.

11. Microbial Flora

• Examples: Lactic acid bacteria, Pseudomonas, Staphylococcus (in raw milk).

• Concentration: <10³-10⁵ CFU/mL in fresh, uncontaminated milk.

• Role: Natural contaminants from udder, air, or equipment.

• Significance: Affect shelf life and safety; pasteurization eliminates most.

Factors Affecting These Substances

• Species: Horse milk has more immune factors; sheep milk has less water.

• Diet: Forage increases β-carotene and flavor compounds; high-protein feed boosts NPN.

• Lactation Stage: Colostrum is rich in enzymes, immune factors, and NPN.

• Processing: Pasteurization reduces enzymes, hormones, and microbes; skimming removes pigments in fat.

• Storage: Gases dissipate, organic acids increase with fermentation.