Dairy Chemistry & Milk Constituents

Diagnostic and functional biochemical roles of indigenous milk enzymes:

• Lipoprotein lipase hydrolyzes milk triglycerides to free fatty acids, producing Rancid flavour $\to$ 5.
• Lactoperoxidase catalyzes the oxidation of thiocyanate by hydrogen peroxide to hypothiocyanite ($OSCN^-$), exerting broad Bactericidal activity (LP system) $\to$ 6.
• Catalase originates from leukocyte somatic cells; high activity serves as a primary Mastitis indicator $\to$ 1.
• Plasmin is a heat-stable native alkaline serine protease that survives UHT heating and slowly hydrolyzes caseins during storage, causing Age gelation in UHT milk $\to$ 2.
• Lysozyme possesses striking 3D structural and sequence homology with the milk protein alpha-lactalbumin $\to$ 3.
  F. Alkaline phosphatase is thermally destroyed just slightly beyond Mycobacterium tuberculosis / Coxiella burnetii, serving as the official statutory Indicator of HTST pasteurisation $\to$ 4.

Cellular and physiological sources of indigenous milk enzymes:

• Blood plasma: Enzymes transudate directly from bovine bloodstream across mammary tight junctions into milk (e.g., plasminogen/plasmin, catalase, thrombin).
• Milk fat globule membrane (MFGM): Derived from the apical plasma membrane of mammary epithelial cells during secretion, carrying high concentrations of xanthine oxidase, alkaline phosphatase, and 5'-nucleotidase.
• Somatic cells: Leukocytes and sloughed epithelial cells lyse to release intracellular lysosomal enzymes (acid phosphatase, catalase, esterases).
• Casein micelle: A self-assembled supramolecular protein-mineral complex composed of $\alpha_{s1}, \alpha_{s2}, \beta, \kappa$-caseins and colloidal calcium phosphate. It is a product structure, NOT a biological or cellular source of enzymes (although some enzymes like plasmin associate with it, the micelle itself is not the source).

Biological and physiological roles of specialized milk constituents:

• Conjugated Linoleic Acid (CLA, rumenic acid $c9, t11\text{-}C_{18:2}$) is a potent natural Anticarcinogenic fatty acid in milk fat that inhibits carcinogenesis, atherogenesis, and tumorigenesis $\to$ (IV).
• Lactoferrin is an iron-binding glycoprotein that deprives pathogenic bacteria of essential ferric iron ($Fe^{3+}$) and directly disrupts bacterial membranes, exerting broad Antimicrobial activity $\to$ (I).
• $\beta$-Lactoglobulin belongs to the lipocalin family of calyx-shaped carrier proteins with a hydrophobic internal cavity that functions as a Vitamin carrier for retinol (Vitamin A) and hydrophobic ligands $\to$ (II).
• $\alpha$-Lactalbumin acts as the essential regulatory/specifier B-protein subunit of the lactose synthase enzyme system, modifying galactosyltransferase specificity to catalyze Lactose biosynthesis in the mammary epithelial Golgi apparatus $\to$ (III).