After the milk has been drawn from the udder, for a certain period, there may be certain decrease in the member of bacteria as determined by plate count. This action as a rule does not continue beyond three hours after the milk is drawn. High temperature of 140-176ºF for 30 min appears to destroy the germicidal property. Several theories have been advanced to explain this property of milk. Certain bacteria die on account of unfavorable conditions. Some of the bacteria may be ingested by leukocytes in milk. Agglutinins present in milk may clump bacteria and thus cause low Plate count. Broadly there are three types of natural defense mechanisms in milk system:

·         Immune system

Milk comprises of inhibitors called immunoglobulin -antibodies against specific antigens.

ü  IgA: Produced locally in the udder

ü  IgG: transferred to milk from blood

                     Primary practical significance of antibody in milk is the protection of new born through passive immunity against various diseases. However antibody also serves to reduce the severity of udder diseases. For e.g.: by neutralizing the toxins elaborated during the disease process or by acting as opsonins to facilitate the engulfment of bacteria by PMNG cells

·         Phagocytosis (engulfment and destruction of invading organisms)

Phagocytes mainly PMN in milk engulf the bacteria. Total leukocyte count of normal udder ranges from 100,000 to 500,000 cells/ml out of which 10% are PMN. Infected udder may have 10,000,000 cells/ml, (90% PMN) that confer protection to udder against infections.

·         Bacteriostatic & bacteriocidal proteins and enzymes in milk

Lactoferrin, Lysozyme and LP in milk is mainly responsible for germicidal properties.

ü  Lactoferrin: It is an iron binding protein similar to transferrin in blood which prevents growth of bacteria by sequestering any free iron. Once iron is bound to lactoferrin, it is no longer available to bacteria and thus growth stops.

ü  Lysozyme: Non specific inhibitors present in milk whose concentration is about 300 times more in human milk than in cow milk. This enzyme hydrolyzes beta 1-4 linkage of peptidoglycan (linkage between N-acetyl muramic acid and N- acetyl glucosamine) of bacterial cell wall which ultimately leads to lysis of bacteria.

ü  LP system: This system comprises of three components: the enzyme-LP, the substrate-thiocyante and the promoter-hydrogen peroxide. The system is inactive in the absence of any of the three components. LP is synthesized in mammary gland. However, thiocyanate & hydrogen peroxide conc. become the limiting factor for the system. SCN is present in varying amounts depending upon the nutrition of cow hydrogen peroxide is usually the more limiting factor.

Mechanism of Action of LPS: LP oxidizes SCN in presence of hydrogen peroxide to an unstable oxidation product hypothiocyanate (OSCN +), which is antibacterial in nature and damages the inner cell membrane causing leakage and cessation of nutrient uptake, leading eventually to death of the organism. LP system might be utilized as a “cold sterilization” process to render milk safe for consumption especially at places where refrigeration is scarce.