Germicidal action of milkSynonyms: Native inhibitory

substances in milk ; Antimicrobial system in raw milk ;Natural defense mechanism in milk

• Germicidal property of milk has been noted by several workers.

• 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 hrs. after the milk is drawn.

• High temp. of 1400 to 1760F for 30 min. appear 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, Viz.,

• Immune system.• Phagocytosis (engulfment and

destruction of invading organisms)• Bacteriostatic & bacteriocidal proteins

and enzymes in milk- capable of either killing or arresting growth of bacteria.

• Immune system: Comprises of inhibitors called

immunoglobulin -antibodies against specific antigens.

IgA: Produced locally in the udder IgG: transferred to milk from

blood.

• Primary practical significance of a.b. in milk is the protection of new born through passive immunity against various diseases.

• However a.b. also serve to reduce the severity of udder diseases 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: • Phagocytes mainly PMN in milk engulf the

bacteria• TLC of normal udder- 100,000 to 500,000

cells/ml of milk of this 10% are PMN • Infected udder may have 10,000,000

cells/ml, (90% PMN) that confer protection to udder against infections

Proteins & enzymes: whey proteins: • Non specific defense mechanism

having board spectrum of activity in milk these are:

Lactoferrin; Lysozyme and LP system.

Lactoferrin• An iron binding protein similar to

transferrin in blood.• 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.

Lysozyme: • Non specific inhibitor present in milk• conc. 300 times more in human milk than

in cow milk. • This enzyme hydrolyzes beta 1-4 linkage

of peptidoglycan (linkage between N-acetyle muramic acid and N- acetyl glucosamine) of bacterial cell wall weakening of cell wall lysis of bacteria.

LP system : • most imp. non specific inhibitor • 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 • Always present in sufficient concentration. • However, thiocyanate & H202

conc. become the limiting factor for the system

• SCN is present in varying amounts depending upon the nutrition of cow

• H202 is usually the more limiting factor. • Certain Species of LAB are known to

produce under aerobic conditions sufficient H2o2 to activate LPS system . But it does not last very long and no significant improvement in keeping quality of milk is attained

• However extraneous addition of SCN and H2o2have been shown to improve keeping quality of milk significantly.

• SCN & H2o2 (@ 10-15 & 7.5-10ppm respectively, final levels in milk)

• preservative effect depends on temp. • preservative effect lasts for:• 7-8hrs @ 250C • 11-12hrs @ 15-200C • 24-26hrs @ 15 -160C

Mechanism of Action of LPS:

• LP oxidizes SCN- in presence of H2o2 to an

unstable oxidation product hypothiocyanate (OSCN+), which is antibacterial in nature

• 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.