The natural / inherent antimicrobial agents in milk prevent
microbial growth in fresh milk. This explains the reason why freshly drawn milk
will take some time before it coagulates. These agents also have the ability to
protect the cow from mastitis
infection.
This nature of milk also protects the consumers from
dangerous metabolites that would otherwise result from microbial activity in
milk.
Research has shown that the ability of milk to impart
these antimicrobial properties depend on certain factors in milk such as
lactoperoxidase, lactoferrin, lysozyme, and N-Acetyl-ß-D-Glucosaminidase
(NAGase). The composition of these factors vary from species to species.
Lactoferrin is a glycoprotein that binds iron in milk.
Mammalian milk contains this protein in large quantities (about 1 gram per
liter), and may increase (50 to 100 grams per liter in bovine milk) during late
lactation period.
This protein has been shown to have important biological
functions including anti-inflammatory, synergistic to immunoglobulin secretions
that are necessary for immunity, antibacterial, and protection against
gastro-intestinal infections.
Most bacteria such as coliforms, Staphyloccocus
aureus, and Lysteria monocytogenes require iron for
growth. Lactoferrin can bind the iron making it unavailable to these bacteria
hence depriving them of a crucial growth factor leading to their decimation.
Lactoferrin can also exert a direct (non-iron dependent)
bactericidal effect on Vibrio cholerae and other streptococcal
mutants. However, lactoferrin does not affect the lactic acid bacteria present
in the small intestine.
Lactoperoxidase enzyme is present in milk at the rate of
0.03 grams per liter. The content is usually lower in colostrum but increases
rapidly after parturition.
The enzyme (lactoperoxidase) combines with thiocyanate and
hydrogen peroxide to form the lactoperoxidase system, which is lethal to
bacteria.
Lactoperoxidase catalyzes the reaction between hydrogen
peroxide and thiocyanate to form the temporary hypothiocyanate.
This temporary substance (hypothiocyanate) oxidizes vital
bacterial enzymes leading to death of the bacteria.
Kales have been found to contain higher concentrations of
thiocyanate content. Feeding kales will lead to a proportionate rise in
thiocyanate content of milk.
In the presence of hydrogen peroxide (which is also
naturally present in milk), the lactoperoxidase system is activated. This will
lead to natural preservation of fresh milk without any intervention.
This should give you enough time to look for secondary
preservation methods to increase the shelf life of milk. However, as the
concentration wears out, the milk will begin to deteriorate.
Lysozyme is naturally present in cows (can be found in the
stomach tissue or body fluids) and it can be manifested as either c-lysozyme or
g-lysozyme.
It disrupts the glycosidic bonds between two peptidoglycan
constituents leading to the leakage of the cell protoplasm hence bacterial
death. Lysozyme is effective in the presence of lactoferrin or immunoglobulin.
N-Acetyl-ß-D-glucosamindase (NAGase) is a lysosomal enzyme that is usually produced in
large quantities by an inflamed udder.
NAGase levels in milk rise proportionally during an
infection. It works closely with lactoferrin and is present in high quantities
during the late lactation to dry period.
This is the time when udder health is at its best. NAGase
has been shown to inhibit Actionmyces pyogenes, Pseudomonas aeroginosa,
Staphylococcus aureus, and Strepto-coccus agalactiae.
It was, however, found to be non-inhibitory to Escherichia
coli and Enterobacter aerogenes.
