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.