Protein in Milk
Protein represent 3-4% of the composition of milk and component may be fractioned out of milk by ultracentrifugation.
Casein is the primary protein of milk, comprising approximately 80% of the milk protein. The casein are actually a group of similar proteins, which can be separated from the other milk proteins by acidification to a pH of 4.6 (the isoelectric point).
At this pH, the caseins aggregate, since they are hydrophobic, are poorly hydrated and carry no net charge.
The other milk protein, being more hydrophobic remain dispersed in the aqueous phase.
In milk, the casein fractions associate with each other and with colloidal calcium phosphate to form stable spherical structures known as casein micelles.
The more hydrophobic alpha, and beta casein fractions mainly in the interior of the micelles, whereas the more hydrophobic kappa-casein mainly on the micelle surface.
It is the kappa –casein that gives the micelles their stability in milk under normal handling conditions.
This due to the negative charge and hydration of the kappa-casein coupled with the fact that the charge hydrophobic carbohydrate section of the molecule tends to protrude from the micelle surface in hair-like structures which confer steric (or spatial) stability on the micelles.
A second protein fraction of milk is the whey or serum. It makes up approximately 20% of milk protein and includes the lactalbumins and lactoglobulins. Whey protein are more hydrated than casein and are denatures and precipitate by heat rather than by acid.
Other protein components of milk include enzymes such as lipase, protease, and alkaline phosphatases, which hydrolyze triglycerides, proteins and phosphate esters, respectively.
The average measures of protein quality, including biological value, digestibility, net protein utilization, protein efficiency and chemical score, for milk and milk products.
Protein in Milk
