What proteins are in milk?
There are two major categories of milk proteins that are broadly defined by their chemical composition and physical properties.
The primary group of milk proteins are the caseins and make up the largest structures in the fluid portion of the milk - the casein micelles. Each casein micelle is roughly spherical and about a tenth of a micrometer across. The casein family contains phosphorous and this high phosphate content of the casein family allows it to associate with calcium and form calcium phosphate salts. Caseins have an appropriate amino acid composition that is important for growth and development of the nursing young. Collectively, they make up around 76% - 86% of the protein in milk by weight.
Milk contains dozens of other types of proteins besides caseins and including enzymes. These proteins are more water soluble than caseins and do not form larger structures. These proteins remain suspended in whey, remaining when caseins coagulate into curds and are collectively called as whey proteins.
Other whey proteins are the immunoglobulins (antibodies found in colustrum) and serum albumins - a serum protein. Whey proteins also include enzymes, hormones, growth factors, nutrient transporters, disease resistance factors and others. Whey protein family consists of approximately 50% beta-lactoglobulins, 20% alpha-lactalbumin, lactoferrins, transferrins, and many minor proteins. They also contain a large amount of sulphur containing amino acids. Whey proteins make up approximately 20% of the proteins in milk by weight. Lactoglobulin is the most common whey protein by a large margin.