A2 milk is cow’s milk that contains only the A2 variant of beta-casein protein. In referring to A2 milk, we are talking only about the specific type of protein it contains, called beta casein, and nothing else (not fat or carbohydrates).
According to the literature, more than 10,000 years ago, and before they were domesticated, cows produced only the A2 beta casein protein and not the A1 beta casein protein. However, some 8,000 years ago a natural single-gene mutation occurred in Holsteins, resulting in production of the A1 beta casein protein in this breed. This mutation in the beta casein gene led to 12 genetic variants, of which A1 and A2 are most common. The mutation was passed on to many other breeds, principally because Holsteins are used to genetically improve the production of other breeds. Slowly, the A1 beta casein variant became dominant in milk. While dairy herds in much of Asia, Africa, and part of Southern Europe remain naturally high in cows producing A2 milk, the A1 version of the protein is common among cattle in the Western world.
A point of reference is that A2 milk products are made from dairy cows that produce only the A2 beta casein protein, whereas today’s cow’s milk contains both A2 and A1 beta casein proteins. The most common variants among Western cattle are A1, A2, and B.
In general, milks from Guernsey, Jersey, Asian herds, human milk, and others (sheep, goat, donkeys, yaks, camel, buffalo, sheep, etc.) contain mostly A2 beta casein. Milks from Holstein Friesian contain mostly A1 beta casein. The Holstein breed (the most common dairy cow breed in Australia, Northern Europe, and the United States) carries A1 and A2 forms of beta caseins in approximately equal amounts. More than 50 percent of the Jersey breed carries the A2 beta casein variant, but with considerable variation among the herd, and more than 90 percent of the Guernsey breed carries the A2 beta casein variant.
Two major protein groups are present in cow’s milk – approximately 82 percent of protein is casein and approximately 18 percent is whey protein. Both groups have excellent nutritional benefits.
Caseins are a group of proteins. Among the caseins, beta casein is the second most abundant protein (about one-third of the caseins) and has an excellent nutritional balance of amino acids.
The beta casein group has two common variants: A1 and A2 beta casein. Most milk contains a mixture of these proteins. Approximately 60 percent of the beta casein is A2, and 40 percent is A1.
The proportion of A2 and A1 beta casein in milk can vary with different breeds of dairy cattle – A2 milk contains only A2 beta casein.