Milk contains about 85% water. The remaining 15% is the milk sugar lactose, protein, fat and minerals. Bovine milk comprises of two major proteins, casein and whey protein. Casein protein accounts more proportion than whey protein in milk. Casein protein helps in the transportation of Ca2+ and P. Moreover, bioactive peptides are originated from casein which benefits the human health in various ways.
Casein protein genes are located on the bovine chromosome 6.
There are three subtypes of casein protein:
i. alpha-casein (αs1 – casein/CSN1S1 gene & αs2 – casein/CSN1S2 gene)
ii. beta-casein (β-casein/ CSN2 gene)
iii. Kappa-casein (k-casein/ CSN3 gene)
Whey protein constitutes
i. Beta- lactoglobulin (β-LG)
ii. Alpha- lactalbumic (α-LA)
iii. Blood Serum Albumin (BSA)
iv. Immunoglobulin (Ig)
v. Beta- casein derived protease peptones
vi. Minor proteins
Among the three subtypes of casein, the beta-casein is the second abundant protein in milk and has excellent nutritional balance of amino acids. β-Casein gene contain 13 variant (A1, A2, A3, A4, B, C, D, E, F, H1, H2, I, G), among them A1 and A2 are the most common, while B is less common and A3 and C are rare.
The A1 and A2 variants of beta casein differ at amino acid position 67 with histidine in A1 and proline in A2 milk as a result of single nucleotide difference.
Fig.: A2 and A1 beta casein differ only by one amino acid at 67th position in 229 amino acid chain
During the digestion process, digestive enzymes act differently toward A1 and A2 beta-casein proteins. Beta-casomorphin-7 (BCM-7) is a bioactive seven-amino peptide that is released from the A1-beta-casein protein by digestive enzymes, but these enzymes are unable to break the A2 protein because it contains proline. As a result, digestion of A2 proteins does not release BCM-7. BCM-7, known as the “devil” in A1 milk, interacts with the human gastrointestinal tract, internal organs, and brainstem.
Fig.: Release of BCM 7 from A1 beta casein during digestion
A2 cows are those that produce A2 milk, whereas A1 cows are those that produce A1. A1 milk is mostly produced by Holstein and Ayrshire cattle breeds, whereas Jersey, Guernsey, Asian, and African cows produce A2. Sheep, goat, yaks, buffalo, camels, donkeys and Asian cows naturally contain more A2 beta casein protein.
A2 milk is safe for consumption because it doesn’t contain the devil in milk BCM-7. Cardiovascular disease and Type 1 diabetes are less common in populations that consume milk with higher beta-casein A2. People consuming A2 milk had better stool consistency, less incidence of bloating and less abdominal pain.