A2 milk is the type of beta casein in animal milk, which is A2 β- casein.Since the early dairy cows were producing A2 milk, and the relative A1 milk is caused by gene mutation in the later artificial feeding process, so A2 milk is considered to be more original. [1]Compared with A1 milk, A2 milk can significantly relieve gastrointestinal discomfort in the process of human digestion and absorption, and many gastrointestinal discomfort caused by lactose intolerance are actually due to A1 protein  [1].Meanwhile, a new randomized data based on Chinese preschool children's digestion of ordinary milk (containing A1 β - casein) and milk containing only A2 β - casein showed that using A2 milk instead of ordinary milk (containing A1 β - casein) could reduce gastrointestinal symptoms associated with milk intolerance in Chinese preschool children and improve their cognitive performance accordingly.

There are 30-35% β- caseins in milk, which are many variants. The most common two variants are A1 and A2. But the difference between them is that one of the 209 amino acids constituting the protein is different.The 67th position of the amino acid chain is proline and the casein is A2- -casein, and the 67th histidine is A1 -β- casein.Cell experiments show that these two proteins are decomposed at different sites under the action of digestive enzymes, resulting in the difference of digestion process between two proteins in vivo. This difference results in the release of beta-casomorphin-7 (also known as bcm-7, a 7-amino acid polypeptide chain) from A1 casein after normal enzymatic hydrolysis, while a2 - β - casein does not.Instead of producing BCM-7, A2 produces BCm-9, which is structurally closer to the -casein of breast milk than A1, and is more conducive to the growth and development of infants and young children.Bcm-7 is a kind of endorphin, which can interact with many systems of the body. Several studies have shown that bcm-7 is associated with increased risk of type I diabetes, immune response, digestive dysfunction, autism and respiratory dysfunction in some infants.

Scientists believe that the mutation of A1 gene occurred 5000-10000 years ago, that is, when cows were brought to Europe, proline at 67th position in the base chain was replaced by histidine during artificial feeding, and was genetically distributed to the whole population.At present, only 30% of the Western dairy cows widely used in dairy processing can produce pure A2 milk. In order to produce A2 milk, enterprises need to carry out professional gene testing on dairy cows, screen out pure breed A2 cows, and use the milk produced by these cows for later processing.^[5]Due to the rare breed, special selection process and other factors, A2 milk is more precious.

Not all A2 sources are called organic sources. Organic milk source is the "most healthy and natural" dairy products produced according to organic standards and strictly certified by the third party. Organic milk emphasizes "completely natural" and "no pollution in the whole process". It is strictly forbidden to use chemical fertilizers, pesticides, hormones, growth regulators, feed additives, food additives and other synthetic chemicals in the production and processing process. The packaging, storage and transportation of organic milk should also strictly comply with the relevant standards of organic food. In addition, it also requires the manufacturer to establish a sound quality tracking and review system. Therefore, for the majority of consumers, organic milk is the "safest" dairy products. ^[6]

[1]New ingredients of milk powder for novice mothers----A2 milk China business website

[2]Chen long, Fu WangYan, Fang qiongyan, Tang Guo, Liu Lun, Xiang Songtao. Genotyping of β - casein in and research progress of β - casein-7 in milk [J]. Chinese dairy industry, 2019 (08): 29-34

[3] Truswell, A.S. (2005), "The A2 milk case: a critical review", European Journal of Clinical Nutrition 59: 623–631, doi:10.1038/sj.ejcn.1602104, retrieved 29 June 2014 .

[4] Swinburn, Boyd (13 July 2004). "Beta casein A1 and A2 in milk and human health" (PDF). Report to New Zealand Food Safety Authority

[5] what is A2 β- casein? MSN Chinese network

[6] Organic milk nutrition and ordinary milk have no difference, but the price is higher than ordinary milk.  China Food Report.2015-09-21