AlphaS1casein

Alpha s1-casein, abbreviated αs1-casein, is a calcium phosphate–stabilized phosphoprotein and a primary component of the milk casein fraction. It is encoded by the CSN1S1 gene and produced by mammary epithelial cells. In cow milk it is among the most abundant caseins and contributes to the formation and stability of casein micelles, which carry calcium and phosphate.

Alpha s1-casein is highly phosphorylated, enabling strong binding of calcium ions and interaction with other caseins to assemble micelles. Its expression level and amino-acid sequence vary with genetic variants, and in cattle multiple alleles of CSN1S1 (for example A, B, C and others) lead to different αs1-casein content and milk protein profiles. Such polymorphisms can influence milk yield, micelle properties, coagulation in cheese production, and curd texture. In goats and some sheep breeds, null or low-expressing CSN1S1 alleles result in reduced αs1-casein and altered dairy characteristics.

Health and industry: αs1-casein is a major cow’s milk allergen for some individuals, contributing to milk protein allergy. Industrially, its level affects rennet coagulation, gel strength, and cheese yield, making CSN1S1 status a consideration in breeding and processing.

Species variation and research: cows, goats, and sheep show allelic diversity and expression differences. CSN1S1 remains a focus in dairy genetics and dairy product manufacturing.