s2casein

S2-casein, also known as alpha-S2-casein, is a milk protein belonging to the casein family. In bovines it is encoded by the CSN1S2 gene and is present in cow milk as part of the casein fraction. S2-casein is also found in the milk of some other ruminants, though the amount and exact isoforms can vary by species and breed.

Structurally, S2-casein is a phosphoprotein and a relatively acidic, relatively high‑proline component of casein. It contributes to the formation of casein micelles, complex colloidal assemblies in milk that incorporate calcium phosphate. Through its presence in the micelle, S2-casein participates in milk stability, texture, and processing properties such as gelation and coagulation, which are important for cheese production.

Genetic variation plays a role in the expression of S2-casein. The CSN1S2 gene exhibits allelic diversity in cattle and other ruminants, leading to different S2-casein isoforms. These polymorphisms can influence milk-processing characteristics, including rennet coagulation behavior, cheese yield, and possibly digestion to some extent. Analytical methods such as electrophoresis and mass spectrometry are used to identify and characterize S2-casein variants in milk.

In dairy science, S2-casein is one component among the major milk proteins that determine the functional properties of milk and dairy products. It is typically discussed alongside other caseins (alpha-S1, beta, and kappa) in studies of milk composition, micelle structure, and breed-specific milk quality.