Beta casein proteins – A comparison between caprine and bovine milk

Dairy products from a range of species, including caprine products, have been labelled as A2. Yet these simple terms do not accurately capture the complexity of casein protein sequences or properties, particularly when used to describe non-bovine proteins. This review examines the current state of knowledge of the beta casein (β-CN) proteins in caprine and bovine milk. It explores differences in the naming, sequence and function of caprine and bovine β-CN proteins and questions whether caprine milk can be considered 'A2' or 'A2-like'. None of the twelve caprine β-CN alleles or proteins have been scientifically named 'A2' to date and the caprine β-CN silent allele A1 differs in sequence to bovine β-CN A1. While the bovine β-CN proteins A1 and A2 differ by one amino acid at position 67 (histidine in A1 and proline in A2), all caprine β-CN proteins have proline at this position. These caprine proteins could be considered 'A2-like' based on this criterion alone but there are several other differences in sequence within and outside the β-casomorphin-7 (BCM-7) region that differentiate caprine β-CN proteins from bovine β-CN proteins. These sequence differences in caprine β-CN protein sequence may affect functional properties or the physiological effects of β-casomorphins (BCMs) potentially generated during consumption although there is little information from studies performed to date. Given the current state of understanding of caprine β-CN proteins, it appears inappropriate to label caprine dairy products as 'A2-like'. These knowledge gaps offer promising areas for future research that could provide new insights into the differences between caprine and bovine animals and their milk and dairy products including the bioactivity, functionality and digestibility of different proteins.