Detecting β-Casein Variation in Bovine Milk

In bovine species, -casein (-CN) is characterized by genetic polymorphism. The two most common protein variants are -CN A(2) (the original one) and A(1), differing from A(2) for one amino acid substitution (Pro(67) to His(67)). Several bioactive peptides affecting milk nutritional properties can originate from -CN. Among them, -casomorphin-7 (BCM7) ranging from amino acid 60 to 66 can be released more easily from -CN variants carrying His(67) (A(1) type) instead of Pro(67) (A(2) type). Nowadays, A2 milk is produced in different countries claiming its potential benefits in human health. The aim of this study was to further develop and apply an isoelectric focusing electrophoresis (IEF) method to bulk and individual milk samples in order to improve its use for -CN studies. We succeeded in identifying A2 milk samples correctly and quantifying the percentage of A(2), A(1), and B variants in bulk samples not derived from A2 milk as well as in individual milk samples. The method allows us to quantify the relative proportion of -CN variants in whole milk without eliminating whey protein by acid or enzymatic precipitation of caseins. The aim of this study was also to study the different behavior of -CN and -lactoglobulin (-LG) in the presence of trichloroacetic acid (TCA). The higher sensitivity of -CN to TCA allows quantifying -CN variants after TCA fixation because -LG is not visible. Monitoring -CN variation in cattle breeds is important in order to maintain a certain balance between Pro(67) and His(67) in dairy products. Overall, the debate between A1 and A2 milk needs further investigation.