Impact of β-casein phenotype on the physical properties of skim milk powders and their subsequent digestion characteristics

This study investigated the physical properties of skim milk powders containing beta-casein A1/A1, A1/A2 or A2/ A2 phenotypes and determined their digestion properties. Rehydrated skim milk powders all had a type A heat coagulation profile (HCT); although A2/A2 milk had a different local maximum and minimum HCT, compared to A1/A1 and A1/A2 milks. Differences in the rehydration properties of milk powders appeared to be related to the presence of beta-casein A1 in reconstituted A1/A1 and A1/A2 milks since they were characterised by smaller casein micelles and greater levels of kappa-casein, compared to A2/A2 milk. All reconstituted samples displayed protein aggregation and coagulum formation within the first 5 min of gastric digestion, at which time the pH ranged from 6.0 to 5.5. During digestion of reconstituted A2/A2 milk, casein breakdown was slower, compared to that of A1/A1 or A1/A2 milks. The final gastric clot obtained from the A2/A2 sample possessed a tight protein network, containing a greater level of calcium and aggregated beta-sheets. In this regard, the dry weight of the separated clot after the final gastric phase was significantly higher by 29 and 68 % in A2/A2 digesta compared to that in A1/A1 or A1/A2 digests, respectively. Overall, this study shows that for rehydrated skim milk powders there was a significant difference in the level of gastric protein-breakdown between milks containing beta-casein A1 and milk containing only beta-casein A2. This may have significant implications during in vivo gastric digestion and emptying.