Physicochemical and simulated gastric digestion properties of A1/A1, A1/ A2 and A2/A2 yoghurts

This study aimed to determine the physicochemical properties of yoghurts from skim milk containing either (3-casein A1/A1, A1/A2 or A2/A2 and establish their behaviour during simulated gastric digestion. Yoghurts produced from milk containing (3-casein A1/A1 and A1/A2 had significantly higher storage modulus, water holding capacity, and lower syneresis compared to yoghurt produced from A2/A2 milk. Microscopy images also showed a more porous microstructure and greater pore size in A2/A2 yoghurts. The main conformational variability between the yogurts was approximately 35 % lower levels of aggregated (3-sheets in A2/A2 yoghurt, compared to the other yogurt systems. The A2/A2 sample was also characterised by larger casein micelles, lower levels of x-casein, and higher levels of (3-lactoglobulin in the yoghurt serum phase. During gastric digestion, coagulum formation in all three yoghurts occurred within the first 5 min between the pH ranges of 4.3 to 3.8. However, protein breakdown in A1/A1 and A1/A2 yoghurts occurred faster, with the final gastric clot characterised as a loose protein network, as opposed to that of the A2/A2 digesta. Overall, the use of A2/A2 milk in yoghurt production results in prolonged gelation times, with altered gastric digestion properties compared to yogurts containing (3-casein A1.