Influence of changes in pH during milk coagulation process on ultrasonic phase velocities of milk

摘要

An ultrasonic phase velocity is an effective index for monitoring degree of milk fermentation, because the pattern of phase velocity change with time agrees with of the acidity change. However, it is not clear why phase velocity has a high correlation with milk acidity or pH. We have focused on the changes in hydration state of casein micelle due to pH for investigating the relations between phase velocity and pH. In this study, we investigated the changes in densities of milk and casein micelle during fermentation. The milk density decreased largely at a pH slightly larger than 4.6, and then continued to decrease with progress of fermentation. It means that the increase in ultrasonic phase velocity is the direct result of the decrease in milk density. The limiting density number decreased with decrease in pH and became the lowest at pH 4.6. The limiting density number which indicates the casein micelle density followed a similar pattern as in milk density. The zeta potential of casein micelle increased with decrease in pH. It means that the hydration state of casein micelle is changing during fermentation. Therefore the increase in ultrasonic phase velocity before hardening of milk could be caused by the decrease in density of casein micelle including hydration layer.