The turbidity of heated milk in relation to particle size and protein distributions between the casein micelles and the serum phase

At pH 6.50, milk turbidity increased on heating at 90 °C for up to 10 min before plateauing. Turbidity at each heating time was lower as the pH increased so that at pH 7.10, the turbidity initially decreased before plateauing. On heating, the particle size and total sedimentable protein increased at low pH and decreased at higher pH before plateauing. Sedimentable whey protein increased whereas sedimentable αS+β-casein and κ-casein decreased as pH at heating increased. There was a positive correlation between turbidity and total sedimentable protein whereas the relationship with the protein fractions was weaker. There was a moderate correlation between size and turbidity, whereas that between size and sedimentable protein was weaker unless samples at short heating time were excluded. These results indicate that the turbidity of heated milk is due to changes in the distribution of caseins and whey proteins between the casein micelles and the serum phase.