The influence of pH and heat treatment on the colour and stability of ultra-high-temperature sterilized milk

Abstract

SummaryWhen the pH of milk was varied within the range 7·1 to 6·3 by addition of acid or alkali or through bacterial action, the reflectance of the milk after subsequent ultra-heat-treatment (UHT) was at a maximum of about pH 6·70. Below this value the reflectance dropped rapidly with decrease in pH. The cause of this decrease was the development of increasing amounts of sediment in the product. At pH 6·4–6·5, at least 90% of the casein and 40% of the whey proteins had been precipitated. The addition of 0·1% sodium di-hydrogen phosphate or 0·1% sodium citrate to the raw milk prevented the formation of the sediment. The role of calcium appeared important as small additions of calcium chloride or EDTA altered the patterns of sediment formation and reflectance with changing pH. Addition of 0·3% EDTA prevented sediment formation as the pH dropped.