575. Colour changes in heated and unheated milk: III. The effect of variation in milk composition on the whitening and browning of separated milk on heating

Abstract

The effect of natural variations in the pH, and in the casein, soluble-protein and lactose contents on the whitening and browning of separated milk has been studied by holding samples of known composition, obtained from individual cows, at a temperature of 110°C. for various times and determining their reflectances after treatment.The rate of browning, measured as a rate of loss of reflectance at a wave-length of 4260 Å., was found to depend on the pH of the milk. Apparent variations of the rate of browning with protein content are considered to have been caused by associated variations of pH with protein content in the milk from the different cows. The rate of browning varied over a range of 2:1 for the samples examined.The whitening of milk, measured as the maximum rise of reflectance occurring on heating, was found to be much more variable, and over the same range of samples varied by 5·5:1. The soluble-protein content and pH affect the amount of whitening, but there appears to be some other factor of importance which was not covered by the chemical analyses that were made.A considerable range of reflectances is shown by different milk samples after a fixed heat treatment. These differences are determined more by variations in the amount of whitening than by variations in the rate of browning.