Viscosity changes in concentrated skim-milk treated with alkali, urea and calcium-complexing agents: I. The importance of the casein micelle

Abstract

The viscosity of concentrated skim-milk changed markedly with time upon the addition of urea, calcium-complexing agents or alkali, the viscosity-time curve passing through a distinct maximum. Also, the characteristic opacity of the milk disappeared in the presence of these reagents. These phenomena are explained in terms of a postulated expansion of the casein micelles followed by their disintegration into ‘molecular’ units. Electrostatic repulsions within the casein micelles are considered to be the cause of expansion. Calcium, either in the ionic atmosphere of the casein micelle or as calcium bridges within the micelle, and hydrogen bonds are regarded as two of the major factors responsible for maintaining the stable structure of the casein micelle. The authors wish to thank Mr E. F. Woods, Division of Protein Chemistry, C.S.I.R.O., Melbourne, and various staff members of the Dairy Research Section, C.S.I.R.O., Melbourne, for helpful discussion during the preparation of the manuscript. The technical assistance of Miss Audrey K. Morris is also gratefully acknowledged.