Milk protein-carbohudrate interactions in sterilised milk products

Abstract

The present work deals with the nature of milk-protein carbohydrate interactions during the heat treatment of milk. Starches of different botanical origin were added (0.5-1.5% w/w) to milk (9% TS or 22% TS) and the mixtures were preheated at 70°C to ensure gelatinization of the starch granules. The heat stability of the milk-starch mixtures was defined as the time required for the coagulation of the milk proteins to occur on heating at 140°C (skim milk of 9% TS) or at 120°C (concentrated milk 22% TS), over the pH range, 6.5-7.1. On addition of native starches there was a reduction in the coagulation time of milk. The various starches hadsimilar effects on the coagulation time of milk, the destabilisation being proportional to the amount of starch added. The destabilisation of milk was not associated either with the amylose to amylopectin ratio of the starches sor to the granular structure of the starch. Further experiments probed the effects on heat stability of changing the molecular structure of starch in two ways: a) by the addition of acid modified starches and b) by the addition of cross-linked starches. Both types of treatment promoted a reduction in the water-binding capacity of the starch and both resulted in an improvement of the coagulation time of the milk-carbohydrate mixture. However,complete recovery of the heat stability was achieved only in the case of unconcentrated milk. Most of the research to date on milk proteincarbohydrate interactions has involved acidic polysaccharides, whereas there is limited information for non-ionic gums. Due to the importance of the heat stability of milk in the dairy industry it was useful to initiate this study by investigating the effect of various forms of carbohydrates on the stability of the milk proteins upon heating.