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Heat-induced changes in milk proteins in high-carbohydrate media
Casein micelles in milk are unique association colloids, sterically stabilized by a polyelectrolyte brush consisting of κ-casein, and are crucial structure elements in the creation of desirable texture and stability in dairy products such as cheese and yoghurt. Casein micelles show remarkable colloidal stability but when treatment conditions extreme or solvent quality is reduced, the micelles lose their colloidal stability and aggregate. This study investigated the colloidal stability of casein micelles and whey protein at high temperature and in the presence of a high level of carbohydrate, conditions commonly encountered in caramels, sweetened condensed milk and Dulche de Leche.
Adding 10-50% carbohydrate to milk reduced their colloidal stability of casein micelles. These effects are more extensive for carbohydrates of lower molar mass. Heating milk at >110°C increased casein micelle size and turbidity as a result of the aggregation of casein micelles, with contributions from heat-induced denaturation and aggregation of whey proteins. Heat-induced increases in particle size and turbidity were more extensive at higher heating intensity, with increasing carbohydrate concentration and decreasing molar mass of the carbohydrate. The presence of a free aldehyde group in reducing sugars also strongly influence the heat stability of milk through Maillard reaction products. The positive effect of Maillard reaction products on the heat stability of milk was derive from the formation of reductones, which can facilitate the covalent cross-linking of milk proteins and hence increase the heat stability.
When protein denaturation was impaired by the presence of carbohydrates. Denaturation temperature increased with increasing carbohydrate content, effects being larger for low-molecular mass carbohydrates. In, the size of heat-induced whey protein aggregates could be tailored by a combination of carbohydrate type and concentration.
The results presented facilitate the extension of our understanding the behaviour of milk proteins in environments strongly deviating from natural physiological conditions. Such insights can be applied to understand and tailor the behaviour of milk proteins in environments of high carbohydrate content, e.g., caramels, sweetened condensed milk and Dulche de Leche, and facilitate the design of rules for attaining maximum milk protein functionality in these systems.
Keywords: casein; whey protein; heat; carbohydrate; heat stability; denaturation