Aseptic Rennet Coagulation of Ultra-High Temperature Processed Milk Concentrates

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Milk was concentrated by pressure-driven filtration (UF or reverse osmosis) to 9.5, 15, 18, or 20% TS and then sterilized by UHT processing to 140°C for 4s using plate heat exchangers. Following sterilization, samples were aseptically inoculated with rennet to coagulate the milk, which was then stored at room temperature (22°C). We investigated the influence of TS, milk fat, extent of heat treatment, rennet dosage, storage temperature, and storage time on gel strength and syneresis as indicators of suitability for manufacturing a shelf-stable dairy dessert.As the TS content of the UF milk concentrate increased, gel strength also increased, and the optimum was obtained when milk was concentrated by UF to 18% TS. A small amount of added fat increased gel strength, but an increase in fat content from 1.5 to 3.5% decreased gel strength. Gel strength and syneresis increased as the rennet dosage was increased, and the optimum was in the range of 0.005 to 0.035 RU (rennet units)/ml of milk concentrate. Storage of the renneted UHT concentrates at 21°C yielded firmer gels and more syneresis than did storage at 4°C. Products stored for longer periods were also firmer, although syneresis increased during storage. Increasing the extent of whey protein denaturation by heating the concentrates to 81°C for 30min before UHT processing helped reduce syneresis of the gel during storage.