Aseptic rennet coagulation of ultra-high temperature processed milk concentrates.
Smith M. | McMahon D.J.
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 degrees C for 4 s using plate heat exchangers. Following sterilization, samples were aseptically inoculated with rennet to coagulate the milk, which was then stored at room temperature (22 degrees 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 degrees C yielded firmer gels and more syneresis than did storage at 4 degrees 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 degrees C for 30 min before UHT processing helped reduce syneresis of the gel during storage.
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