Contribution of milk enzymes, starter and rennet to proteolysis during storage of quarg

The relative contributions of rennet, starter and plasmin to proteolysis du ring storage of the acid-coagulated cheese quarg were investigated. In the experimental design used, each agent was, in separate treatments, either om itted (rennet), replaced by a nonproteolytic substitute (glucono-delta-lact one instead of starter) or alternatively either increased by the addition o f exogenous enzyme or decreased by addition of a specific inhibitor (plasmi n). A final experimental variable involved heating milk to 90 degrees C for 10 min prior to acidification. All quarg samples were thermised after acid ification, to reduce levels of adventitious microflora and to extend shelf life. The experimental treatments used had little effect on composition or yield of quarg, with the exception of high heat treatment, which increased yield and moisture content due to whey protein denaturation. Omission of re nnet during manufacture significantly reduced proteolysis during ripening o f quarg, and altered both urea-polyacrylamide gel electrophoresis and rever sed-phase HPLC patterns of proteolysis. Omission of starter had a considera bly lesser effect. The role of plasmin was also minor, resulting in product ion of the large water-insoluble gamma-caseins. The principal ripening agen t in thermised quarg appeared to be rennet, while starter enzymes and plasm in played a lesser role. (C) 1999 Elsevier Science Ltd. All rights reserved .