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

The relative contributions of rennet, starter and plasmin to proteolysis during storage of the acid-coagulated cheese quarg were investigated. In the experimental design used, each agent was, in separate treatments, either omitted (rennet), replaced by a nonproteolytic substitute (glucono-δ-lactone instead of starter) or alternatively either increased by the addition of exogenous enzyme or decreased by addition of a specific inhibitor (plasmin). A final experimental variable involved heating milk to 90°C for 10 min prior to acidification. All quarg samples were thermised after acidification, 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 rennet during manufacture significantly reduced proteolysis during ripening of quarg, and altered both urea-polyacrylamide gel electrophoresis and reversed-phase HPLC patterns of proteolysis. Omission of starter had a considerably lesser effect. The role of plasmin was also minor, resulting in production of the large water-insoluble γ-caseins. The principal ripening agent in thermised quarg appeared to be rennet, while starter enzymes and plasmin played a lesser role.