The Significance of Rennin and Pepsin in Rennet
A test of the importance of rennin, and other fractions of rennet in cheese manufacture was obtained by comparing cheese made with crystalline rennin, commercial rennet and the supernatant solution remaining after removal of about 80 per cent of the clotting activity from rennet. The amount of these solutions used in cheese manufacturing was based on conventional estimates of their clotting strength in a substrate prepared by dissolving 60 g. of nonfat dry milk (NDM) in 500 ml. of 0.01 M CaCl2. Cheese milk was coagulated with these clotting solutions and the strength o the resulting coagulum was measured with a Cherry Burrell curd meter. Eight lots of cheese made with each clotting solution were analyzed for moisture, total nitrogen and soluble nitrogen; and the cheese graded at 2, 10 and 34 weeks.
All the cheese were of good commercial quality, and there appeared to be no differences among the cheese made with the three solutions. Good quality cheese was made with crystalline rennin, and the non-rennin fraction of rennet did not contribute to cheese quality. The only component in commercial rennet essential for cheese manufacture under these conditions appeared to be the enzyme rennin.
There was no obvious reason why the crystalline rennin treatment produced the highest level of soluble nitrogen, and the supernatant the lowest level with the commercial rennet intermediate between the two. It was hypothesized that pepsin in the rennet and the supernatant solution caused these differences. Measurement of the clotting strength of a mixture of rennin and pepsin in the CaCl2 substrate did not predict accurately the clotting strength in skim milk. The relative clotting activity of pepsin and rennin in the calcium enriched substrate is not maintained in skim milk.
A method was developed to determine the pepsin activity in commercial rennet samples. Ten samples tested by this method revealed that pepsin contributed from none to 3.7 per cent of the milk clotting strength.
Evidence was obtained that the conversion of prorennin to rennin in fresh rennet extract containing 15 per cent NaCl might not be autocatalytic as had been supposed. Partially purified prorennin could not be activated in the presence of 15 per cent NaCl at pH 4.7. The addition of pepsin to prorennin under these conditions produced activation approximately proportional to the amount of added pepsin. The addition of rennet to prorennin, under the same conditions, produced activation of the prorennin at a rate which depended on the measured pepsin content of the rennet. It was concluded that pepsin plays a major role int eh activation of prorennin under conditions of commercial rennet manufacture.