Effects of a Low Concentration of Added Plasmin on Ultra-High Temperature Processed Milk

Abstract

The relationship between proteolysis and gelation was studied in UI-IT- processed milk following the aseptic addition of the enzyme plasmin at a concentration of .I5 m&. Individual 250-ml containers of commercially processed (direct steam injection, 134.4'C for 14.2 s) milk were used. The milk was injected with plasmin 1 wk after processing and stored at room temperature (-23'C). Over a 6-mo period, the milk was examined for changes in appearance, pH, apparent viscosity, gel formation, enzymatic activity, and casein breakdown. Control milk samples did not gel during the test period. The milk containers that received the plasmin addition began to form a gel at 90 d of storage, and this gelation was accompanied by an increase in apparent viscosity. In the samples with added plasmin, enzyme activity was detected using the chromogenic substrate, H-D-valyl-L leucyl-L-lysyl-4-nitroanilide (S-225 l), and casein breakdown was apparent as examined by SDS-PAGE. It appeared that the added plasmin preferentially attacked & and a-caseins over K-casein.