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

¹ Department of Food Science, Purdue University, West Lafayette, IN 47907

The relationship between proteolysis and gelation was studied in UHT-processed milk following the aseptic addition of the enzyme plasmin at a concentration of .15 mg/L. 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-2251), and casein breakdown was apparent as examined by SDS-PAGE. It appeared that the added plasmin preferentially attacked ß- and -caseins over -casein. The evidence supports a relationship between a low level of plasmin activity and the gelation of UHT milk.

Key Words: ultra-high temperature milk • gelation • plasmin • proteolysis

Submitted on May 11, 1990
Accepted on October 5, 1990

This article has been cited by other articles:

				R. Lopez-Fandino and A. Olano Review: Selected indicators of the quality of thermal processed milk / Revision: Indicadores seleccionados para el control de calidad de la leche tratada termicamente Food Science and Technology International, January 1, 1999; 5(2): 121 - 137. [Abstract] [PDF]