Plasmin levels in fresh milk whey and commercial whey protein products

Growth of psychrotrophic bacteria in nonfat dry milk at refrigeration tempe ratures was shown previously in our laboratory to cause a shift in plasmin (a native milk protease) from the casein to the whey fraction. The whey fra ction from cheesemaking is commonly used to make whey protein concentrates and isolates, which then are used as functional ingredients in various food systems. Plasmin activity in whey protein products may cause breakdown of food proteins to have desirable or undesirable effects on food quality. Thi s raised questions about the level of plasmin in commercial whey protein pr oducts and factors that affect this plasmin level. Therefore, the objective s of this study were to determine: 1) plasmin concentrations in sweet and a cid whey protein products as influenced by Pseudomonas growth during storag e of fresh milk, and 2) plasmin concentrations in commercial whey protein p roducts. Whey type (sweet or acid) had a significantly (P < 0.05) greater effect on whey-associated plasmin activity than did Pseudomonas fluorescens M 3/6 gro wth. Acid whey protein products had significantly (P < 0.05) higher plasmin concentrations than sweet whey. Plasmin activities associated with acid an d sweet whey protein products were both significantly (P < 0.0001) affected by the growth of Pseudomonas fluorescens M 3/6. The interaction effect bet ween bacterial growth and whey type on plasmin activity was not significant (P = 0.2457). Plasmin activity in the reconstituted commercial whey protei n concentrates (i.e., sweet and acid) varied considerably (16.3 to 330 mu g /g of protein), but was significantly lower (2.1 to 4.4 mu g/g of protein, P < 0.05) in whey isolates. These quantitative data were supported by plasm in activity visualized by casein SDS-PAGE.