Investigations into the activity of enzymes produced by spoilage-causing bacteria: a possible basis for improved shelf-life estimation

Microbial enzymes are major causes of qualify deterioration and food spoilage. Understanding of the enzymatic processes which have taken place or may still occur in food is required for making valid, well-founded shelf-life evaluations We explored the activity of extracellular lipolytic and proteolytic enzymes using bacteria-free enzyme filtrates of selected spoilage-causing bacteria (Pseudomonas aeruginosa, P. fluorescens Aeromonas caviae, A. hydrophila, Proteus mirabilis P. vulgaris, Bacillus cereus, B. subtilis, Staphylococcus aureus, S. epidermidis, Micrococcus luteus, Clostridium perfringens and Serratia marcescens) under the combined influence of various pH values (4.0-7.3) temperatures (2-37 degrees C), water activities (a(w) 0.8-0.98) and of different heating programmes (60 degrees C 60 min(-1) 65 degrees C 30 min(-1) 71 degrees C 15 min(-1), 75 degrees C 5 min(-1)), which can be of value in evaluating the activity of microbial enzymes in food The activity of the lipases and proteases were tested by a modified agar-diffusion method. Tween, tributyrin, gelatin and casein were used as substrates. The collected 77600 data are the theoretic basis for a better estimation of enzyme activity in raw and heated food under the mentioned conditions. Selected results of the enzymatic activity of P. fluorescens were chosen for this publication to demonstrate the variety of the data Generally, results indicated that the combination of temperature, pH and a(w) influences the enzyme activity to a greater degree than a single environmental factor Maximum enzymatic reactions occurred at temperatures between 30 and 37 degrees C, at pH 7.3 and a(w) from 0.95 to 0.98. In general a decrease in temperature, pH or a(w) was associated with a loss of enzyme activity. But considerable reactions of lipases and proteases were also detectable under non-optimal conditions, for instance even at 2 degrees C or at a(w) 0.8 (P. fluorescens). Heating at 65 degrees C 30 min(-1) or 75 degrees C 5 min(-1) did nor destroy enzymes completely (P. fluorescens). The remaining activity is highly remarkable. Further, the range of enzymatic activity can be much higher than the range of bacterial growth. Growth of the tested strains stopped long before the enzymatic activity was undetectable when tested under the same conditions. All results will be accessible by a user-friendly program and will contribute to more accurate evaluation of shelf-life. (C) 1999 Academic Press.