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 spoila ge. Understanding of the enzymatic processes which have taken place or may still occur in food is required for making valid, well-founded shelf-life e valuations We explored the activity of extracellular lipolytic and proteoly tic enzymes using bacteria-free enzyme filtrates of selected spoilage-causi ng bacteria (Pseudomonas aeruginosa, P. fluorescens Aeromonas caviae, A. hy drophila, Proteus mirabilis P. vulgaris, Bacillus cereus, B. subtilis, Stap hylococcus aureus, S. epidermidis, Micrococcus luteus, Clostridium perfring ens and Serratia marcescens) under the combined influence of various pH val ues (4.0-7.3) temperatures (2-37 degrees C), water activities (a(w) 0.8-0.9 8) 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 activ ity of the lipases and proteases were tested by a modified agar-diffusion m ethod. Tween, tributyrin, gelatin and casein were used as substrates. The c ollected 77600 data are the theoretic basis for a better estimation of enzy me activity in raw and heated food under the mentioned conditions. Selected results of the enzymatic activity of P. fluorescens were chosen for this p ublication to demonstrate the variety of the data Generally, results indica ted that the combination of temperature, pH and a(w) influences the enzyme activity to a greater degree than a single environmental factor Maximum enz ymatic reactions occurred at temperatures between 30 and 37 degrees C, at p H 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 rea ctions of lipases and proteases were also detectable under non-optimal cond itions, for instance even at 2 degrees C or at a(w) 0.8 (P. fluorescens). H eating 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 rema rkable. Further, the range of enzymatic activity can be much higher than th e range of bacterial growth. Growth of the tested strains stopped long befo re the enzymatic activity was undetectable when tested under the same condi tions. All results will be accessible by a user-friendly program and will c ontribute to more accurate evaluation of shelf-life. (C) 1999 Academic Pres s.