摘要：Background and Objective: Psychrotrophic bacteria produce extracellular proteases, resulting in deterioration and reduced shelf life of dairy products. In this study, 21 species of psychotropic bacteria isolated from buffalo milk were selected and the thermal resistance of the proteases produced by these bacteria was evaluated. Materials and Methods: The isolates were tested to evaluate proteolytic activity of buffalo milk agar. The cell-free supernatants from the growing of isolates were obtained for the quantification of enzymatic activity under different pH values (5.5, 7.0 and 8.0). Thermal resistance and the clotting ability of proteolytic enzymes in buffalo and bovine milk substrates were also evaluated. One-way ANOVA test with a critical probability of p < 0.05 followed by the Tukeys test was used to evaluate the results. Results: All strains were able to produce proteolysis in buffalo milk agar; additionally, all cell-free supernatants showed enzymatic activity , with values of >1 U mL1 under at least one of the pH tested. Five isolates produced cell-free supernatants resistant to pasteurization (63.5°C/30 min), following which they were able to coagulate buffalo and bovine milk. The crude enzyme of P. fluorescens PL5.4 showed the greatest enzymatic activity within a wide pH range (4-10) and at an optimum temperature of 40°C. The cell-free supernatant of this isolate resisted to tests with detergents and organic solvents. However, it was not possible to identify the type of protease. Conclusion: The results of this study showed the negative impact of the presence of psychrotrophic bacteria producing proteolytic enzymes in buffalo milk. This is because the enzymes studied caused changes in milk samples, revealing a negative impact on the production of derived products. This is significant, since the buffalo milk produced in Brazil is directed to the production of dairy products.
