Spoilage potential of psychrotrophic bacteria isolated from raw milk and the thermo-stability of their enzymes

Abstract

The storage and transportation of raw milk at low temperatures promote the growth of psychrotrophic bacteria and the production of thermo-stable enzymes, which pose great threats to the quality and shelf-life of dairy products. Though many studies have been carried out on the spoilage potential of psychrotrophic bacteria and the thermo-stabilities of the enzymes they produce, further detailed studies are needed to devise an effective strategy to avoid dairy spoilage. The purpose of this study was to explore the spoilage potential of psychrotrophic bacteria from Chinese raw milk samples at both room temperature (28 °C) and refrigerated temperature (7 °C). Species of Yersinia, Pseudomonas, Serratia, and Chryseobacterium showed high proteolytic activity. The highest proteolytic activity was shown by Yersinia intermedia followed by Pseudomonas fluorescens (d). Lipolytic activity was high in isolates of Acinetobacter, and the highest in Acinetobacter guillouiae. Certain isolates showed positive β-galactosidase and phospholipase activity. Strains belonging to the same species sometimes showed markedly different phenotypic characteristics. Proteases and lipases produced by psychrotrophic bacteria retained activity after heat treatment at 70, 80, or 90 °C, and proteases appeared to be more heat-stable than lipases. For these reasons, thermo-stable spoilage enzymes produced by a high number of psychrotrophic bacterial isolates from raw milk are of major concern to the dairy industry. The results of this study provide valuable data about the spoilage potential of bacterial strains in raw milk and the thermal resistance of the enzymes they produce.

Keywords: Spoilage enzyme; Psychrotrophic bacteria; Raw milk; Thermo-stability.

Fig. 1

Number of isolates that produce different types of enzymes at 7 and 28 °C

Fig. 2

Abilities of isolates to produce protease, lipase, β-galactosidase, and phospholipase at 28 °C (a) and 7 °C (b) The number in the bracket implies the total number of strains for each genus

Fig. 3

Proteolytic activity of psychrotrophic bacteria isolated from raw milk (a) Isolates belonging to Pseudomonas incubated at 28 °C; (b) Isolates belonging to other predominated species incubated at 28 °C; (c) Isolates belonging to Pseudomonas incubated at 7 °C; (d) Isolates belonging to other predominated species incubated at 7 °C. Data are expressed as mean±standard deviation (n=3)

Fig. 4

Lipolytic activity of psychrotrophic bacteria isolated from raw milk (a) Isolates belonging to Pseudomonas incubated at 28 °C; (b) Isolates belonging to other predominated species incubated at 28 °C; (c) Isolates belonging to Pseudomonas incubated at 7 °C; (d) Isolates belonging to other predominated species incubated at 7 °C. Data are expressed as mean±standard deviation (n=3)

Fig. 5

Distribution of isolates based on their residual activity (R) of proteases (a) and lipases (b) after the heat treatment at 70 °C for 15 min