Influence of temperature shifts on survival, growth, and toxin production by psychrotrophic and mesophilic strains of Bacillus cereus in potatoes andchicken gravy

A study was done to determine the influence of temperature on growth and to xin production characteristics of psychrotrophic and mesophilic strains of Bacillus cereus when inoculated into mashed potatoes and chicken gravy cont aining various concentrations of sodium chloride and held at temperatures d ifferent from those at which cells had been cultured. Logarithmic growth ph ase cells (10 h, 30 degrees C) of psychrotrophic (F3802A/84) and mesophilic (B4ac-1) strains of Bacillus cereus were inoculated into rehydrated commer cially processed instant mashed potatoes and chicken gravy supplemented wit h 0, 2, or 4% sodium chloride. Growth, survival, and diarrheal toxin produc tion in potatoes and gravy held at 30, 37, and 10 degrees C (strain F3802A/ 84) or 30, 40, and 10 degrees C (strain B4ac-1) were monitored. Both strain s grew in bath foods containing no added sodium chloride or 2% sodium chlor ide when held at 30, 37, or 40 degrees C for 2 days. Strain B4ac-1 grew bet ter than strain F3802A/84 in foods containing 4% sodium chloride. Maximum a mounts of enterotoxin (1024 ng/g) were produced by strain B4ac-1 in chicken gravy held at 30 and 40 degrees C. Strain F3802A/84 grew to populations of 7 log(10) CFU/g in foods containing no added sodium chloride or 2% sodium chloride at 10 degrees C. Strain F3802A/84 produced the highest amount of e nterotoxin (1024 ng/g) at 30 degrees C in chicken gravy containing 0.7 or 2 % sodium chloride however, little or low amounts of toxin (4-16 ng/g) were produced in chicken gravy at 10 degrees C. Compared to strain B4ac-1, cells of strain F3802A/84 subjected to a downward shift in incubation temperatur e (10 degrees C) grew more rapidly in chicken gravy. Strain B4ac-1 produced the highest amount of toxin (1024 ng/g) at 30 degrees C in gravy containin g 4% sodium chloride and at 40 degrees C in gravy containing 0.7% sodium ch loride. Toxin was not detected in inoculated mashed potatoes. Results of th is study indicate that shifts in incubation temperature influence growth an d toxin production by psychrotrophic and mesophilic strains of B. cereus di fferently. It is important to store pasteurized, ready-to-eat foods at a te mperature low enough to prevent the growth of B. cereus. (C) 1999 Elsevier Science B.V. All rights reserved.