Bacillus cereus in a whey process

Abstract

A cheese dairy and its whey manufacturing line were examined for Bacillus cereus. Colonies typical of B. cereus were detected in 120 (17%) samples out of 720 analysed. Only 3% of the sampled raw milk contained B. cereus (≥10 cfu ml −1) whereas in evaporated whey concentrate B. cereus was present in 76% of the samples. Nitrate reductase negative and weakly casein hydrolysis isolates were rare in raw milk and the early parts of the process but these defective biotypes became increasingly frequent towards the end of the whey process. The composition of whole cell fatty acids of B. cereus isolates originating from the whey part of the process was different from that of the type strain and of the isolates originating from the raw materials of cheese making. The B. cereus strains in concentrated whey were 100% similar to the type strain in 16S rDNA sequence (500 bp) although they were not or only poorly recognized as B. cereus by a commercial whole cell fatty acid library. All of B. cereus isolates in raw milk were sensitive to one or more of the B. cereus group phages ( n=17) whereas 43% of the isolates from the whey process were sensitive to none. None of the 23 strains originating from the whey processing lines grew at ≤8°C, although strains with minimum growth temperatures of 5.3°C and 7.0°C were present in the raw materials. Our results indicate that the B. cereus population of the warm (>30°C) parts of the cheese dairy process was separate from that of cold (2°C to 4°C) part of the process.