Identification of contamination sources of Bacillus cereus in pasteurized milk

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Lin, Shirley

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University of Guelph

Abstract

This research investigated the potential of Fourier transform infrared spectroscopy (FTIR) and the Microbial Identification System (MIDI) to rapidly identify Bacillus cereus and to trace the sources of B. cereus in pasteurized milk. Ten B. cereus strains tested produced identical characteristic infrared absorbance peak between wavenumbers 1738-1740 cm\sp−1 on brain heart infusion (BHI) and trypticase soy agar (TSA) by using FTIR. The infrared absorbance peak can be used to differentiate B. cereus from other bacteria. A total of 232 milk samples and 122 environmental swabs were collected in two dairy plants between March and September 1996. The incidence and the average counts of B. cereus in the positive heat-treated raw milk, pasteurized milk and final products were over 80% and 1.1×10\sp5 CFU/ml, respectively, after enrichment at 8\sp∘C for 14 days. A total of 546 B. cereus isolates from 183 milk samples and 3 environmental swabs were classified by their fatty acid profiles using MIDI. The results suggested that the B. cereus spores in raw milk were the major source of B. cereus in pasteurized milk and the post-pasteurization contamination from the dairy environment was possibly another minor source of B. cereus in pasteurized milk.

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Fourier transform infrared spectroscopy, Microbial Identification System, identification, Bacillus cereus, trace, pasteurized milk

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