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Bacillus cereus in a whey process

T S Pirttijärvi1, L M Ahonen, L M Maunuksela

  • 1Department of Applied Chemistry and Microbiology, Biocenter, University of Helsinki, Finland. tuija.pirttijarvi@helsinki.fi

International Journal of Food Microbiology
|December 16, 1998
PubMed
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Bacillus cereus contamination increased significantly from raw milk to whey concentrate in a cheese dairy. Different B. cereus strains were found in warm and cold processing areas, impacting detection methods and phage sensitivity.

Area of Science:

  • Food Microbiology
  • Dairy Science
  • Bacterial Contamination

Background:

  • Bacillus cereus is a common foodborne pathogen.
  • Its presence in dairy products is a concern for food safety.
  • Understanding B. cereus dynamics in cheese manufacturing is crucial.

Purpose of the Study:

  • To investigate the prevalence and characteristics of Bacillus cereus in a cheese dairy and its whey manufacturing line.
  • To differentiate B. cereus populations in different processing stages and temperatures.
  • To assess the impact of processing on B. cereus biotypes, fatty acid profiles, phage sensitivity, and growth characteristics.

Main Methods:

  • Sampling of raw milk and various stages of the whey process.
  • Isolation and identification of Bacillus cereus.

Related Experiment Videos

  • Analysis of B. cereus biotypes (nitrate reductase, casein hydrolysis).
  • Whole cell fatty acid profiling.
  • 16S rDNA sequencing.
  • Phage sensitivity testing.
  • Determination of minimum growth temperatures.

    • Main Results:

    • Bacillus cereus was detected in 17% of 720 samples, rising from 3% in raw milk to 76% in evaporated whey concentrate.
    • Defective biotypes (nitrate reductase negative, weak casein hydrolysis) became more frequent in later whey processing stages.
    • B. cereus isolates from whey showed different fatty acid profiles and were poorly recognized by commercial libraries compared to raw material isolates.
    • While 100% 16S rDNA similarity was observed, phage sensitivity varied, with 43% of whey isolates being insensitive.
    • Whey processing isolates did not grow at or below 8°C, unlike some raw material strains.

    Conclusions:

    • The B. cereus population in the warm sections (>30°C) of the cheese dairy process is distinct from that in the cold sections (2-4°C).
    • Processing conditions, particularly temperature, influence B. cereus characteristics, including biotype, phage susceptibility, and growth potential.
    • Standard identification methods like fatty acid analysis may misidentify B. cereus strains adapted to specific dairy processing environments.