title: Growth/no growth models for heat-treated psychrotrophic Bacillus cereus spores under cold storage.
abstract: The microbiological safety of refrigerated and processed foods of extended durability (REPFED) is linked to spore-forming pathogens, more specifically Clostridium botulinum and Bacillus cereus. In this study two sets of growth/no growth (GNG) models are presented for the spores of two B. cereus strains. The models incorporate both product (water activity (a(w)) and pH) and process parameters (pasteurization value at 90 °C (P(90)) or heating temperature). The first model evaluates the effect of four different P(90)-values (P(90)=0, 4, 7 or 10 min, all applied at 90 °C) on the germination and subsequent growth of B. cereus spores under different conditions of pH and a(w) at 10 °C. These models show that a heat treatment not only increases the time to growth (TTG), but also significantly increases the minimal a(w) and pH necessary for germination and subsequent growth: e.g. at a(w) 0.995 and without heat treatment (P(90)=0), strain FF355 B. cereus spores were predicted to germinate and grow at pH 5.3. With a P(90) of 10 min, the minimal pH increased to 5.7. The second set of models for B. cereus spores compares the effect of three heat treatments with the same P(90)-value (10 min) but applied at different temperatures (85, 87 and 90 °C), on the germination and subsequent growth at 10 °C. The second model shows that lower heating temperatures (85 and 87 °C) had less effect on the TTG and minimal a(w) and pH than a higher temperature (90 °C). Finally, the first set of models was validated in broth using spores of seven psychrotrophic B. cereus strains, to evaluate the effect of strain variability on the model predictions. The results of the validation (% growth) were compared to the predicted growth probability. The results showed that the models were prone to fail-dangerous results (i.e. predicting no growth when growth was observed: 17%-34%). Using a very low threshold for growth (0.1% predicted chance of growth was considered to be complete growth), the models were more fail-safe (11%-34%) than fail-dangerous (0.4%-14%).
authors.affiliation: Laboratory of Food Microbiology and Food Preservation, Department of Food Safety and Food Quality, Part of Food2Know, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, B-9000 Ghent, Belgium. jeff.daelman@ugent.be
authors.forename: Jeff
authors.initials: J
authors.lastname: Daelman
authors.forename: An
authors.initials: A
authors.lastname: Vermeulen
authors.forename: Tine
authors.initials: T
authors.lastname: Willemyns
authors.forename: Rebecca
authors.initials: R
authors.lastname: Ongenaert
authors.forename: Liesbeth
authors.initials: L
authors.lastname: Jacxsens
authors.forename: Mieke
authors.initials: M
authors.lastname: Uyttendaele
authors.forename: Frank
authors.initials: F
authors.lastname: Devlieghere
mesh_terms: D001409:Bacillus cereus / Q000254:growth & development*; D003080:Cold Temperature; D015169:Colony Count, Microbial; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D016015:Logistic Models; D008954:Models, Biological; D015203:Reproducibility of Results; D013171:Spores, Bacterial / Q000254:growth & development; D013997:Time Factors