Model for Combined Effects of Temperature, pH and Water Activity on Thermal Inactivation of Bacillus cereus Spores

Abstract

Initially, the effect of water activity (a w ) on heat resistance of Bacillus cereus spores (decimal reduction time) was investigated. A linear relationship was found between log D and 1-a w . The combined effects of temperature (85-105°C), pH (4.5-6.5) and water activity (0.80-1) were then studied. A four parameter model was fitted to the data. This model appeared to be parsimonious with each parameter having a biological significance. Interactions between factors were observed but they accounted for <2.4% of the total variation and they were not taken into account by the model. The obvious analogy between growth and survival kinetics induced some researchers to base thermal resistance models on predictive microbiology. Fernandez et al. (1996) developed a linear and a quadratic model for describing the heat resistance of spores vs temperature and pH. Davey et al. (1978) were the first to develop a model for predicting the combined effects of process temperature and medium pH on thermal resistance of spores. Their four parameter model was explained to be an extension of the Arrhenius' equation. Mafart and Leguérinel (1998) proposed an extension of Bigelow's equation and developed a model with three parameters for describing the effects of temperature and pH. The water activity was taken into account for the first time by Reichart (1994) who derived a semi-empirical model for the death rate of Escherichia coli. The application of their five parameter model is difficult because it requires hydrogen and hydroxyl ion concentrations to be known. Another five parameter model was proposed by Cerf et al. (1996) from the experimental data of Reichart (1994). This model is an extension of the Davey's model that included water activity in addition to temperature and pH:LnK ϭ C 0 ϩ(C 1 /T) ϩ C 2 pH ϩ C 3 pH 2 ϩC4a w 2where T is the absolute temperature, a w is the water activity and C 0 , C 1 , C 2 , C 3 , C 4 are empirical coefficients without biological significance.Our objective was to develop, using a similar approach, a new model including, in addition to temperature and pH, water activity by an extension of the first version of Mafart's model (Mafart and Leguérinel, JOURNAL OF FOOD SCIENCE MICROBIOLOGYThe authors are affiliated with

The occurrence of enterotoxigenic isolates of B. cereus in foodstuffs

“…It is, however, suggested that the highest risk for consumers is foodstuffs with long expiration periods, such as cured meats for grilling, spreads and specialities, etc. Formerly published data (Gaillard et al 1998) confirm our thesis that pH and cold temperature cannot be considered as quite secure factors for the restriction of the growth and activity of B. cereus. The isolates from ready-to-cook products and from swabs were characterised by lower quantities of enterotoxin production in comparison with the group of isolates coming from meat products.…”

Behavioural pattern of vegetative cells and spores of Bacillus cereus as affected by time-temperature combinations used in processing of Indian traditional foods

“…Although pasteurization is known to inactivate the vegetative cells, spores of B. cereus are known to be highly heat resistant and are usually unaffected by physical, chemical and biological factors (Gaillard et al 1998;González et al 1999). In general, investigations have addressed these aspects relating to either vegetative cells or spores and not both the phases together of one and the same species, except for a not very recent study (Byrne et al 2006).…”

Effects of hydration on molecular mobility in phase-bright Bacillus subtilis spores

“…The characteristically high heat resistance of dormant bacterial spores is explained by the low water concentration in the core Abbreviations : CPMAS, cross-polarization magic angle spinning ; DPA, dipicolinic acid ; SPMAS, single-pulse magic angle spinning ; STEM, scanning transmission electron microscopy ; TEM, transmission electron microscropy. (Nakashio & Gerhardt, 1985 ;Beaman & Gerhardt, 1986 ;Setlow, 1994 ;Gaillard et al, 1998). Water contents of spore protoplasts were found to vary among Bacillus species between 26 and 55 % (Nakashio & Gerhardt, 1985 ;Lindsay et al, 1985 ;Beaman et al, 1984 ;Beaman & Gerhardt, 1986).…”