The influence of non-lethal temperature on the rate of inactivation of vegetative bacteria in inimical environments may be independent of bacterial species.

The influence of non-lethal temperature on the survival of two species of food-borne bacteria under growth-preventing pH and water activity conditions was investigated. Specifically, inactivation rates of four strains of Escherichia coli and three strains of Listeria monocytogenes were determined in culture broth adjusted to pH 3.5 and water activity 0.90, to prevent growth of both species, and for temperatures in the range 5-45 degrees C at 5 degrees C intervals. Sixty-three inactivation rates were obtained, plotted on Arrhenius co-ordinates, and lines of best-fit determined by simple linear regression. Differences in the mean inactivation rate of each species at a given temperature were not significant (p < 0.05) with the exception of the rates at 25 degrees C. The inactivation rate responses of both species were comparable to those reported by McQuestin et al. (Appl. Environ. Microbiol., 75:6963-6972, 2009) for a variety of E. coli strains under a wide range of growth-preventing pH and water activity conditions. The results support the hypothesis that non-lethal temperature is a key factor governing the rate of inactivation of vegetative bacteria in foods when other hurdles prevent their growth and indicate that the temperature effect may also be independent of bacterial species.

Kinetics of growth and inactivation of Salmonella enterica serotype Typhimurium DT104 in pasteurised liquid egg products.

The potential impact of post-pasteurisation contamination of liquid egg products with the multi-antibiotic resistant pathogen Salmonella enterica serotype Typhimurium definitive type 104 (DT104) was assessed by determining the viability of this bacterium in whole egg, albumen and 10% w/w sugared and salted yolk incubated at 4-42 degrees C. Results indicated that populations of S. Typhimurium DT104 were slowly inactivated in all four products when stored at 4 degrees C. However, based on the typical shelf-lives of cold-stored liquid egg, less than 0.6 log-kill would be achieved in those products prior to their use. Incubation at temperatures pertaining to abuse situations (10, 15, 20 and 25 degrees C) revealed an increasing potential for growth of S. Typhimurium DT104 in whole egg, albumen and sugared yolk, as indicated by trends in growth rate, lag duration and maximum population density. At even higher temperatures (30, 37 and 42 degrees C), growth rates of S. Typhimurium DT104 in whole egg and sugared yolk continued to increase. The same was true for S. Typhimurium DT104 in albumen except that growth was not observed at 42 degrees C and instead populations were inactivated within 30 h. At no temperature tested was S. Typhimurium DT104 able to grow in salted yolk. The influence of these growth and inactivation patterns on the risk of salmonellosis in relation to product type and storage temperature is discussed.

Growth and survival of antibiotic-resistant Salmonella typhimurium DT104 in liquid egg products.

Since 11 September 2001, quality and food safety are no longer the concerns of only consumers, industry, regulatory agencies, or other government officials. Liquid foods that are prepared or stored in bulk, including liquid egg products, are considered to be at potential risk for sabotage. Because of their versatility, low price, and functional properties, many of these products are being marketed. Four of the most common products of this type are whole egg, egg albumen, 10% sugared yolk, and 10% salted yolk. Although all of the serotypes of Salmonella enterica may cause illness, multidrug-resistant Salmonella Typhimurium DT104 has become widespread and can cause severe illness that is difficult to treat. Studies were conducted to determine growth patterns of Salmonella Typhimurium DT104 in four commercial liquid egg products held at 4, 10, 20, 30, 37, and 42 degrees C for 0 to 384 h. All experiments were performed in duplicate and repeated twice. Standard methods were used to estimate cell numbers, and log CFU per gram of egg product was plotted against time. The number of cells of Salmonella Typhimurium DT104 increased to 8 to 9 log CFU/g in whole egg and 10% sugared yolk, increased by 1 log CFU/g in liquid albumen, but decreased by 3 log CFU/g in 10% salted yolk. Data from this study have been archived in the ComBase database to further assist policy makers or other scientists interested in Salmonella growth characteristics in liquid eggs. However, based on data generated in this study, Salmonella Typhimurium DT104 probably does not constitute a food threat agent in liquid eggs.

Quantification of the relative effects of temperature, pH, and water activity on inactivation of Escherichia coli in fermented meat by meta-analysis.

Outbreaks of Escherichia coli infections linked to fermented meats have prompted much research into the kinetics of E. coli inactivation during fermented meat manufacture. A meta-analysis of data from 44 independent studies was undertaken that allowed the relative influences of pH, water activity (a(w)), and temperature on E. coli survival during fermented meat processing to be investigated. Data were reevaluated to determine rates of inactivation, providing 484 rate data points with various pH (2.8 to 6.14), a(w) (0.75 to 0.986), and temperature (-20 to 66 degrees C) values, product formulations, and E. coli strains and serotypes. When the data were presented as an Arrhenius model, temperature (0 to 47 degrees C) accounted for 61% of the variance in the ln(inactivation rate) data. In contrast, the pH or a(w) measured accounted for less than 8% of variability in the data, and the effects of other pH- and a(w)-based variables (i.e., total decrease and rates of reduction of those factors) were largely dependent on the temperature of the process. These findings indicate that although temperatures typically used in fermented meat manufacture are not lethal to E. coli per se, when other factors prevent E. coli growth (e.g., low pH and a(w)), the rate of inactivation of E. coli is dominated by temperature. In contrast, inactivation rates at temperatures above approximately 50 degrees C were characterized by smaller z values than those at 0 to 47 degrees C, suggesting that the mechanisms of inactivation are different in these temperature ranges. The Arrhenius model developed can be used to improve product safety by quantifying the effects of changes in temperature and/or time on E. coli inactivation during fermented meat manufacture.

Temperature governs the inactivation rate of vegetative bacteria under growth-preventing conditions.

Novel studies, in combination with a meta-analysis of available data, were undertaken to explore the kinetics of non-thermal inactivation of Escherichia coli with particular attention to inactivation in fermented meats and including analogous broth-based model systems. The analyses were based on rates of inactivation and specifically investigated the influence of temperature, pH and water activity at levels that alone, or in combination, prevented growth. When independently-derived inactivation data, obtained using different test conditions and diverse E. coli strains, were presented as Arrhenius plots, temperature was found to have a strong effect on the rate of inactivation, explaining 60% of the variance in the data. The slope of the Arrhenius plot changed, however, at temperatures above approximately 47 degrees C, corresponding to the maximum for growth of E. coli. A strong and consistent effect of pH or water activity on inactivation rate was not observed upon meta-analysis of collated data, but the relative effect of both factors was quantified in an analogous broth-based system. We also observed that inactivation rates of three strains of Listeria monocytogenes in the range 5 to 40 degrees C did not differ systematically from those of four strains of E. coli when growth was prevented by low pH and water activity. The observations of a consistent slope of Arrhenius plots for non-thermal inactivation rate of bacteria under diverse environmental conditions and for different strains and species, but which differ from slopes associated with thermal inactivation, raise the intriguing possibility of a mechanism of inactivation at sub-lethal temperatures, distinct from thermal inactivation, that is common to many vegetative bacteria.