Long-Term Survival and Thermal Death Kinetics of Enterohemorrhagic Escherichia coli Serogroups O26, O103, O111, and O157 in Wheat Flour

Abstract

Wheat flour has been associated with outbreaks of enterohemorrhagic (EHEC), but little is known on EHEC's survival during storage and thermal processing. The objective of this study was to determine long-term viability and thermal inactivation kinetics of EHEC serogroups O26, O103, O111, and O157. Wheat flour samples were inoculated with a cocktail of five strains of a single serogroup and stored at 23 and 35°C. Inoculated samples were heated at 55, 60, 65, and 70°C. Viability was determined by plate counting. Decimal reduction time () and first decimal reduction time (δ) values were calculated with log-linear and Weibull models, respectively. At 23°C, EHEC counts declined gradually for 84 days and samples tested positive from 84 to 280 days. The thermal resistance ( and δ) values ranged from 7.5 to 8.2 and 3.1 to 5.3 days, respectively, but there were no significant differences among serogroups ( ≤ 0.05). At 35°C, no EHEC was quantifiable by day 7 and no positive samples were detected after 49 days. Heating at 55 and 65°C resulted in δ-value ranges of 15.6 to 39.7 min and 3.0 to 3.9 min, respectively, with no significant difference among serogroups either. Z values were 12.6, 6.7, 10.2, and 13.4°C for O26, O103, O111, and O157, respectively. Thermal death kinetics of EHEC in flour were better described using the Weibull model. Survival and inactivation rates of four serogroups were remarkably similar. These findings indicated that all EHEC serovars tested remained viable for at least 9 months at room temperature and survived for up to 60 min at 70°C in wheat flour. Enterohemorrhagic (EHEC) and have recently caused several gastroenteritis outbreaks and recalls of wheat flour. Because EHEC can cause illness with very low doses and there is very scarce information regarding their ability to survive storage and heating in flour, the present study was undertaken to assess the long-term survival of EHEC serogroups O26, O103, O111, and O157 in flour. These findings are relevant, as we report that EHEC can survive for more than 9 months in wheat flour during storage. In addition, results obtained suggest that thermal inactivation at 65°C for 30 min or 2 months of storage at 35°C may be feasible strategies to mitigate the risk of most EHEC serovars in wheat flour.

Salmonella and Enterohemorrhagic Escherichia coli Serogroups O45, O121, O145 in Wheat Flour: Effects of Long-Term Storage and Thermal Treatments

“…Thus in the present paper, we included assessing Salmonella long-term survival at room temperature and thermal death kinetics at 55, 60, 65, and 70°C in wheat flour. In addition, Salmonella survival was studied at 35°C for comparison with similar data on EHEC obtained in our previous study (Forghani et al, 2018). These temperatures were selected because it was unlikely that they had negative influence on the wheat flour properties (Ozawa et al, 2009; Khamis, 2014; Sudha et al, 2016) for the amount of time that they were used in the study.…”

“…The information available about EHEC long-term survival in wheat flour was very limited until our recent study (Forghani et al, 2018), in which we reported survival and thermal inactivation of EHEC serovars O26, O103, O111, and O157 in wheat flour. Such information was important to the industry due to the fact that product composition such as the unique composition of wheat flour may affect the rate of microbial inactivation (Santillana Farakos et al, 2014).…”

“…A number of studies have reported that vegetative bacterial species can survive during prolonged storage in dry foods (Eglezos, 2010; Forghani et al, 2018). The traditional process given to wheat grains to produce flour does not normally involve an antimicrobial step which results in a relatively stable microbiological load during flour production (Manthey et al, 2004).…”

“…In an effort to advance our understanding about the risk of EHEC in wheat flour, we recently reported on the long-term survival and thermal inactivation kinetics of EHEC O26, O103, and O157 in wheat flour (Forghani et al, 2018). That study helped assess the likelihood of EHEC survival during storage, helping build fundamental knowledge on the subject and also tested the viability of EHEC reduction in flour by thermal treatments using linear and Weibull models (Santillana Farakos et al, 2013).…”

Microbiological Survey of Wheat Flour Sold at Retail in Canada, 2018 to 2019

“…Wheat flour is an agricultural product which can be contaminated with bacterial pathogens during primary production and the milling process (34,37). The low-moisture content (aw <0.6) of wheat flour does not support the growth of bacterial pathogens, however some bacterial pathogens can survive in this low-moisture environment (21,25) and can multiply in highermoisture raw flour-based products such as batter or cookie dough (43). Ready-to-Eat (RTE) wheat flour-based products are usually subject to thermal treatments such as baking, steaming, and frying, all of which would contribute to the inactivation of bacterial pathogens before consumption.…”