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

Abstract

<i>Salmonella</i> and enterohemorrhagic <i>Escherichia coli</i> (EHEC) are of serious concern in wheat flour and its related products but little is known on their survival and thermal death kinetics. This study was undertaken to determine their long-term viability and thermal inactivation kinetics in flour. Inoculation was performed using mixtures of EHEC serogroups O45, O121, O145 and <i>Salmonella</i> followed by storage at room temperature (23°C) or 35°C (for <i>Salmonella</i>). Plate counting on tryptic soy agar (TSA) and enrichment were used to assess long-term survival. For thermal studies, wheat flour samples were heated at 55, 60, 65, and 70°C and cell counts of EHEC and <i>Salmonella</i> were determined by plating. The δ-values were calculated using the Weibull model. At room temperature, EHEC serovars and <i>Salmonella</i> were quantifiable for 84 and 112 days, and were detectable for the duration of the experiment after 168 and 365 days, respectively. The δ-values were 2.0, 5.54, and 9.3 days, for EHEC O121, O45, and O145, respectively, and 9.7 days for <i>Salmonella</i>. However, the only significant difference among all values was the δ-value for <i>Salmonella</i> and serogroup O121 (<i>p</i> ≤ 0.05). At 35°C, <i>Salmonella</i> counts declined to unquantifiable levels after a week and were not detected upon enrichment after 98 days. Heat treatment of inoculated wheat flour at 55, 60, 65, and 70°C resulted in δ-value ranges of 20.0-42.9, 4.9-10.0, 2.4-3.2, and 0.2-1.6 min, respectively, for EHEC. The δ-values for <i>Salmonella</i> at those temperatures were 152.2, 40.8, 17.9, and 17.4 min, respectively. The δ-values obtained for <i>Salmonella</i> at each temperature were significantly longer than for EHEC (<i>p</i> ≤ 0.05). Weibull model was a good fit to describe the thermal death kinetics of <i>Salmonella</i> and EHEC O45, O121 and O145 in wheat flour. <b>HIGHLIGHTS</b> -EHEC and <i>Salmonella</i> can survive for extended periods of time in wheat flour.-Long-term storage inactivation curves of EHEC and <i>Salmonella</i> were similar.-EHEC was more sensitive to heat than <i>Salmonella.</i>-Weibull model was a good fit to describe thermal death kinetics of EHEC and <i>Salmonella.</i>-Flour storage at 35°C may be a feasible method for microbial reduction.