Effects of Inoculation Procedures on Variability and Repeatability of Salmonella Thermal Resistance in Wheat Flour

Abstract

Limited prior research has shown that inoculation methods affect thermal resistance of Salmonella in low-moisture foods; however, these effects and their repeatability have not been systematically quantified. Consequently, method variability across studies limits utility of individual data sets and cross-study comparisons. Therefore, the objective was to evaluate the effects of inoculation methodologies on stability and thermal resistance of Salmonella in a low-moisture food (wheat flour), and the repeatability of those results, based on data generated by two independent laboratories. The experimental design consisted of a cross-laboratory comparison, both conducting isothermal Salmonella inactivation studies in wheat flour (~0.45 water activity, 80°C), utilizing five different inoculation methods: (i) broth-based liquid inoculum, (ii) lawn-based liquid inoculum, (iii) lawn-based pelletized inoculum, (iv) direct harvest of lawn culture with wheat flour, and (v) fomite transfer of a lawn culture. Inoculated wheat flour was equilibrated ~5 days to ~0.45 water activity and then was subjected to isothermal treatment (80°C) in aluminum test cells. Results indicated that inoculation method impacted repeatability, population stability, and inactivation kinetics (α = 0.05), regardless of laboratory. Salmonella inoculated with the broth-based liquid inoculum method and the fomite transfer of a lawn culture method exhibited instability during equilibration. Lawn-based cultures resulted in stable populations prior to thermal treatment; however, the method using direct harvest of lawn culture with wheat flour yielded different D-values across the laboratories (α = 0.05), which was attributed to larger potential impact of operator variability. The lawn-based liquid inoculum and the lawn-based pelletized inoculum methods yielded stable inoculation levels and repeatable D-values (~250 and ~285 s, respectively). Also, inoculation level (3 to 8 log CFU/g) did not affect D-values (using the lawn-based liquid inoculum method). Overall, the results demonstrate that inoculation methods significantly affect Salmonella population kinetics and subsequent interpretation of thermal inactivation data for low-moisture foods.

Inoculation Protocols Influence the Thermal Resistance of Salmonella Enteritidis PT 30 in Fabricated Almond, Wheat, and Date Products

“…The Syamaladevi et al (24) experiment was similar to this study, except for the inoculum preparation. These results support the premise that inoculation procedures impact thermal resistance of Salmonella in wheat flour (10).…”

“…0.05) and were lower than the other product types because the inoculum contained fewer cells. However, prior results have shown that initial inoculation level does not affect thermal resistance of Salmonella Enteritidis PT 30 in low-moisture products (10); therefore, comparisons of thermal resistance between pre-and postfabrication samples should not be affected by these differences in initial population.…”

“…In conclusion, these results have shown that thermal resistance of Salmonella Enteritidis PT 30 depends on the inoculation protocol, product type, and product structure. In all known prior studies with fabricated products (e.g., peanut butter (9,16), wheat flour (10,23), and dried fruits (1)), postfabrication inoculation protocols were applied to inoculate products, determine inactivation kinetics, and validate the processes. This suggests that some published data may not accurately reflect actual scenarios in which a raw material is contaminated and then fabricated into an ingredient or finished product, which may influence thermal resistance.…”

“…Similarly, Keller et al (13) reported that Salmonella growth procedures, including temperature and growth media, also impacted Salmonella thermal resistance in peanut butter. More recently, when Hildebrandt et al (10) used five different methods for inoculating Salmonella into wheat flour, significantly different survival kinetics were obtained at the same water activity (0.45 a w )a n d temperature (808C). Additionally, a mist-inoculation procedure was shown to result in lower Salmonella survival than did a sand-inoculation procedure for stored dried fruit (1).…”

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

“…Therefore, in this study we used a cocktail of 5 different EHEC strains of each serogroup to better account for the possible natural variability among strains (25). The inoculation method may influence the thermal resistance and survival of inoculated bacteria in challenge food studies (26). Liquid inoculation was reported to result in instability of Salmonella during equilibration, which can be expected since desiccation is an environmental stress adversely affecting viable bacterial numbers (27).…”