4.4 Article

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

Journal

JOURNAL OF FOOD PROTECTION
Volume 79, Issue 11, Pages 1833-1839

Publisher

INT ASSOC FOOD PROTECTION
DOI: 10.4315/0362-028X.JFP-16-057

Keywords

Error; Flour; Low moisture; Pathogen; Thermal resistance; Water activity

Funding

  1. U.S. Department of Agriculture (USDA)
  2. National Institute of Food and Agriculture (NIFA)
  3. National Integrated Food Safety Initiative (NTFSI) [2011-51110-30994]
  4. CONACyT (Consejo Nacional de Ciencia y Tecnologia, Mexico)

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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 (similar to 0.45 water activity, 80 degrees 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 days similar to 5 to similar to 0.45 water activity and then was subjected to isothermal treatment (80 degrees C) in aluminum test cells. Results indicated that inoculation method impacted repeatability, population stability, and inactivation kinetics (alpha=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 (alpha=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 (similar to 250 and similar to 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.

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