title: 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.
authors.affiliation: Department of Biosystems and Agricultural Engineering, Michigan State University, East Lansing, Michigan 48824.
authors.forename: Ian M
authors.initials: IM
authors.lastname: Hildebrandt
authors.affiliation: Department of Biosystems and Agricultural Engineering, Michigan State University, East Lansing, Michigan 48824.
authors.forename: Bradley P
authors.initials: BP
authors.lastname: Marks
authors.affiliation: Department of Food Science and Human Nutrition, Michigan State University, East Lansing, Michigan 48824.
authors.forename: Elliot T
authors.initials: ET
authors.lastname: Ryser
authors.affiliation: Department of Biological Systems Engineering, Washington State University, Pullman, Washington 99164, USA.
authors.forename: Rossana
authors.initials: R
authors.lastname: Villa-Rojas
authors.affiliation: Department of Biological Systems Engineering, Washington State University, Pullman, Washington 99164, USA.
authors.forename: Juming
authors.initials: J
authors.lastname: Tang
authors.affiliation: Department of Biosystems and Agricultural Engineering, Michigan State University, East Lansing, Michigan 48824.
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Garces-Vega
authors.affiliation: Department of Biosystems and Agricultural Engineering, Michigan State University, East Lansing, Michigan 48824.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Buchholz
mesh_terms: D015169:Colony Count, Microbial; D005433:Flour; D005516:Food Microbiology; D012475:Salmonella / Q000187:drug effects; D014908:Triticum
keywords: Error; Flour; Low moisture; Pathogen; Thermal resistance; Water activity