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IAFP’s European Symposium on Food Safety

IAFP's European Symposium on Food Safety
Nantes, France
April 24, 2019 – April 26, 2019

Session: Survival and Potential for Pathogenicity Changes for Listeria monocytogenes in Low-moisture Foods
Wednesday, 24 April, 2019
Jeffrey Farber, PhD
University of Guelph

Low-moisture foods (LMFs) are characterized by a water activity (aw) below 0.85 and are emerging as novel vehicles for foodborne illness. Although the growth of bacterial pathogens is inhibited by low aw, they have been shown to survive/persist for long periods of time in some LMFs. This presents a public health concern, especially when LMFs are consumed without undergoing any microbial inactivation steps. The main purpose of our study is to assess the survival of Listeria monocytogenes on artificially-inoculated LMFs. Foods were inoculated with a 4-strain cocktail of Lm at an initial concentration of 8 log10 CFU/g by immersion (pistachios) or misting (chocolate liquor, corn flakes). They were then dried at 30°C, and stored at both 23°C, 30–35% relative humidity (RH) and 4°C, 30–35% RH. For dried fruits (raisins, apples, strawberries) we used a dry-inoculum, and research was done to determine the best carrier for the recovery and enumeration of L. monocytogenes. Bacterial counts were done on tryptic soy agar with 0.6% (w/v) yeast extract, and/or Oxford agar. Analysis of significant differences in survival was determined by using a two-way repeated measures ANOVA. In general, the organism survived best at both temperatures in pistachios and chocolate liquor and worst in dried apples. Monthly sampling of LMFs was done for up to a year. A new multiplex PCR method was developed to track and differentiate between the 4 L. monocytogenes serotypes (1/2a, 1/2b, 3a, and 4b) used to inoculate the LMFs. The raisins and pistachios appeared to contain the most diverse microbiomes. As the presence of any L. monocytogenes on ready-to-eat foods can potentially lead to a food recall, research regarding the survival of foodborne pathogens on LMFs is important to understand the environmental factors underlying pathogens survival and is also important for predictive modeling used in health risk assessments.

This work was supported by the ILSI North America Food Microbiology Committee.

 

Poster: Survival of Listeria monocytogenes on Pistachios, Corn Flakes and Chocolate Liquor at 4 and 23°C
Thursday, 25 April, 2019
Vivian Ly
University of Guelph

Introduction: Low-moisture foods (LMFs) such as dried fruits, cereals and confections, are characterized by water activity (aw) below 0.85 and have been recognized as being vehicles for foodborne illness. Although the growth of bacterial pathogens is inhibited by low aw, they have been shown to survive/persist for long periods of time in some LMFs. This presents a public health concern, especially when LMFs are consumed without undergoing any microbial inactivation steps. Purpose: The main purpose of this study is to assess the survival of Listeria monocytogenes on artificially-inoculated LMFs (dry-roasted shelled pistachios, chocolate liquor and corn flakes). Methods: Foods were inoculated with a four-strain cocktail of L. monocytogenes at an initial concentration of 8 log CFU/g by wet inoculation (pistachios) or misting (chocolate liquor, corn flakes). LMFs were then dried at 30°C, equilibrated, and stored at either 23°C, 30-35% relative humidity (RH) or 4°C, 55-81% RH for up to one year. Bacterial enumerations were done on tryptic soy agar with 0.6% (w/v) yeast extract, with the exception of pistachios, for which Oxford agar was used due to interfering background microbiota. Analysis of significant populations was determined by using a two-way repeated measures ANOVA. Results: During the initial drying/equilibrium period, populations of L. monocytogenes declined by 1.2 to 1.9 log CFU/g on LMFs. Populations declined significantly (P≤0.05) by 2.7, 5.3, and 6.4 log CFU/g on dry-roasted pistachios, corn flakes, and chocolate liquor stored at 23°C, respectively. At 4°C, L. monocytogenes populations remained stable on LMFs throughout the storage period. Significance: As the presence of any L. monocytogenes on ready-to-eat foods can lead to food recalls, research regarding foodborne pathogens on LMFs is very important to understand the environmental mechanisms underlying pathogens survival and also has great relevance for predictive modeling used in microbial health risk assessments.

This work was supported by the ILSI North America Food Microbiology Committee.

Learn more about IAFP’s European Symposium on Food Safety.