Summary
In the past decade, nuts and nut products have been established as potential source of foodborne illness; outbreaks associated with consumption of raw almonds, inshell hazelnuts, peanut butter, pine nuts, and walnuts have been documented in the North America and Australia. Until recently, very little was known about the ecology of foodborne pathogens in nut production and processing environments impeding the development of targeted commodity-specific intervention programs. The proposed research builds on recent research in this laboratory pertaining to the survival of Salmonella in pistachios. Points during postharvest handling of pistachios where foodborne pathogens may be reduced, controlled or amplified will be identified. These data and industry expert opinion will be used to construct a pistachio risk model to estimate risks from harvest to storage; this model will allow the pistachio industry to develop harvest management strategies that reduce the potential for product contamination. Characterization of the heat resistance of foodborne pathogens in inoculated pistachios under dry and moist heat conditions will provide the scientific foundation for process validation in the pistachio industry.
Technical Abstract
Nuts and other low-moisture foods have generally been considered low-risks for foodborne illness because they are consumed in a dry state where water activity (available moisture) is too low to support microbial growth. However, it is increasingly recognized that many foodborne pathogens can cause illness at very low concentrations, such that microbial growth is not required. In the past decade, outbreaks of salmonellosis and E. coli O157:H7 gastroenteritis associated with consumption of a range of low moisture foods including several tree nuts have been documented in North America and Australia. The proposed research builds on previous research on the ecology of Salmonella in the pistachio postharvest environment. This proposal will, through laboratory studies, identify points during postharvest handling of pistachios where foodborne pathogens may be reduced, controlled or amplified. Emphasis will be placed on understanding the behavior of inoculated Salmonella, E. coli O157:H7, and L. monocytogenes during simulated postharvest holding (prehulling), post hulling holding, and drying. These data and industry expert opinion will be used to construct a pistachio risk model to estimate risks from harvest to storage; this model will allow the pistachio industry to develop harvest management strategies that reduce the potential for product contamination. Characterization of the heat resistance of Salmonella, E. coli O157:H7, L. monocytogenes, and E. faecium in inoculated pistachios under dry and moist heat conditions will provide the scientific foundation for process validation in the pistachio industry.
Research Objectives
Objective 1. To identify points during postharvest handling of pistachios where foodborne pathogens (Salmonella, E. coli O157:H7 and L. monocytogenes) may be reduced, controlled or amplified.
Objective 1a. To evaluate growth or survival of foodborne pathogens on in-hull pistachios after harvest including the impact of pathogen type, temperature of exposure, harvest container (bins or trailers), time of harvest (first or second shake), and posthuller stream (floaters and sinkers).
Objective 1b. To evaluate growth or survival of foodborne pathogens on pre-dryer hulled pistachios including impact of pathogen type and post float tank stream (floater vs sinker).
Objective 1c. To evaluate the reduction of foodborne pathogens during drying under a range of drying times, temperatures and target moisture levels used by the pistachio industry.
Objective 1d. To develop a quantitative risk model to assess the parameters between the point where pistachios are shaken from the tree to the point that dehydration in the silo is complete.
Objective 2. To determine the impact of pistachio moisture and nut form (kernel or inshell) on the heat sensitivity of Salmonella, E. coli O157:H7, L. monocytogenes, and E. faecium inoculated pistachios.
Findings & Recommendations
The data generated in this study support Salmonella as the appropriate target pathogen for pistachios and provide strong evidence that thermal processes validated for this organism will reduce E. coli O157:H7 and L. monocytogenes by similar or greater levels. The data support the use of Enterococcus faecium NRRL 2354 as a surrogate organism for validating thermal processes in reducing Salmonella on pistachios. A model developed with industry input and data generated in this study predicted that delays between harvest and hulling and between hulling and drying pistachios can significantly impact levels of Salmonella in sinker and, especially, in floater pistachios. The model predicted significantly greater levels of Salmonella in floaters than in sinkers.