Multiple outbreaks of foodborne illness (Salmonella and E. coli O157:H7) have been associated with tree nuts and tree nut products since the early 2000s. In response to these outbreaks and many others in fresh produce and peanut butter, Congress created and passed the Food Safety Modernization Act (FSMA) to require a preventive, rather than a reactive, approach to improve food safety from production to consumer purchase. One outcome of FSMA, the Preventive Controls Rule, requires processors to implement strategies to reduce microbiological contamination. The most prudent approach for a processor is the implementation of a validated process proven to achieve a desired level of decontamination. Fumigation of tree nuts with propylene oxide (PPO) is a potential process that could be used to reduce contamination of Salmonella spp. The purpose of this study was to determine the efficacy of PPO at reducing Salmonella spp. on raw hazelnuts (in-shell and kernels) in commercial processing facilities.
Two commercial-scale trials were conducted to determine the efficacy PPO at two different concentrations: 0.65 and 1.00 kg/m³. In-shell hazelnuts and kernels were inoculated with a cocktail of five Salmonella strains previously associated with tree nuts or Enterococcus faecium ATCC 3459. Inoculated samples (25-50 g) were strategically placed within commercial packaging containing bulk hazelnuts and transported to commercial PPO facilities for fumigation and off-gassing (5 days). Samples were transported to the laboratory and analyzed for Salmonella survivors using standard serial dilution and spread plating techniques on Hektoen Enteric (HEK) with incubation at 37°C for 24-48 hrs. Samples with low levels of survivors were analyzed using a most probable number (MPN) technique. Salmonella spp. concentrations in travel controls were compared to surviving concentrations in PPO and PPO+off-gassing samples to determine efficacy of treatments.
A PPO treatment of 0.65 kg/m³ with off-gassing achieved average reductions of 3.67 and 4.83 log CFU/g for kernel and in-shell hazelnuts, respectively. Increasing the PPO concentration to 1.00 kg/m³ increased the average reductions of Salmonella spp. to 5.66 and 6.51 log CFU/g, respectively. PPO processing variables other than PPO concentration that impact PPO efficacy were discovered. Results in the second study indicated potential development of gas layers (PPO and air/nitrogen) in the chamber that likely contributed to variability in Salmonella spp. survivors due to three-dimensional location. The degree of chamber fill may also impact the efficacy of PPO. PPO has the potential to be a viable solution for the hazelnut industry, however, additional commercial-scale processing variables (gas dispersion, chamber fill, nut type and form, packaging) need to be further examined to establish consistent and effective PPO processes.
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