Growth and Survival of Enterobacteriaceae and Inoculated Salmonella on Walnut Hulls and Maturing Walnut Fruit

Abstract

Postharvest contamination of in-shell walnuts may occur when the fruit is dropped to or harvested from the orchard floor or as the outer hull is removed with mechanical abrasion and water. To evaluate the effect of maturity on the potential for microbial contamination, 'Howard' walnut fruits were collected weekly from the tree canopy, from 6 to 7 weeks before to 1 week after typical commercial harvest. The numbers of microorganisms able to form colonies on plate count agar, MacConkey agar (presumptive Enterobacteriaceae), or violet red bile lactose agar (presumptive coliforms) were compared on whole walnut fruits collected by hand directly from the tree or after exposure to the orchard floor for 10 min or 24 h. Salmonella Enteritidis PT 30 was inoculated at <1 to 8 log CFU/g onto 5-g hull pieces (from walnut fruit of different maturities) and stored at ambient temperature (23 to 26°C) in unsealed bags (38 to 90% relative humidity [RH] within bag) or in low humidity (20 to 45% RH) or high humidity (68 to 89% RH) for up to 14 days. Salmonella at 2 or 5 log CFU/ml was inoculated onto hulls before or up to 14 days after blending with water. As the walnut fruit matured, the indigenous bacterial levels on the surface increased, irrespective of whether fruit was collected from the tree or the ground. The RH influenced the growth of inoculated bacteria on hull pieces: Salmonella declined to <0.3 log CFU/g within 24 h at low RH but multiplied from 2 to 6 log CFU/g over 14 days of storage at >40% RH. Salmonella populations declined to <1 CFU/ml within 24 h in freshly blended green hulls but survived or multiplied in blended brown hulls or in blended green hulls that had been stored for 24 h or more before being inoculated.