Survival of Salmonella Enteritidis PT 30 on Inoculated Almonds after Commercial Fumigation with Propylene Oxide

Abstract

Propylene oxide (PPO) is commonly used to reduce microbial populations in U.S. bulk raw almonds, but the process has not been validated for reduction of foodborne pathogens. The reduction of Salmonella Enteritidis phage type (PT) 30 inoculated onto almonds was evaluated after exposure to a standard commercial PPO treatment. Almonds were inoculated with Salmonella Enteritidis PT 30 to approximately 8.0 log CFU/g after drying. Inoculated almonds were placed in bags designed for gaseous sterilization and positioned in the center of 900-kg bins or 22.7-kg boxes of warmed almonds. Almonds were further warmed to an initial temperature of 23 to 34°C, treated with PPO (0.5 kg/m3 for 4 h), and held for 0 or 2 days at 38 to 43°C followed by storage for 2 to 5 days at 15 to 18°C. Salmonella Enteritidis PT 30 was recovered by vigorously shaking 100 g of almonds in 100 ml of Butterfield's phosphate buffer, plating onto tryptic soy or bismuth sulfite agar, and incubating at 35°C for 24 or 48 h, respectively. Populations of Salmonella Enteritidis were consistently reduced by >5.0 log CFU/g (5.2 to >8.6 log CFU/g) when initial counts were compared with counts obtained 5 days after PPO treatment. Reductions of 1.2 to 4.4 log CFU/g occurred during post-PPO storage. Reductions were not significantly improved (P < 0.05) when almonds were held at 38 to 43°C after PPO treatment. PPO residues were >400 ppm immediately after removal from the PPO chamber and declined to <300 ppm during post-PPO storage. PPO is an effective treatment for reducing populations of Salmonella Enteritidis PT 30 on bulk almonds.