In-package atmospheric cold plasma inactivation of Salmonella in freeze-dried pet foods: Effect of inoculum population, water activity, and storage

Abstract

Recently, Salmonella has been linked to numerous low water activity (aw) food recalls. As an emerging technology, atmospheric cold plasma (ACP) has the potential to reduce Salmonella in low aw foods. The objective of this study was to evaluate the effects of initial inoculum population of Salmonella (~ 10, 9 and 8 log CFU/cm2), treatment time (2.5, 5, 7.5 and 10 min), water activity (0.13, 0.34, 0.54), and post-treatment storage (0, 7 and 14 days at 21 °C) on the reduction of cocktails of S. Typhimurium and S. Senftenberg on freeze-dried pet food treats. The top surface of pet food was spot inoculated with cocktails of Salmonella with initial cell populations of 10, 9 and 8 log CFU/cm2. Inoculated samples were first equilibrated to target aw values, packaged with modified atmosphere (78% N2 + 21% O2 + 1% CO2), and then in-package ACP treatments were performed. The initial inoculum population, treatment time, and post-treatment storage had significant (P < 0.05) effects on Salmonella inactivation efficacy of ACP. Regardless of the initial inoculum population and storage time, Salmonella equilibrated to 0.13 aw pet food exhibited the greatest resistant to ACP treatment. ACP treatment for 10 min followed by 7 days storage reduced the Salmonella cells in 0.54 aw pet food, to below the limit of detection (>4.5 log reduction), when the initial inoculum population was 8.2 log CFU/cm2. Regardless of aw of petfood, peroxide and thiobarbituric acid reactive substance values of 10 min ACP-treated pet foods were significantly increased to 3.44 meq/ kg of fat and 1.53 mg MDA/ kg pet food from 0.87 meq/ kg of fat and 0.50 mg MDA/ kg pet food, respectively. Post-ACP treatment storage of 10 min ACP-treated samples for 14 days further increased the lipid oxidation to 2.58 from 1.53 mg MDA/ kg of pet food, irrespective of the aw of samples. This study showed that the Salmonella reduction efficacy of in-package ACP treatment in low aw environment is influenced by various process and product factors. Industrial relevanceFreeze drying is not considered a kill step to inactivate microorganisms on foods. In the future, in-package ACP technology could be used as a potential method for post-packaging decontamination of freeze-dried pet foods in industrial settings after obtaining necessary regulatory approvals.