Purpose: The purpose of this study was to quantify Salmonella survival during long-term almond storage, and the subsequent effect on thermal and x-ray resistance.
Methods: Almonds were inoculated with Salmonella PT30 and conditioned to 0.45 aw. For thermal treatment, individual almonds were vacuum-packed in plastic bags, heated in an isothermal water bath (~80°C), pulled at time steps, and cooled in ice baths. For x-ray treatment, four gram samples were irradiated utilizing a 70 kV x-ray irradiator at doses targeting one- to five-log reductions. Samples for both treatments were enumerated to determine initial inoculation and D10-values. The remaining almonds were stored in airtight canisters. Almonds were sampled, equilibrated to 0.45 aw, treated, and enumerated after 0, 7, 15, 27, and 66 weeks of storage.
Results: The initial inoculum level in the almonds (8.53±0.15-log CFU/g) decreased (P<0.05) after 27 weeks (7.80±0.11-log CFU/g). However, thermal resistance of Salmonella was not significantly (P>0.05) different after 27 weeks (D80°C ~23.8±2.2 min). In contrast, the resistance to 1.31 kGy of x-ray irradiation was increased (P<0.05) from week 0 (2.97±0.342-log reductions) to week 27 (1.24±0.359-log reductions).
Significance: The population of Salmonella in low-moisture food decreased over six months. Although Salmonella thermal resistance did not change during this period, irradiation resistance increased significantly. These findings suggest mechanism-specific effects of storage on subsequent resistance to lethal treatments.