Fate of Heat-Shocked Salmonella enterica and Escherichia coli O157:H7 Cells on Under- and Adequately Cooked Pork Gyros Trimmings During Postcooking Storage Until Serving

This study assessed the fate of heat-shocked cells (HS) of Salmonella enterica and Escherichia coli O157:H7 during subsequent postcooking storage of under- and adequately cooked pork gyros at 30 °C (under broiler) and 40 °C (in heating chambers), conditions usually applied in grill houses before serving. Frozen, semi-cooked gyros trimmings (2‒3 mm thick) were used. Undercooked gyros was derived from thawing (30 min at room temperature) (a_w 0.950 ± 0.010; pH: 6.21 ± 0.13), and adequately cooked samples followed the package instructions (microwaved at 700 W for 2 min) (a_w 0.808 ± 0.032; pH: 6.20 ± 0.09). Individual strains of pathogens were heat-shocked at 47.5 °C for 60 min; controls were non‒heat-treated cells. Afterwards, cells' heat tolerance was checked at 57 °C by estimating inactivation rates (k_max; min^-1). HS showed lower k_max (4-8 fold for Salmonella; 3-11 fold for E. coli O157:H7) than controls (p < 0.05), indicating increased heat resistance. Thereafter, HS Salmonella or E. coli O157:H7 composites were surface inoculated (3.5‒4.0 log CFU/g) on under- or adequately cooked gyros and stored at 30 and 40 °C for 6 h (n = 6) to evaluate "cross-protection" associated with a_w changes that occur during storage. Controls were inoculated with non-HS cells. In undercooked gyros at 30 °C, E. coli O157:H7 population levels (controls and HS) did not change (p ≥ 0.05) after 6 h. In adequately cooked gyros stored at 40 °C (a_w 0.808‒0.829), HS E. coli O157:H7 persisted, whereas the controls declined, resulting in 1.8 logs higher than controls (p < 0.05). Salmonella (controls and HS) remained close to initial levels at 30 °C (under- and adequately cooked) and 40 °C (adequately cooked) or increased by 0.8‒1.1 logs at 40 °C (undercooked) (p < 0.05), indicating that no additional cross-protection was induced. Such findings may emphasize that even adequately cooked gyros can permit pathogen persistence or recovery during postcooking short-term storage and until serving.