Purpose: This study investigated the impacts of oven relative humidity (RH) and matrix composition on the inactivation of Salmonella during baking.
Methods: Model high-protein and high-fat foods, formulated with wheat flour, soy protein, and soy oil, were inoculated with Salmonella enterica serovar Agona 447967 to a level of ~ nine log CFU/g, and mixed to form a homogenous dough. Dough samples (57 mm diameter by 6 mm thick) were baked (three samples per dwell time, six dwell times per condition) in a laboratory-scale oven at 120°C (10% RH) and 85°C (20%, 35% RH, 50% RH), respectively. Temperature and water activity (aw) were measured at the surface and geometric center of the product during baking. Processed samples were collected in sterile bags and immediately cooled in an ice-water bath. Salmonella was enumerated on trypticase soy agar supplemented with yeast extract and incubated at 37°C for 24 h.
Results: Reductions of 5.12±023, 5.11±0.36, 4.55±0.31, and 4.78±0.51 log CFU/g were achieved after 40 min at 120°C/10% RH, 90 min at 85°C/20% RH, 50 min at 85°C/35% RH, 8 min at 85°C/50% RH, respectively, in the high-protein model food. Similar results were achieved in the high-fat matrix. The aw at the geometric center of the product (initially at aw =0.98) did not change appreciably during baking. However, aw at the product surface, the location of least lethality, decreased significantly (P<0.05) during baking; the decreases were more pronounced at the lowest oven RH.
Significance: Surface drying caused by low oven RH conditions increased Salmonella survival in food products. Therefore, both oven temperature and RH should be monitored during baking process.