ABSTRACTQUANTIFYING WATER EFFECTS ON THERMAL INACTIVATION OF SALMONELLA IN LOW-MOISTURE FOODSBy Francisco Javier Garcés-Vega Thermal processing is the most used technology to control pathogens in the food supply. However, thermal processing of low moisture foods (LMF) faces challenges, given the enhanced thermal resistance of Salmonella.Product water is recognized as a controlling factor in thermal inactivation of Salmonella in/on LMF, such as almonds. Water activity (aw) describes the state of water; however, aw is temperature dependent and characterized by hysteresis between sorption states. Moisture content (%MC) describes the amount of water in a product; it is not temperature dependent, and might be a more convenient metric than aw to account for water in thermal inactivation processes.Food products, and microorganisms in/on those products, are influenced by process conditions involving heat and mass transfer phenomenon. Process temperature and humidity typically are significant factors in these inactivation systems. The effect of process humidity was recently quantified and reported, but information about the effect of process air velocity on microbial inactivation processes for LMF is very limited.The goal of this study was to improve the understanding of the role of water in thermal inactivation of bacteria in LMF. The specific objectives were to: (1) evaluate the relationships of two water metrics with thermal resistance of Salmonella on LMF; (2) describe and quantify the interactive effects between product water, process humidity, and air velocity on the thermal resistance of Salmonella on LMF, and (3) propose and evaluate a model to describe the effects of air velocity, aw, and/or %MC on the thermal resistance of Salmonella on LMF.Inoculated almonds were equilibrated to two moisture (%MC) levels but the same aw, and two aw levels but the same %MC. Equilibrated products were vacuum packaged and thermally treated in a water bath at 80°C. Survivors were recovered and enumerated, the resulting inactivation curves were used to fit the log-linear inactivation model, and the inactivation kinetics were compared. D values ranged from 15.7 to 18.0 min, and the RMSE was 0.25 to 0.69 log CFU/g. No differentiated effect attributable preferentially to aw or %MC was seen in the inactivation kinetics (P > 0.05), likely because of the variability of the parameters. However, there are other effects of the two water metrics that should be further studied.To assess the effect of process condition, inoculated almonds were equilibrated at 25 and 65% RH. After equilibration, samples at each moisture content were treated in a laboratory-scale convection oven at four different conditions (121°C, 2 air velocities, and 2 humidities (~3 and 30% moisture by volume)), for 7 durations in triplicate. Survivors were recovered and enumerated. The resulting 24 inactivation curves were used to globally estimate the parameters of six inactivation models. A model incorporating temperature, process humidity, and air velocity performed best, with a RMSE of 0.51 log N/N0.The separate effects of aw and %MC on the inactivation kinetics of Salmonella in LMF remain inconclusive. Further analysis is needed to identify which metric is best for modeling and validating thermal inactivation processes. However, the effect of air velocity was significant, indicating a velocity effect independent of the influence on heating rate, due to the relative impact of product and process moisture on bacterial inactivation. Validation studies with other products will be important to further test the magnitude of the impact of aw, %MC, and air velocity on thermal inactivation processes for low-moisture foods.
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