Apple Drying Process Lethality Prediction Using Response Surface Methodology

This study evaluated the effects of drying temperature, bed depth, and airflow on Salmonella lethality achieved during hot-air drying of inoculated apple cubes to a fixed water activity level, using a three-factor, three-level Box-Behnken design. Apples were dried under 15 conditions, and Salmonella inactivation was estimated at a product water activity (a_w) of 0.60, a level representative of a relatively higher-a_w dried apple product. Across all trials, Salmonella inactivation correlated linearly (R² = 0.91-0.97) with a_w reduction, allowing a_w to be used as a practical proxy for drying process lethality estimation. Estimated Salmonella inactivation at a_w 0.60 ranged from 2.25 ± 0.11 to 4.97 ± 0.21 log CFU/4 cubes. A refined response surface methodology (RSM) model captured the significant linear effects of temperature, airflow, and bed depth, and a marginal interaction between airflow and bed depth. Higher temperature and airflow enhanced lethality, while a deeper bed reduced lethality. However, the positive effect of airflow diminished at greater bed depths. Model validation under three randomly selected drying conditions showed strong agreement between model predictions and experimental observations, with a low root mean square error of prediction (0.371 log CFU/4 cubes), an accuracy factor of 1.101, and a bias factor of 1.073 (7.3% overprediction). These results demonstrate that water activity reduction can be used as a reliable indicator of microbial lethality in hot-air apple drying and that the developed RSM model provided a robust tool to predict Salmonella inactivation across practical processing conditions.