Modeling the inactivation of Salmonella enterica at the droplet scale during drying in hot air flow

Spray-drying is a crucial process for ensuring the microbial safety of powdered infant formula (PIF). This study investigated the inactivation of five Salmonella enterica strains in different food matrices: whole milk, two milk-based PIFs, and one soy-based PIF, during a single droplet drying in hot air flow, mimicking spray-drying. Microbiological analyses were performed by counting the diminution of cultivable counts on plates. Using a Central Composite Design approach, experimental points were selected and the experimental design successfully allowed to capture the nonlinear interactions between drying parameters and microbial inactivation. Our results revealed that the maximum inactivation for sub-stressed cells was limited to a 2-log reduction, a value significantly lower than the 4-log reduction reported for non-stressed cells under similar conditions. A Principal Component Analysis further identified desiccation sub-stress as a key discriminating factor, statistically confirming that the physiological state of the cells outweighs the influence of the food matrix or strain diversity. This limited inactivation demonstrates a robust cross-protection effect induced by desiccation, suggesting that pre-drying conditions in industrial settings may critically enhance S. enterica resilience. These findings provide a deeper understanding of inactivation kinetics at the droplet scale, which mimics industrial spray-drying, and offer a quantitative framework for optimizing drying processes to enhance food safety.