Evaluation of Hot-Air Drying To Inactivate Salmonella and Enterococcus faecium on Apple Pieces

Hot-air drying processes are used to provide specific quality attributes to products, such as dehydrated apple pieces. To comply with the U.S. Food and Drug Administration Food Safety Modernization Act, there is a need to understand microbial lethality during these processes. The objective of this study was to determine the level of inactivation provided by hot-air drying on a Salmonella cocktail inoculated onto apple cubes and to evaluate the performance of Enterococcus faecium as a surrogate. A cocktail of Salmonella serovars (Agona, Tennessee, Montevideo, Mbandaka, and Reading) and E. faecium were individually inoculated onto cored, peeled Gala apple cubes at 9.2 ± 0.3 and 8.8 ± 0.1 log CFU per sample, respectively. Apple cubes were dried at 104 or 135°C in ∼1.5-kg batches using a hot-air dryer with a vertically directed heat source and without mixing. Three subsamples, consisting of four inoculated cubes, were enumerated at each time point (n ≥ 5) from multiple product bed depths. Water activity decreased throughout the duration of the study, with samples drying faster at 135 than 104°C. Samples at the bottom bed depth, closer to the heat source, dried faster than those at the higher bed depth, regardless of temperature. Significant microbial inactivation was not seen immediately. It took >10 min at the bottom bed depth or >40 min of drying at the top bed depth, regardless of temperature (P < 0.05). By the end of drying, average Salmonella inactivation of greater than 5 log CFU per sample was achieved. At temperature conditions evaluated, E. faecium inactivation was slower than Salmonella, indicating that it would likely serve as a good surrogate for in-plant validation studies. Case hardening did not inhibit microbial inactivation in the conditions tested. Hot-air drying under the conditions evaluated may provide a preventive control in the production of dehydrated products, such as apples.