Scientific Study

This study investigated the effects of inoculation method, water activity (a_w), packaging method, and storage temperature and duration on the survival of Salmonella on almonds as well as their resistance to subsequent thermal treatments. Whole almond kernels were inoculated with a broth-based or agar-based growth Salmonella cocktail and conditioned to a_w of 0.52, 0.43 or 0.27. Inoculated almonds with a_w of 0.43 were treated with a previously validated treatment (4 h of dry heat at 73 °C) to determine the potential differences in heat resistance resulted from the two inoculation methods. The inoculation method did not significantly (P > 0.05) impact the thermal resistance of Salmonella. Inoculated almonds at a_w of 0.52 and 0.27 were either vacuum packaged in moisture-impermeable mylar bags or non-vacuum packaged in moisture-permeable polyethylene bags before stored at 35, 22, 4, or -18 °C for up to 28 days. At selected storage intervals, almonds were measured for a_w, analyzed for Salmonella population level, and subjected to dry heat treatment at 75 °C. Over the month-long storage of almonds, Salmonella populations remained almost unchanged (<0.2 log CFU/g) at 4 °C and -18 °C and declined slightly (<0.8 log CFU/g) at 22 °C and more substantially (1.6-2.0 log CFU/g) at 35 °C regardless of the inoculation method, packaging method, and almond a_w. When stored at 35 °C, almonds with initial a_w of 0.52 had significantly higher (P < 0.05) Salmonella reductions than those with initial a_w of 0.27. Prior storage of almonds vacuum packaged in mylar bags at temperatures between -18 °C and 35 °C for 28 days affected their a_w levels but did not significantly (P > 0.05) affect the subsequent thermal resistance of Salmonella at 75 °C regardless of almond a_w and storage duration. Salmonella on almonds with higher a_w was more sensitive to heat treatment than those with lower a_w. To achieve >5 log CFU/g reductions of Salmonella, a dry heat treatment at 75 °C for 4 and 6 h was needed for almonds with initial a_w of 0.52 and 0.27, respectively. When applying the dry heating technology for almond decontamination, the processing time needs to be determined based on initial a_w of almonds regardless of storage condition or age of almonds within the current design frame. https://doi.org/10.1016/j.fm.2023.104269