Isothermal Inactivation of Salmonella and Enterococcus faecium in Dates Impacted by Water Activity Variation at Elevated Temperature

Introduction: Temperature and water activity (a_w) are highly affecting the thermal inactivation efficiency of microorganisms in low-moisture foods; however, the influence of a_w at elevated temperatures on thermal resistance of Salmonella and its surrogate Enterococcus faecium (E. faecium) in dates is not yet available.

Purpose: (1) To compare thermal resistance of E. faecium with a Salmonella Enteritidis PT30 (Salmonella PT30) on date surfaces in the same thermal treatments, and (2) to understand the relationship between temperature-induced changes in a_w and thermal resistance of Salmonella.

Methods: Dates were inoculated with E. faecium or Salmonella PT30 and equilibrated to different a_{w, 25°C} (0.30, 0.45 and 0.60±0.02) to establish 8.5±0.1 CFU/piece. The samples were vacuum-sealed in thin plastic bags, heated isothermally at 75, 80 and 85°C, cooled after treatment, and enumerated to obtain D-values. Equilibrium water sorption isotherms (moisture content vs. a_w) for dates at 20 to 80ºC were generated using a vapor sorption analyzer and a thermal cell with relative humidity sensor, and compared using ANOVA.

Results: E. faecium showed equal or higher thermal resistance than Salmonella PT30 at all a_w levels (e.g., at a_w,25°C=0.45±0.02, 80°C, D_{E. faecium}=4.22±0.72 min, D_{Salmonella PT30} =2.60±0.42 min). A flat isotherm curve was observed for dates below a_w,25°C=0.5, indicating constant moisture content and same a_w at elevated temperatures. It may explain the statistically same D_85°C with different initial a_w (e.g. D_{E. faecium, aw,25°C=0.30}=1.64±0.86 min, and D_{E. faecium, aw,25°C=0.45}=1.60±0.80 min).

Significance:  Improved understanding of the relationship between temperature-induced changes in a_w of dates and the thermal resistance of Salmonella and E. faecium is critically important in designing and validating thermal processing for ensuring dates' safety.