Effects of Temperature, Water Activity, and Structure on Thermal Resistance of Salmonella in Dates and Date Paste

Introduction: Various low-moisture products have been implicated in salmonellosis outbreaks and related recalls. However, few studies have addressed Salmonella reduction in dried fruits, even though it has been shown to be present and able to survive long periods in such products. Additionally, the effect of structure change (whole fruits vs. fruit paste) on Salmonella thermal resistance is not yet well studied.

Purpose: The objective was to quantify the thermal resistance of Salmonella on dates and in date paste during heat treatment at different water activities (a_w).

Methods: Date surfaces and pitted dates (later processed into paste) were inoculated with Salmonella Enteritidis PT30 and equilibrated in controlled-humidity chambers to 0.25, 0.45, or 0.65 a_w. Samples (~1.7 g) were treated isothermally (in triplicate) in sealed containers in a water bath (70, 75, or 80°C) for defined periods. Salmonella survivors were recovered on modified trypticase soy agar, incubated for 48 h, and enumerated. D-values were determined from linear regression of the survivor curves for each treatment.

Results: D-values decreased (P < 0.05) with increasing temperature (8.5, 2.8, and 1.1 min, respectively) at 0.25 a_w. At 80°C, D-values were not affected (P > 0.05) by a_w (1.1, 1.3, and 1.0 min, respectively). D-values for Salmonella were greater in date paste than on date surfaces (P < 0.05) at 0.45 a_w and 80°C (3.4 and 1.3 min, respectively).

Significance: In most food products, a_w plays a significant role in the thermal resistance of Salmonella, but this appeared less true for dates. Other compositional factors may be more important than a_w in this product type. Future studies should continue exploring the effects of these factors to ensure reliable thermal resistance data for process design and validation.