Effect of Product Structure and Water Activity on X-ray Inactivation of Salmonella in Low-water Activity Foods

Introduction: Salmonella contamination of low-moisture foods is a socioeconomically important issue evidenced by the multiple outbreaks and recalls in recent years. Thus, it is critical to develop effective mitigation strategies to significantly reduce foodborne pathogens while maintaining the integrity of the food product.

Purpose: The purpose of this study was to assess the efficacy of X-ray irradiation in inactivating Salmonella in almonds (kernels, meal, butter), dates (whole fruit, paste), and wheat (kernels, flour) at various water activities (a_w).

Methods: Almond kernels were inoculated with Salmonella Enteritidis PT30, conditioned to 0.25, 0.45, or 0.65 a_w, and ground into coarse meal. Dates were spot inoculated on the surface of the whole date and conditioned to the target a_w. Bagged samples (~1-4 g) were irradiated utilizing a 70 kV X-ray irradiator at doses targeting 1-5 log reductions. Samples were then stomached, diluted, and plated on modified tryptic soy agar, and the survival counts were used to determine radiation D₁₀-values by linear regression.

Results: D₁₀-values for whole dates (2.17 and 2.03 kGy at 0.45 and 0.65 a_w, respectively) were unexpectedly insensitive to a_w (P = 0.64). However, the D₁₀-values for almond products were significantly (P < 0.05) lower than for the dates (e.g., 0.51 kGy for 0.45 a_w almonds and 0.74 kGy for 0.65 a_w almond meal).

Significance: As a nonthermal intervention technology, X-ray irradiation is a viable tool that can effectively reduce bacterial population in low-moisture food products without significant impact on overall food quality, as shown in a prior study.  However, the effects of product structure and a_w are complex and probably need to be considered on a case-by-case basis.