Inoculation methods in pathogen inactivation studies ideally represent conditions that might occur in real-world scenarios. Surface contamination in or on low-moisture foods affects Salmonella thermal resistance, which is critically important for process validation applications. The objective of this study was to quantify the effect of inoculation protocol on the thermal resistance of Salmonella Enteritidis PT 30 in fabricated low-moisture foods. Almond meal, almond butter, wheat meal, wheat flour, and date paste were inoculated via prefabrication and postfabrication protocols. In the prefabrication protocol, kernels and fruits were surface inoculated and equilibrated to a target water activity (a) (0.40 for almond and wheat products, 0.45 for date products) before fabricating meal, butter, flour, or paste and then reequilibrating the samples to the target a. In the postfabrication protocol, meal, butter, flour, and paste were fabricated before inoculation and equilibration. All inoculated and equilibrated samples were subjected to isothermal treatment (80°C), pulled sequentially during processing, cooled, serially diluted, and plated to enumerate survivors. Log-linear and Weibull-type models were fit to the Salmonella survivor data and were compared via the corrected Akaike information criterion. Pre- and postfabrication protocols resulted in significant differences ( P < 0.05) in Salmonella thermal resistance in all products. Overall, the thermal resistance of Salmonella Enteritidis PT 30 in almond products was greater ( P < 0.05) than in wheat products, which was also greater ( P < 0.05) than in date paste. Additionally, Salmonella was more thermally resistant in almond products and date paste when inoculated pre- rather than postfabrication; however, the opposite was true for wheat products. These results indicate that the means of inoculation can significantly affect thermal resistance of Salmonella in low-moisture foods.

Salmonella survival and thermal resistance on the surface of almond kernels were evaluated after periods of storage. Almond kernels were inoculated with Salmonella Enteritidis PT 30 and equilibrated to 0.45 water activity. Samples were separated into two groups (I and II) and stored in sealed metal cans at room temperature. Group I samples (stored 7, 15, 27, and 68 weeks) were re-equilibrated in controlled humidity chambers to 0.45 water activity before performing the thermal treatments after each storage period, but group II samples (stored 70 and 103 weeks) were thermally treated immediately after the cans were opened. For thermal treatments, individual almond kernels were vacuum sealed in thin plastic bags, heated isothermally in a water bath (80°C) for nine intervals, immediately cooled in an ice bath, and assayed for surviving Salmonella. Log-linear and Weibull models were fit to the inactivation data. Salmonella population decreased (P &lt; 0.05) more than 2 log CFU/g during the long-term storage. Salmonella survival in group II at 70 weeks (7.3 log CFU/g) was higher (P &lt; 0.05) than in group I (which had been re-equilibrated multiple times) at 68 weeks (6.2 log CFU/g). However, the thermal resistance of Salmonella Enteritidis PT 30 did not decrease (P &gt; 0.05) for up to 68 weeks of storage, and the log-linear model best described the thermal inactivation data. Overall, the results suggest that re-equilibrating almonds (group I) multiple times may have increased the rate of reduction of Salmonella populations during long-term storage. However, Salmonella thermal resistance on almonds appears to be essentially unaffected by long-term storage, which is important information for designing and conducting validation studies for pathogen control processes.

Recent outbreaks and recalls of low-moisture foods contaminated with Salmonella have been recognized as a major public health risk that demands the development of new Salmonella mitigation strategies and technologies. This study aimed to assess the efficacy of X-ray irradiation for inactivating Salmonella on or in almonds (kernels, meal, butter), dates (whole fruit, paste), and wheat (kernels, flour) at various water activities (aw) and storage periods. The raw materials were inoculated with Salmonella Enteritidis PT30, conditioned to 0.25, 0.45, and 0.65 aw in a humidity-controlled chamber, processed to various fabricated products, and reconditioned to the desired aw before treatment. In a storage study, inoculated almond kernels were stored in sealed tin cans for 7, 15, 27, and 103 weeks, irradiated with X ray (0.5 to 11 kGy, targeting up to a ∼2.5-log reduction) at the end of each storage period, and plated for Salmonella survivors to determine the efficacy of irradiation in terms of D10-value (dose required to reduce 90% of the population). Salmonella was least resistant (D10-value = 0.378 kGy) on the surface of almond kernels at 0.25 aw and most resistant (D10-value = 2.34 kGy) on the surface of dates at 0.45 aw. The Salmonella D10-value was 61% lower in date paste than on whole date fruit. Storage of almonds generally had no effect on the irradiation resistance of Salmonella over 103 weeks. Overall, these results indicate that product structure (whole, meals, powder, or paste), water activity (0.25 to 0.65 aw), and storage period (0 to 103 weeks) should be considered when determining the efficacy of X-ray irradiation for inactivating Salmonella in various low-water-activity foods.

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