The water activity, pH and other compositional parameters such as the moisture, fat and NaCl contents of various food emulsions were determined: butters, margarine (W/0) and mayonnaises (O/W). Water activity was determined by a gravimetric method (PEC procedure) and an instrumental method (dew-point hygrometer). The mean a(w) values obtained for the samples assayed were 0.904 +/- 0.050 (butters), 0.914 +/- 0.028 (margarines) and 0.947 +/- 0.013 (mayonnaises). The a(w) of the butter and margarine samples could be predicted with an error less than 0.02 units from the NaCl content of their aqueous phase (m) by using the following simple linear regression equation: a(w) = 0.954 - 0.036m. This equation could not be used to predict the a(w) of the mayonnaises, which, however, were obtained by using the Chen equation for mixtures of solutes.

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.