Effect of Almond Product Structure on X-ray Inactivation Kinetics of Salmonella

Introduction: Microbial contamination of low-moisture foods is a difficult food safety challenge, reflected in several nationwide Salmonella outbreaks and/or recalls involving different types of nuts and various other dry ingredients. Therefore, effective processing interventions that minimally impact end-product quality are needed to reduce the risk of Salmonella.

Purpose: The purpose of this study was to assess the efficacy of X-ray irradiation for inactivation of Salmonella on three structurally different almond products - whole kernels, almond meal, and almond butter under the same environmental conditions.

Methods: Almond kernels were inoculated with Salmonella Enteritidis PT30 and processed into meal and butter using a blender, after which the three structurally different products were conditioned in a controlled-humidity chamber. Each sample was bagged inside the chamber and irradiated at two dose levels using an x-ray food irradiator at 70 kV (Rayfresh, Ann Arbor, MI). Thereafter, the irradiated samples were stomached, serially diluted, and plated on modified trypticase soy agar to enumerate survivors. Data from three replicates were plotted with the D-values determined by linear regression.

Results: After 4 weeks of storage, almond kernel, meal, and butter yielded Salmonella populations of 7.22, 7.54, and 7.61 CFU/g which were less than 1 log below the initial inoculation level. The water activity at the time of irradiation was 0.333, 0.339, and 0.315, and the radiation D-value was 0.430 (R²=0.92), 0.363 (R²=0.97), and 0.318 (R²=0.98) kGy for almond kernels, meal, and butter, respectively.

Significance: X-ray irradiation was an effective means to inactivate Salmonella Enteritidis PT30 in/on various almond products. However, the process needs to be calibrated for different types of products, given that the efficacy can be affected by structural differences, even at identical composition and water activity.