Purpose: The objective was to quantify the effect of product temperature, product moisture, and process humidity on Salmonella lethality on in-shell pistachios.
Methods: In-shell pistachios were inoculated with Salmonella Enteritidis PT30 (~8.5 log CFU/g), equilibrated in controlled-humidity chambers (0.45 or 0.65 aw), and, in some cases, exposed to a pure-water or 27% NaCl brining treatment for 30 s (0.95 and 0.75 aw, respectively) prior to thermal treatment. Samples (15 g) were heated in a computer-controlled, laboratory-scale, moist-air convection oven, following a full-factorial experimental design (in duplicate) with process temperatures of 104.4 and 118.3ºC, process humidities of ~3, 15, and 30% v/v (corresponding to dew points of ~23.8, 54.4, and 69.4ºC, respectively), and air speed of 1.3 m/s. Salmonella survivors, moisture content, and aw were quantified at six time points during each treatment, targeting cumulative lethality of ~3 to 5 log. Inactivation rates were modeled as a function of time, product temperature, product moisture, and process dewpoint.
Results: Increasing product temperature or process dewpoint increased Salmonella inactivation rates (P<0.05). For unbrined and brined treatments, analyzed separately, initial product aw did not affect inactivation rates (P>0.05). However, when comparing unbrined against brined treatments, inactivation rates were greater (P<0.05) for the brined pistachios.
Significance: Product and process moisture both appear to impact pathogen reduction in low-moisture products, and this project quantifies those impacts for Salmonella inactivation in pistachios. Considering these factors is critically important when designing and validating thermal processes as pathogen-reduction steps.