The Effect of Process Air Velocity, Humidity, and Product Moisture on Salmonella Inactivation on Almonds

Introduction:  Understanding microbial inactivation is necessary to develop robust models that can be used to validate processes. Most modeling efforts in low-moisture foods have been based on data from iso-conditions or assumed that relevant factors remain constant. However, industrial processes are highly dynamic, which can significantly affect inactivation response.

Purpose: The objective was to evaluate the effects of process air velocity on the inactivation response of Salmonella on almonds.

Methods: Inoculated almonds with Salmonella Enteritidis PT30 at seven to eight log CFU/g were equilibrated to water activity (a_w) of 0.38 or 0.65 (2.8 and 6.2% moisture content, dry basis, respectively). Samples (13 g) were processed at two air velocities (i.e., high and low) and two process humidities (<3% Mv and 30% Mv), at 121°C. Dynamic product surface temperature, moisture content, process dew point, and a_w were recorded. Inactivation data were used to estimate parameters of a Bigelow-type log-linear model, via nonlinear regression of the integrated form, including terms for temperature (zT), process humidity (zM), and air velocity (aV).

Results:  Models were fitted with and without the aV term. Model errors (RMSE) ranged from 0.51 to 0.62 log CFU/g. Distinct parameters were obtained for each model; the aV parameter (-0.39) indicated that air velocity had a large effect on microbial inactivation, independent of the T(t) effect. Analysis of residuals indicated there also is an effect of product moisture content (P<0.05), of which the best model form is still unclear.

Significance: Overall, the results suggest that air velocity impacts the resistance of Salmonella in thermal inactivation processes, independently of temperature effects, likely reflecting velocity effects on the relative influence of process humidity and product moisture.