Purpose: The objective was to quantify the effect of product structure on thermal resistance of Salmonella Enteritidis PT30 inoculated onto whole almonds subsequently ground into almond meal and almond butter.
Methods: Almonds were inoculated with Salmonella Enteritidis PT30 (~108 CFU/g) and equilibrated to ~0.4 aw. After equilibration, almonds (100 g) were ground in a food processor (45 s) to produce an almond meal sized between U.S. standard sieves #20 and 80. Almond butter was produced by further milling almonds (200 g) for 15 min, with dry ice added every 2 min to control product temperature (<40°C). All products were re-equilibrated to ~0.4 aw. The inoculated almonds were individually vacuum-packed in thin layer plastic bags, and meal and butter samples (thickness < 1 mm) were packed in aluminum test cells. Samples were heated in an isothermal water bath (~80°C), with almonds pulled every 10 min for 1.25 h, and meal and butter pulled every 15 min for 2.5 h; all were cooled immediately in an ice bath, diluted in peptone water, and plated on modified TSA to enumerate survivors.
Results: Initial Salmonella populations and sample water activities were not significantly different (P > 0.05) after grinding and milling. However, D80°C values, determined by linear regression of the Salmonella survivor curves, were greater (P< 0.05) in almond meal (60.7 min) and almond butter (66.0 min) than on the whole almonds (19.1 min).
Significance: Changing the product structure, given equivalent aw and composition, significantly impacted Salmonella thermal resistance. Therefore, it is extremely important to use product-specific inactivation parameters when validating pasteurization processes.