Effects of moisture content and mild heat on the ability of gaseous chlorine dioxide against Salmonella and Enterococcus faecium NRRL B-2354 on almonds

Abstract

Contamination of almonds with pathogenic Salmonella can be detrimental to consumers’ health and causes deterioration of the product. This study evaluated the effect of gaseous chlorine dioxide (ClO2) with mild heat and moisture against pathogenic Salmonella enterica and its surrogate inoculated on almonds. Almonds were inoculated with pathogenic Salmonella enterica and nonpathogenic surrogate Enterococcus faecium NRRL B-2354. A dry precursor method was used to produce gaseous ClO2 to treat almonds in an airtight desiccator chamber. A 20-g precursor dose was used to generate ClO2 to treat almonds with, (i) original moisture content (MC, 4%), (ii) increased MC (7%), and (iii) increased MC (7%) and mild heat (40 °C ± 1.5). Almonds were exposed to these treatment conditions for 1, 2, 3, and 4 h. Chlorine dioxide treatment demonstrated bacterial reductions of 1.2 log CFU/g on Salmonella at 3 h and 0.9 log CFU/g on E. faecium NRRL B-2354 at 4 h. Gas treatment with the addition of moisture on almonds resulted in bacterial log reductions of 1.8 CFU/g on Salmonella, and 1.0 CFU/g on E. faecium NRRL B-2354 at 3 h. Gaseous ClO2 with additional moisture and heat contributed the highest bacterial reduction on almonds at 3 h (2.0 CFU/g on Salmonella and 1.6 CFU/g on E. faecium NRRL B-2354). Bacterial reductions observed on Salmonella were statistically (P > 0.05) similar to E. faecium NRRL B-2354. Scanning electron microscopy (SEM) analysis displayed the complex 3-dimensional surface of almonds in comparison to coupons. Bacterial cells were observed to be covered in lipids and settled in crevices on almond surfaces, potentially harboring the bacteria from gaseous ClO2 treatments. The present study demonstrated the importance of combination treatments, including gaseous ClO2, mild heat, and moisture, to reduce pathogens Salmonella enterica on almonds. These results help to understand the effect of combined mild heat and moisture, which can be optimized for potential processing of almonds.