Thermal reduction of bacillus in naturally contaminated mesquite flour with two different water activities

Authors: Fan, Xuetong; Baik, Jessica; Gurtler, Joshua

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2020
Publication Date: 10/30/2020
Citation: Fan, X., Baik, J.I., Gurtler, J. 2020. Thermal reduction of bacillus in naturally contaminated mesquite flour with two different water activities. Journal of Food Protection. Vol. 84, No. 3, 2021, Pages 490–496. https://doi.org/10.4315/JFP-20-268.
DOI: https://doi.org/10.4315/JFP-20-268

Interpretive Summary: Mesquite flour, which has high sugar content and a unique aroma, has been recalled due to the presence of high populations of Bacillus cereus. Thermal processing is the most common technology used for inactivating human pathogens in food; however, heat treatment of mesquite flour for the inactivation of Bacillus has not been reported. In this study, the thermal resistance of Bacillus spp. in naturally contaminated mesquite flour was evaluated as affected by water activity, treatment time and temperature. Our results demonstrated that both vegetative cells and spores of naturally present Bacillus. in the flour were highly resistant to heat, while increasing water activity increased heat sensitivity of the bacteria. The information reported here will help develop strategies to minimize contamination of this low moisture food with Bacillus cereus.

Technical Abstract: Mesquite flour with high sugar content has been found to have high populations of Bacillus cereus. The purpose of the present study was to evaluate the thermal resistance of presumptive Bacillus spp. in naturally contaminated mesquite flour. Flours with and without adjusted water activity (aw) were treated at various temperatures (100-140°C) and times (up to 2 h). Total mesophilic bacteria and presumptive Bacillus spp. were enumerated for both vegetative cells and spores using TSA and selective media, respectively. Results showed that naturally contaminated presumptive Bacillus spp. and other mesophilic bacteria in mesquite flour were highly resistant to heat. To reduce the initial populations (4.75 and 3.55 log CFU/g) of presumptive Bacillus spp. to non-detectable levels (<1.18 log CFU/g), thermal treatments of 120°C and 110°C for 2 h were needed for the vegetative cells and spores, respectively. D-values (time required for 90% reduction) at 100°C of total mesophilic bacteria were 5.6 and 4.0-fold higher (0.63 and 0.92 h) than those of presumptive Bacillus spp. for both spores and vegetative cells, respectively. With increasing treatment temperature, the D-value between total mesophilic bacteria and B. cereus became smaller. When the aw of flour was adjusted from 0.34 to 0.71, the D-values for Bacillus were dramatically decreased. Treatment of 100°C for 1 and 0.5 h reduced the populations to nondetectable levels for vegetative cells and spores, respectively. Our results demonstrate that both vegetative cells and spores of naturally present presumptive Bacillus spp. in flour are highly resistant to heat, while increasing aw increased their heat sensitivity.