Thermal Reduction of Bacillus spp. in Naturally Contaminated Mesquite Flour with Two Different Water Activities

Abstract

Mesquite flour with endogenous high sugar content is often contaminated with Bacillus cereus. The purpose of the present study was to evaluate the thermal resistance of 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 Bacillus spp. were enumerated using tryptic soy agar and Brilliance Bacillus Cereus Agar media, respectively. Results revealed that naturally contaminated Bacillus spp. and other mesophilic bacteria in mesquite flour (aw=0.34) were highly resistant to heat. To reduce the initial populations (4.75 log CFU/g) of Bacillus spp. to non-detectable levels (<1.18 log CFU/g), thermal treatments of 120°C for 2 h were required. D100°C-values for total mesophilic bacteria were 5.6 fold higher than those of Bacillus spp. 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 decreased significantly. Treatment at 100°C for 1 h reduced Bacillus spp. populations to nondetectable levels. Our results demonstrate that naturally present Bacillus spp. in flour are highly resistant to heat, while increasing the aw increased their heat sensitivity. The high thermal resistance of microbes in mesquite flour warrants further investigations.