Ionizing Irradiation Inactivated Bacillus cereus in Naturally Contaminated Mesquite Pod Flour

Introduction: Mesquite pod flour has unique cinnamon/chocolate flavor with high sugar content (~40%), and is becoming increasingly employed in conventional and gluten-free applications.  As the flour is made from the pods of wild trees (Prosopis species), it is prone to contamination with human pathogenic bacteria such as Bacillus cereus. Thermal treatment, though effective in killing bacteria, changes its unique flavor.  Therefore, non-thermal processing technologies are needed to reduce the population of microorganisms in the flour.

Purpose: The objective of the study was to investigate the efficacy of ionizing radiation in inactivating B. cereus and mesophilic bacteria in naturally contaminated mesquite pod flour. 

Methods: Two types of mesquite pod flours (P. alba and P. pallida) naturally contaminated with B. cereus were irradiated with gamma rays at doses up to 10 kGy. The survival population of B. cereus spores and vegetative cells along with native microflora were determined using selective chromogenic media (after heat activation for spores). 

Results: Results showed that the total plate counts were 4.5 and 5.7 log CFU/g and the populations of B. cereus were 3.8 and 5.4 log CFU/g in the non-irradiated P. alba and P. pallida flours, respectively.  The populations of B. cereus spores were 3.4 and 4.9 log CFU/g in P. alba and P. pallida flours, respectively, which accounted for at least 80 % of mesophilic spores. The D₁₀-values (radiation dose required to achieve 90% reduction) of B. cereus and spores were in the range of 0.94 - 1.55 kGy. Populations of microflora, mesophilic spores, B. cereus and B. cereus spores decreased with increasing radiation doses. At 6 kGy, the populations fell below 1 log CFU/g.

Significance: Our results demonstrated that irradiation at 6 kGy was sufficient to reduce B. cereus population to undetectable level, and the technology may be used to enhance the microbial safety of the low moisture food.