Purpose: To determine the efficacy of targeted directional microwave (TDM) to reduce molds and L. monocytogenes on packaged flour tortillas.
Methods: In separate trials, flour tortillas were spot-inoculated with a two-strain mold and a five-strain L. monocytogenes cocktail at final inoculation level of 104 CFU/g. In duplicate, the samples packed in individual bags were subjected to five treatments using TDM with varying amounts of microwave energy, exposure time, and temperature change. Samples were serially diluted and plated onto dichloran rose-bengal chloramphenicol agar (for mold enumeration) and modified oxford agar (for L. monocytogenes enumeration) followed by incubation at 25°C for 4 days and 37°C for 24 hours, respectively. Non-inoculated tortillas without preservatives were also treated and set aside at room temperature to observe signs of mold growth.
Results: There were significant reductions (p<0.05) of both molds and L. monocytogenes on flour tortillas after treatments all but one TDM treatment. Four out of five treatments had mold reductions of 3.2 to 4.5 log10 CFU/g. A 2.0 to 3.9 log10 CFU/g reduction of L. monocytogenes on treated samples was observed except those from treatment with the lowest energy level. Non-inoculated preservative-free tortillas from two treatments with highest energy levels did not show signs of mold growth 30 days after production date, while all the untreated control samples had visible mold growth 5 days post-production.
Significance: Results suggest that TDM is an effective intervention to control molds and L. monocytogenes on flour tortillas, potentially improving shelf life and product microbial safety.