Microbial levels and reduction strategies for Michigan highbush blueberries

Highbush blueberries were collected from 18 Michigan fields during 2003-2004 and assessed for mesophilic aerobic bacteria (MAB), coliforms, Escherichia coli, yeasts and molds during harvesting and processing (pre-harvest, post-harvest, blower-exit, water tank-exit, and pre-packaging for freezing) along with conveyor belt and chlorinated wash water samples from six processing facilities. Microbial populations generally increased ∼1.5 log between pre- and post-harvest and decreased < 1 log after exiting the water tank (∼10 to 200 ppm chlorine). Thus, microbial levels were higher after processing than before harvesting with populations significantly (P <0.05) higher on conveyor belts and in chlorinated wash water during processing. Chlorine dioxide gas was also assessed as a pre-processing microbial reduction strategy for blueberries. In preliminary work using blueberries inoculated with several foodborne pathogens as well as spoilage yeasts and molds, 12 h of gassing (0.16 mg ClO2 gas/g fruit) in a 20 L bucket decreased all microbial populations >3 logs. When 600 lb pallets of naturally contaminated blueberries were subsequently exposed to 0.13 mg ClO2 gas/g fruit for 12 h, significant (P <0.05) reductions of 2.12, 1.61, 0.72, 1.76, and 1.55 logs CFU/g were seen for MAB, yeasts, molds, coliforms, and E. coli, respectively, compared to ungassed controls. Gassing of additional blueberries with 0.19 mg ClO2 gas/g for 12 h did not affect the appearance, aroma, texture, flavor or overall fruit acceptability.