In response to increasingly stringent microbial specifications being imposed by purchasers of frozen blueberries, chlorine dioxide (ClO2) gas generated by a dry chemical sachet was assessed for inactivation of Listeria monocytogenes, Salmonella spp., and Escherichia coli O157:H7 as well as five yeasts and molds known for blueberry spoilage. Fresh blueberry samples (100 g) were separately inoculated with cocktails of L. monocytogenes, Salmonella, E. coli O157:H7 (three strains each), or yeasts and molds (five strains each) to contain ∼106 CFU/g and exposed to ClO2 (4 mg/liter, 0.16 mg/g) for 12 h in a sealed 20-liter container (99.9% relative humidity) at ∼22°C. After gassing, 25 g of blueberries was added to 225 ml of neutralizing buffer, pulsified for 1 min, and plated using standard procedures to quantify survivors. This treatment yielded reductions of 3.94, 3.62, 4.25, 3.10, and 3.17 log CFU/g for L. monocytogenes, Salmonella, E. coli O157:H7, yeasts, and molds, respectively. Thereafter, 30 lugs of uninoculated blueberries (∼9.1 kg per lug) were stacked on 1.2 by 1.2-m pallets (5 lugs per level × six levels), tarped, and exposed to ClO2 (18 mg/liter, 0.13 mg/g) for 12 h. After gassing, significant (P < 0.05) reductions of 2.33, 1.47, 0.52, 1.63, and 0.48 log CFU/g were seen for mesophilic aerobic bacteria, coliforms, E. coli, yeasts, and molds, respectively, compared with non-gassed controls. No significant differences (P > 0.05) in microbial inactivation were seen between lug levels and, with one exception (mesophilic aerobic bacteria), between the bottom and top surface of individual lugs. Based on these findings, ClO2 sachets may provide a simple, economical, and effective means of enhancing the microbial shelf life and safety of blueberries.
Chlorine dioxide gas treatment improves the quality of hardy kiwifruit (Actinidia arguta) during storage