Effectiveness of silver dihydrogen citrate (SDC) and chlorine dioxide gas (ClO2) as sanitation strategies for picking bags and storage bins at tree fruit production facilities

Foodborne outbreaks and recalls within the tree fruit industry are making producers re-evaluate appropriate cleaning and sanitation practices during harvesting. Without effective sanitation, bacteria can create niches and form biofilms. This study evaluated the efficacy of silver dihydrogen citrate (SDC) and chlorine dioxide (ClO2) gas to control Escherichia coli and Listeria innocua on experimentally inoculated harvesting equipment at commercial apple packinghouses within the Midwest and Pacific Northwest regions. Rifampicin-resistant E. coli and Listeria were grown for either 24 h (sessile form) or 96 h (biofilm form) in 80 µg/ml rifampicin Tryptic Soy Broth (TSB) at 25± 2˚C on high density polyethylene plastic (HDPE), wood, or nylon coupons. Surfaces were allowed to dry and then exposed to ClO2 (100 ppm) for 24 h or SDC (4%) for 2 min. Coupons were swabbed, and remaining populations enumerated. Experiments were conducted in triplicate. ClO2 was the most effective treatment (P<0.05) in controlling sessile E. coli and Listeria on HDPE and nylon in the Midwest, where population was below detectable limit (< 1.95 log CFU/coupon). A lower level of inactivation was observed for biofilms grown on wood after ClO2 treatment (P<0.05). Recovered population was 8.70 and 7.84 log CFU/coupon for E. coli and Listeria, respectively. SDC did not reduce the population of sessile E. coli on HDPE, nylon, or wood in the PNW region. Counts were 9.04, 9.39 and 9.49 log CFU/coupon. Conversely, biofilm population was reduced on HDPE after SDC exposure in both regions and bacteria tested (P < 0.05), where bacteria counts were 5.80 and 5.70 log CFU/ coupon for E. coli and Listeria in the Midwest, and 6.37 and 5.12 log CFU/ coupon in the Pacif Northwest. Validating sanitation procedures in different field conditions can help producers understand the most effective and convenient strategies to mitigate microbial risks for harvesting bins and picking bags sanitation.