Association between bacterial survival and free chlorine concentration during commercial fresh-cut produce wash operation

Authors: Luo, Yaguang - Sunny; Zhou, Bin; VAN HAUTE, SAM - University Of Maryland; Nou, Xiangwu; ZHANG, BOCE - Orise Fellow; TENG, ZI - University Of Maryland; Turner, Ellen; WANG, QIN - University Of Maryland; Millner, Patricia

Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2017
Publication Date: 10/9/2017
Citation: Luo, Y., Zhou, B., Van Haute, S., Nou, X., Zhang, B., Teng, Z., Turner, E.R., Wang, Q., Millner, P.D. 2017. Association between bacterial survival and free chlorine concentration during commercial fresh-cut produce wash operation. Food Microbiology. 70:120-128.

Interpretive Summary: The presence of sufficient amount of anti-microbial agent such as chlorine is critical to prevent pathogen survival and spread during fresh-cut produce wash operations. However, maintaining free chlorine concentration is a practical challenge under actual flume wash conditions that change rapidly in response to fluctuations in the rates of cut produce loading, and water and chlorine replenishment. In this study, research scientists investigated the relationship between the dynamic changes of free chlorine concentration and aerobic bacterial counts in wash water during commercial fresh-cut produce washing. Three major fresh-cut products (Romaine and Iceberg lettuce, and cabbage) were tested, with small to large cut sizes, high to low organic content in the wash water, and difficult to easy maintenance of free chlorine concentrations. Results provide critical information toward optimizing threshold free chlorine concentrations for fresh-cut processing to improve food safety.

Technical Abstract: Determining minimal, effective free chlorine (FC) concentration for preventing pathogen survival and cross-contamination is critical for developing science- and risk-based food safety practices. The correlation between dynamic FC concentrations and bacterial survival was investigated under commercial fresh-cut produce wash operations. Chopped Romaine lettuce, shredded Iceberg lettuce, and diced cabbage were washed in a two-flume system under typical commercial fresh-cut processing conditions. Targeted FC concentrations and pH levels were maintained using sodium hypochlorite and a phosphoric acid-based solution, respectively, via an automated control system. Wash water was sampled every 30 minutes and assayed for chemical oxygen demand (COD), turbidity, total dissolved solids (TDS), and FC; and total mesophilic aerobic bacteria (APC) after chlorine neutralization. Significant increases in COD, turbidity, and TDS were observed over time, with more rapid increases during diced cabbage wash. Combined chlorine (CC) concentration increased consistently over time; FC concentration fluctuated, as impacted by the rates of chlorine replenishment, cut product loading, and water replenishment. Total aerobic bacterial survival showed a strong correlation with real-time FC concentration. With FC below approximately 10 mg/L, increasing FC concentration resulted in sharp declines in the frequency and population of surviving bacteria detected. Increasing FC concentration further resulted in the APC count reduced to below the detection limit (50 CFU/100 mL) in most cases, except a few sporadic positive samples with low cell counts. Results show that maintaining at least 10 mg/L FC in wash water contributes significantly to reduced survival of APC. Tests with culturable indigenous aerobic bacteria in produce wash conditions are valuable for commercial operations, since inoculating produce with human pathogens or even non-pathogenic surrogates during commercial operations is not feasible given the inherent food safety risks.