Operational limits of sodium hypochlorite for different fresh produce wash water based on microbial inactivation and disinfection by-products (DBPs)

Abstract

The goal of the present study was to establish free chlorine operational limits for the disinfection of different types of fresh-cut produce wash water using sodium hypochlorite (NaClO). The proposed operational limits in the wash water are based on both the effectiveness of microbial inactivation and the occurrence of disinfection by-products (DBPs). For this purpose, lab-scale dynamic experiments were performed simulating the conditions of commercial fresh-cut processing lines. Different free chlorine (FC) concentrations (≈0, ≈10, ≈20, and ≈30 mg/L) were maintained in process wash water (PWW) of shredded iceberg lettuce, baby leaves (rocket, red oak leaf, and red swiss chard), shredded red cabbage and diced onions by means of the constant addition of the different doses of NaClO. Physicochemical characteristics of PWWs were monitored during the experiments including FC, total chlorine (TC), pH, temperature, oxidation-reduction potential (ORP), electrical conductivity (EC), chemical oxygen demand (COD), absorbance at 254 nm (UV254), and the presence of DBPs such as trihalomethanes (THMs), haloacetic acids (HAAs) and chlorate. The antimicrobial capacity of FC doses was evaluated by the inactivation of indigenous total aerobic bacteria (TAB). In all cases, FC had a significant antimicrobial effect (p < 0.05), but the operation limit differed depending on the type of PWW. A lower FC operational limit based on TAB counts of 10 mg/L was effective for onion PWW, while levels up to 20–25 mg/L were established for lettuce and cabbage PWWs. However, the levels of TAB in PWW of baby leaves were not controlled even at the highest FC concentration of 30 mg/L. The establishment of a maximum operational limit for FC based on the concentration of DBPs in PWW was not possible as even the lowest FC concentration (10 mg/L) would potentially lead to unacceptable levels of chlorate in the different types of product washed including lettuce, baby leaves, red cabbage and onion.