Reduction of microbial counts during kitchen scale washing and sanitization of salad vegetables

Washing with or without sanitizers is one of the important steps designated to reduce or eliminate microbial hazards in fresh vegetables but the settings, conditions and effectiveness of this step remain contentious. In this study, we investigated kitchen scale salad preparation practices in a field study in Rwandan food service establishments (FSEs) and conducted laboratory trials to identify treatments that can improve reduction of microbial counts during washing and sanitization. In the field study, vegetable samples (n = 112) were taken from 56 FSEs before and after washing with or without sanitizer(s) to determine reduction of counts of Enterobacteriaceae, Listeria spp., and coagulase positive (CP)-staphylococci coupled with observation of the salad preparation practices from start to end. Based on the results obtained during the field study, 8 sanitizers were evaluated in the laboratory to optimize the efficacy of washing of leafy vegetables (corn salad, Valerianella locusta). Findings in the field study revealed that about 61% of the visited FSEs used sanitizers during washing of fresh vegetables, in particular, potassium permanganate (KMnO4) in 39% of FSEs, sanitizing powder (a mixture of polyphosphate, sodium hydrogen carbonate and active chlorine), 13%; sodium hypochlorite (NaClO), 7%; and sodium dichloroisocyanurate (NaDCC) in 2%. Average inactivation ranged from 1.0 log (KMnO4) to 3.1 log (NaDCC). In the laboratory study, average inactivation observed with Listeria spp., Escherichia coli and Aerobic plate count (APC) ranged from 0.7 log (water alone) to 3.0 log (NaDCC). Out of the 8 sanitizers that were evaluated, 5 sanitizers (NaDCC [90 ppm], NaClO [200 ppm], lemon juice [98%], acetic acid [2%] and sanitizing powder [4 g/L]) resulted in significantly higher inactivation compared to water alone. A contact time of 5 min and salad-sanitizer ratio of 1: 20 were considered optimal for kitchen based washing of the studied leafy vegetables with NaDCC and NaClO sanitizers.