Effect of irrigation water quality on the microbiological safety of fresh vegetables

Abstract

Irrigation water is perhaps the leading pre-harvest source of contamination of fresh vegetables in the world. In this thesis, the effect of source water from the Olifants River and the Wilge River on the bacterial quality of water in the Loskop Canal that they feed and also the subsequent contribution to the bacterial contamination of fresh vegetables was determined for a period of twelve months. Also effect of attachment time on the survival of Listeria monocytogenes and the effect of chlorine on L. monocytogenes attached to vegetables were determined. Finally, a step-wise logistic regression analysis was made to determine whether various predictor variables could be used to predict the occurrence of L. monocytogenes, Salmonella spp and intestinal Enterococcus in irrigation water and vegetables (i.e., cauliflower and broccoli). COD and turbidity were higher in the Olifants River and the Wilge River than in the Loskop Canal that they feed, according to the water guidelines set by the World Health Organisation (WHO) and the Republic of South Africa (RSA). The level of the COD and turbidity were significantly different in terms of the two rivers in comparison with the canal. Levels of faecal coliforms and Escherichia coli were also higher than the WHO standard. Staphyloccocus aureus, intestinal Enterococcus, Salmonella, L. monocytogenes were recovered from the two rivers and the canal. Apart from L. monocytogenes that was not recovered from cauliflower, all bacterial pathogens recovered from the surface water were recovered from the vegetables. This study also indicated that L. monocytogenes could attach to both surface and subsurface structures of both tomatoes and spinach within 30 min, and that even after 72 h, it still remained viable. It also indicated that chlorine treatment is more effective against surface L. monocytogenes compared with subsurface inoculated L. monocytogenes. Finally, the logistic regression analysis of the sampled data showed that COD was statistically reliable to indicate a high probability of L. monocytogenes, turbidity reliable to indicate a high probability of intestinal Enterococcus and faecal coliforms and coliforms reliable to indicate a high probability of Salmonella in irrigation water. Low aerobic colony count (ACC) was statistically significant for the prediction of the three pathogens on vegetables.