Impact of irrigation water on the quality of fresh produce.

Abstract

The consumption of minimally processed fresh fruit and vegetables has increased over the past years, mostly because of consumers awareness that fresh produce serves as a good source of vitamins, minerals and fibre. Although fresh produce is important for the human diet it may provide an optimal environment for the growth and proliferation of pathogenic microorganisms, from cultivation to processing. Several outbreaks of disease, associated with the consumption of fresh produce, have been reported worldwide. In addition, fresh produce can become contaminated by heavy metals imposing a public health concern. One of the major sources of contamination is irrigation water, as it may contain pathogens and heavy metals from upstream operations. Irrigation water has been previously shown to be associated with the contamination of fresh produce. Therefore the objective of this study was to evaluate the microbial- and heavy metal- content of irrigation water used by local farmers in KwaZulu-Natal (KZN) over a 12- month period, in order to establish a link between the water quality and the safety of fresh produce, and to develop a suitable method to reduce the microbial contamination of fresh produce during both pre- and post-harvest phases. The microbial quality of the water and fresh produce samples was determined using the membrane filtration and standard spread-plate techniques, respectively. The heavy metal content of the water and fresh produce samples were analysed using inductively coupled plasma optical emission spectrophotometry (ICP-OES). Presumptive Escherichia coli, Salmonella spp., Shigella spp. and coliform counts in the water samples were high during the sampling period. Presumptive E. coli exceeded the DWAF limit of 2×10³ cfu/100 ml for E. coli in irrigation water, in some instances. High counts of presumptive coliforms, Shigella spp. and Campylobacter spp. were recorded in the fresh produce, throughout the sampling period. The roots of the plant demonstrated the highest microbial and heavy metal contamination. Leafy vegetables such as spinach and lettuce were more contaminated than the other fresh produce sampled; for example, Campylobacter spp. exceeded 4.5×10⁵ cfu/g in crisphead lettuce. With regard to the heavy metal content of the irrigation water and the fresh produce, mercury (Hg) exceeded the FAO and WHO limit of 0.001 mg/L, throughout the sampling period, with the highest concentration of 0.057 mg/L obtained from irrigation water. Since the concentrations of Hg in both the irrigation water and fresh produce were the highest during the same period, such as in winter, a clear link can be seen between the irrigation water and fresh produce. The method used during the pre-harvest phase, in order to reduce pathogens from produce, was the effect of Pseudomonas aeruginosa on the uptake of pathogens to the fresh produce. Inhibition assays were employed to determine whether P. aeruginosa could inhibit the pathogens (E. coli, Listeria monocytogenes, Salmonella spp. and Shigella spp.) tested. Only L. monocytogenes was found to be inhibited by P. aeruginosa. A greenhouse experiment was employed to prove that P. aeruginosa could prevent the uptake of this pathogen, via the roots, into the fresh produce by monitoring the concentration of L. monocytogenes in the soil and fresh produce by standard spread-plating. Denaturing gradient gel electrophoresis (DGGE) was also used to monitor the populations of L. monocytogenes and P. aeruginosa in the soil. Colony counts of L. monocytogenes decreased from 6 to 3.5 log cfu/g in the soil during the first 3 weeks of sampling. This decrease was confirmed by DGGE and suggested that this pathogen was inhibited by P. aeruginosa in the soil; hence, this pathogen was also not detected in the plant. During the post-harvest phase the effect of different treatment methods on the quality of the final produce was evaluated using tap water, NaCl, chlorine, hydrogen peroxide, blanching and ultraviolet (UV) light. UV light showed the most promise as the quality of this treated produce was better as compared to the other treated produce. A link between irrigation water qualities with that of produce was evident in this study as the highest microbial counts were recorded in summer for both the water and fresh produce samples. The pre-harvest method for the reduction of pathogens from the produce, which was the effect of P. aeruginosa on the uptake of pathogens to the produce, was limited as this organism had only inhibited L. monocytogenes, of the pathogens tested. Of the post-harvest treatment methods, UV treatment had caused the highest reduction in the microbial load of the fresh produce, with tap water treatment aiding in the survival of these presumptive pathogens. The presence of P. aeruginosa and the use of UV light in reducing microbial counts on fresh produce had both shown promise in this study. However, further studies need to be employed in order to optimise these methods before application. In addition, irrigation water should be routinely monitored and properly decontaminated, if necessary, to prevent the transmission of food-borne pathogens to crops. This may curb the problem of food-borne associated disease outbreaks world-wide as irrigation water has been shown, by the current study, as a link to the contaminated state of fresh produce.