Effect of irrigation intervals and processing on the survival of Listeria monocytogenes on spray irrigated broccoli

Abstract

The first aim of this study was to determine the effect of irrigation intervals on the survival of L. monocytogenes on spray irrigated broccoli under field trial conditions, and subsequent survival of the pathogen on broccoli during postharvest processing procedures. The nonpathogenic L. innocua was used as surrogate organism to L. monocytogenes. Broccoli in the field was treated with irrigation water inoculated with L. innocua, during intervals over a period of five weeks and the growth and survival of the organism was monitored weekly. L. innocua numbers remained similar over intervals that received consecutive inoculations and L. innocua numbers decreased by at least 2.3 log cfu/g after inoculation ceased, which showed an inoculation effect and that time had an influence on organism survival. Cessation of irrigation before harvest was found to effectively reduce pathogen contamination levels on the crop, whilst repeated irrigation with contaminated water contributes to maintenance of L. innocua as well as elevated total microbial counts on the broccoli. A lack of correlation between the L. innocua counts and the recorded environmental temperatures in the field, including temperature and relative humidity, suggested that survival is not solely dependent on and influenced by, nor can it be predicted by these parameters. It was found that the presence of high levels of contamination (with, in this case L. innocua) in irrigation water used for vegetable crops, can be associated with an increased microbial population on the crop surface. Secondly, the effect of processing on organism survival post-harvest was assessed. Washing with water caused a 1 log reduction of L. innocua, whilst washing with 200 ppm chlorinated water facilitated a further 1 log reduction. Cooking reduced L. innocua numbers on broccoli by an average of 1.1 log units and aerobic plate counts by between 1 and 2 log units. A combined treatment of washing with chlorine, storage in MAP (5% CO2, 5% O2) for two days at 4°C and final microwave heating resulted in the lowest pathogen numbers, causing a 5.13 log cfu/g log reduction. Therefore, even though chlorine isprocessing, it does not suffice alone to eliminate pathogens (with L. innocua being representative of L. monocytogenes) from vegetables, just as MAP storage is only effective as part of a hurdle procedure. Cooking is essential in destroying L. innocua present on broccoli and to ensure vegetables that are safe for consumption in terms of pathogenic exposure. With this knowledge on the behaviour of L. monocytogenes on broccoli, the risk associated with the application of contaminated irrigation water to fresh produce can be better understood and the hazard managed. Copyright