EFFECT OF COMPOSTING ON MICROBIAL CONTAMINATION AND QUALITY OF FRESH FRUITS AND VEGETABLES - A MINI-REVIEW

There are 3 main types of compost: animal-based, plant-based and ‘biosolids’. Most of the attention in the literature has been centred on animal-based compost because of concerns related to its potential to serve as a source of microorganisms pathogenic to humans. It will be the focus of this presentation. It is now widely-accepted that non-composted animal waste can have a negative effect on the microbial quality of vegetables and fruits, especially those subject to little or no processing prior to their ingestion. Although a major goal of proper composting is the elimination of pathogenic microbes all that may be achieved is a reduction, usually of 3-4 logs. Thus, even if composted material is applied, there is still a crop contamination risk. This is dependant on several factors, including the amount of crop contact with the soil, physical condition of the crop, soil type and moisture content. For example, root and tuber crops are generally the most exposed to incorporated compost, followed by low-growing leafy vegetables and similar crops. Injured leafy vegetables are more susceptible to compost-borne, human bacterial pathogens than are non-injured plants. Another less appreciated scientific observation is that, given the appropriate conditions, i.e., post-processing storage at 22 or 35°C, or slow compost cooling, significant re-growth of low concentration pathogens, e.g., Escherichia coli 0157:H7, Salmonella spp. and Listeria monocytogenes, can occur in properly-composted material. Furthermore, there are reports that properly-composted materials can serve as a milieu for the proliferation of bacterial pathogens introduced after the composting was completed. During horticultural production, re-introduction could occur via the application of contaminated irrigation water, the deposition of bird or animal fecal material, or as a result of the settling of air-borne materials. One special concern is the possibility of antibiotic resistance occurring in the microbial population of compost derived from the waste of animals exposed to antibiotics.