Factors affecting compost tea as a potential source of Escherichia coli and Salmonella on fresh produce

Authors: Ingram, David; Millner, Patricia

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2006
Publication Date: 11/8/2006
Citation: Ingram, D.T., Millner, P.D. 2006. Factors affecting compost tea as a potential source of Escherichia coli and Salmonella on fresh produce. Journal of Food Protection. 70(4):828-834.

Interpretive Summary: Foodborne illness outbreaks have raised interest in identifying pre- and postharvest sources of contamination on raw fruits and vegetables. Compost tea (CT)is an unheated infusion of compost prepared on-farm by some growers for use as a foliar and fruit spray or soil drench to control foliar and root diseases. When used on fresh produce, farm-to-fork food safety protection demands that CT meet basic microbiological criteria for water quality used in food production. A critical process control point for CT use occurs at the preparation stage even before it is sprayed or drenched because factors used in production may encourage growth and survival of pathogens. While heat-processed compost is suitable for gardens, lawns, and soils, it is not a sterile product. Therefore, even a few bacteria that survive high temperature composting, could grow if favorable conditions, such as might occur in CT production, were provided. This report describes the evaluation of two CT production processes (aerated and non-aerated) on growth/survival of foodborne pathogens and their indicators. Seven commercially available CT nutrient supplements and their individual components were tested. Compost with low to high (10-1000 CFU per gram dry weight) initial counts of E. coli O157:H7 and Salmonella enteritidis were used to assess growth/survival responses to aerated (36 h) and non-aerated (8.5 da) CT processes. Pathogens and indicators were decreased by 8.5 da in non-aerated CT without nutrients. In contrast, use of commercially formulated mixtures or combinations of individual nutrient supplements consistently resulted in growth of pathogens and indicators in aerated CT. At 24-36 h, E. coli O157:H7 in CT increased 10- to 1000-fold and S. enteriditis increased 10- to 10,000-fold. Some nutrient combinations supported small amounts of growth in unaerated CT by 8.5 da. Aeration stimulated more growth of E. coli O157:H7 and fecal coliforms than non-aerated conditions when nutrients were added. Results clearly show that nutrient and other supplements support growth of human pathogens in both aerated and non-aerated CT and should be avoided when CT is used on fresh produce. This information will help inform growers, extension agents, compost- and compost tea equipment producers, scientists, food quality specialists, and others interested in understanding sources of pre-harvest produce contamination and prevention strategies.

Technical Abstract: Compost tea (CT) is an unheated on-farm infusion of compost used as a spray or soil drench to promote plant-growth and control foliar/root diseases. Food safety protection from farm-to-fork demands that CT meet basic microbiological criteria for water quality. The production process is a critical control point for CT use on fresh produce . This report describes effects of two CT production processes (aerated and non-aerated) on growth and survival of foodborne pathogens and their indicators. Seven commercially available nutrient supplements designed for use in CT production were tested individually and in combination. Compost containing low to high (101-103CFU/g) initial concentrations of E. coli O157:H7 and Salmonella enteritidis were used to assess growth and survival responses to aerated (36 h) and non-aerated (8.5 days) processes. Pathogen and indicator populations were decreased by day 8.5 in non-aerated CT without nutrient supplements. In contrast, use of commercially formulated mixtures or combinations of individual nutrient supplements consistently resulted in growth of pathogens and indicators in aerated CT. At 24-36 h, CT populations of E. coli O157:H7 increased 10- to 1000-fold and S. enteriditis increased 10- to 10,000-fold. Some nutrient combinations supported small amounts of growth in unaerated CT by 8.5 days. Aeration stimulated more growth of E. coli O157:H7 and fecal coliforms than non-aerated conditions when nutrient supplements were added. Results clearly show that addition of nutrient and other supplements supports growth of human pathogens in both aerated and non-aerated CT and should be avoided when CT is used on fresh produce.