Purpose: To evaluate fecal contamination in tomatoes grown on small- or medium-sized diversified farms, and elucidate possible contamination sources.
Methods: Fifteen small- or medium-sized farms located in California (Central Valley=6, Central Coast=5, Shasta Cascade=2, North Coast=2) were sampled from July to October 2015; eight farms were certified organic. A total of 305 tomato samples were collected either randomly or as targeted samples due to animal intrusion or other risk factors. Irrigation water (n=78), soil (n=113) and soil amendment (n= 57) samples were also collected. The Most Probable Number (MPN) of Escherichia coli was used as indicator of fecal contamination.
Results: Altogether, 14.29% of samples were positive for E. coli (79/553): 4.26% tomatoes, 9.73% soil, 37.18% irrigation water and 45.61% soil amendment. Average MPN values were: 1914 MPN/100 g for tomatoes (range 0 – 460,000), 2,825 MPN/100 g for soil (0- 150,000), 102 MPN/100 ml for irrigation water (range 0 – 4,200) and 11*106 MPN/100 g for soil amendment (0 – 280*106). The source of irrigation water played an important role with an average MPN (range) of: 0 (0-0) in city water, 2.13 (0-92) in ground water, and 327.7 (0.36- 4,200) in surface water used for irrigation. Also, the type of soil amendment had an influence on average MPN and range: 40,770 (0 – 12*105) for finished compost, 116,887 (0 – 720,000) for unfinished compost and 42.47*106 (0 – 280*106) for raw manure.
Significance: The prevalence of E. coli in tomatoes from small- and medium-sized farms is low. However, irrigation water and soil amendments (especially raw manure) inputs may be a source of fecal contamination. There is a need for food safety training that addresses produce production on small- and medium-sized farms.