Sources of food contamination in a closed hydroponic system

Abstract

This study investigated potential contamination sources in a commercial, closed hydroponic system. Water, substrate and lettuce (Lactuca sativa) samples were evaluated for microbiological indicator populations, including aerobic plate count (APC), coliform bacteria (CB) and yeast and mould (YM). Listeria spp. detection via cultural enrichment and agglutination was negative for all samples. Peat moss substrate (postharvest) had the highest counts for APC (6·8 log CFU per g), CB (4·5 log MPN per g) and YM (5·1 and 4·8 log CFU per g respectively). Roots embedded in plugs demonstrated counts for all populations nearly as high as the substrate. Among water samples, a seedling water reservoir housing germinated plants yielded the highest count for APC (5·1 log CFU per g) and CB (2·4 log MPN per g) likely due to the large numbers of plugs and their close proximity in the reservoir. Harvested lettuce leaves demonstrated higher APC (4·1 log CFU per g) than preharvest leaves (1·7 log CFU per g) due to the transfer of microbes from the root ball. These data suggest that substrates are a significant potential source of contamination in hydroponic systems and likely facilitate microbial transfer to harvested leaves. There is, therefore, the need to further investigate mitigation of potential contamination events. Significance and Impact of the Study Hydroponic production is known to provide safe, clean produce. This study, however, suggests that the hydroponic substrate (peat moss plug) is a possible source of contamination in the hydroponic system. This finding is important as most harvested hydroponic lettuces are packaged and sold with substrate and root ball intact. This implies a high probability of microbial transfer from the root ball to edible harvested lettuce leaves.

Sanitizer efficacy in reducing microbial load on commercially grown hydroponic lettuce

“…That is in line with the low Listeria spp. prevalence usually found in farms and hydroponic environments 4,27–29 . Although CB levels were higher than expected on plugs and roots (>4 log CFU g −1 ), the transfer of these bacteria to leaves by cross‐contamination appears to be minimal in our results.…”

“…prevalence usually found in farms and hydroponic environments. 4,[27][28][29] Although CB levels were higher than expected on plugs and roots (>4 log CFU g −1 ), the transfer of these bacteria to leaves by cross-contamination appears to be minimal in our results. This finding disagrees with other work that showed a high coliform count on both hydroponically grown lettuce leaves (6.2 log CFU g −1 ) and plugs (5.9 log CFU g −1 ).…”

“…A good substrate should be able to balance the oxygen:water ratio around the root system and have a high water‐retention ability. Substrates used in hydroponic production can serve as hosts and harbor microbes with a risk of microbial transfer to the edible portions of the produce 4 . In the case of peat moss, it retains nutrients due to its high cation‐exchange capacity, therefore serving as a good host for microbes 5 .…”

“…Substrates used in hydroponic production can serve as hosts and harbor microbes with a risk of microbial transfer to the edible portions of the produce. 4 In the case of peat moss, it retains nutrients due to its high cation-exchange capacity, therefore serving as a good host for microbes. 5 Its organic material composition makes it sustainable and easily disposed of after use but, at the same time, makes it a suitable matrix for microbial growth.…”

Antibiotic-Resistant Bacteria in Hydroponic Lettuce in Retail: A Comparative Survey

“…However, Wang et al isolated Shiga toxin-producing E. coli from the leaf surface of hydroponic lettuce [ 12 ]. Dankwa also isolated 4.1 log CFU per g of aerobic bacteria by aerobic plate count from the leaves of harvested hydroponic lettuce [ 13 ]. These findings suggested that microbiological contamination of hydroponic produce is possible.…”