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.

BACKGROUND Most hydroponic lettuce growers harvest and package their marketable‐size lettuces with an intact root ball. With a high microbial load on the peat moss substrate, there is a risk of microbial transfer onto the edible portion during packaging and throughout the product's shelf life. Since the produce is believed to have no contact with the substrate, no sanitizer wash is performed before packaging and storage. RESULTS Aerobic plate count (APC) results suggested that reduction in count was influenced by both sanitizer application and storage time. Peroxyacetic acid significantly reduced APC count on leaves, roots, and substrate, with a 1.8 log CFU g−1 initial reduction on the leaf. Fungi and APC levels increased with storage time, with the greatest APC increase in the roots. Leaves had the lowest coliform bacteria (CB), with chlorine slightly reducing CB count. Unlike APC, CB levels decreased during storage on the substrate and root samples. No Listeria positive was confirmed by agglutination test. Further evaluation of different commercial substrates reveals that Com4, a drier‐compacted plug, had the least ability to support growth/survival of all microbial populations enumerated relative to the spongy, wet black plugs. CONCLUSION The ability of peat moss substrates to host microorganisms is influenced by the physical properties of the product. Sanitizer wash efficacy is dependent on the initial microbial load and the length of storage. Chlorine and peroxyacetic acid are effective in reducing microbial populations on the leaves of hydroponically grown lettuce without affecting visual quality during shelf life. © 2020 Society of Chemical Industry

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Objectives Water kefir is a fermented beverage with potential probiotic effects. Its production requires a starter culture known as ‘grains’ and a sweetened water substrate. These grains self-propagate for use in sequential brewing cycles. Changes in microbial composition and its metabolites with repeated grain usage and prolonged storage are unknown. This may result in loss of functional properties or reduced consumer acceptability. This study evaluates the stability and functional components of microbial communities and chemical compositions in water kefir products over repeated brewing cycles and during refrigerated storage. Methods Six cultures were obtained from commercial suppliers (n = 5) or homebrewers (n = 1). Each culture system was brewed following a simplified standard recipe for 20 repeated cycles. Samples were collected at pH 4 and stored at 4 oC for analysis at predetermined time points (2, 6 and 12 weeks). Samples were enumerated for total aerobic bacteria, lactic acid bacteria (LAB), acetic acid bacteria (AAB) and yeast on TSA, MRS, ABS, and APDA media respectively. HPLC was used for simultaneous detection of the major organic acids, sugars, and alcohol in water kefir. Data was subject to ANOVA, MANCOVA, and mapping for visualization. Results Microbial population and chemical profile of water kefir were significantly affected by both repeated brewing cycles and storage. All brewing cycles at the initial 2-week storage time did not significantly affect LAB (range: 5.5–5.7 Log CFU/g) or AAB (range: 5.2–5.4 Log CFU/g) populations. However, significant variations were seen at the 12-week storage in these populations resulting in a reduction of 1.8–2.1 Log CFU/g. Overall, the microbial communities and their metabolite concentration increased until the 10th brewing cycle and then declined to their lowest at the 20th cycle. Hydrolysis of sucrose into component monosaccharides decreased from weeks 2 to 12 for brewing cycles 1 and 10; and increased at brewing cycle 20 over all storage time. Ethanol concentration was significantly lower at the 10th and 20th brewing cycles. Conclusions This data suggests that repeated brewing beyond a certain threshold cycle adversely affects the stability and viability of the microbial culture. There is a need to limit the number of brewing cycles to achieve the optimum health benefits of water kefir. Funding Sources USDA-NIFA.

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