Microbiology of Hydroponically-Grown Lettuce

Abstract

The microbial quality of lettuce (Lactuca sativa L. var. “Ostinata”) cultivated in a hydroponic system was evaluated. Over a 3-month study period, samples of lettuce, nutrient solution, and peat-vermiculite growing mixture from the greenhouse were analyzed for total aerobic bacteria, yeasts, molds, and coliforms. There was a consistent amount of each type of organism occurring within each sample group for a given month, and the numbers of aerobic bacteria and coliforms present were generally similar to those reported for lettuce and leafy vegetable crops propagated by the conventional method of agriculture. Over the study period, the modal values for each type of organism in lettuce were: aerobic bacteria, 7.9 × 106 CFU/g (range of 3.8 × 104 to 2.3 × 108); coliforms, 1.5 × 104 CFU/g (range of none detected to greater than or equal to 5.3 × 107); molds, 2.9 × 103 CFU/g (range of 1.2 × 102 to 5.3 × 104); and yeasts, 2.4 × 104 CFU/g (range of 6.9 × 102 to 2.3 × 106). The primary organisms associated with the growing system were Citrobacter freundii, Enterobacter cloacae, and Enterobacter agglomerans. No organisms of human health concern (i.e. Salmonella spp., Clostrium botulinum, Escherichia coli, or Staphylococcus aureus) were detected in the samples. The bacteriology of lettuce produced for market by this type of hydroponic farming and packaging appears to be generally comparable to that of field-grown lettuce and to present no unique microbiological hazards to consumers.

Sources of food contamination in a closed hydroponic system

“…Importantly, low counts in municipal water (when compared to water samples from other sites in the system) confirm that water acts as a carrier, but not source, for most micro‐organisms in the system. APC in the seedling water reservoir was significantly higher ( P < 0·05) compared to all the other water samples, suggesting that the stagnant water allowed bacteria to multiply in this location (Riser et al ). APC on the fresh, unused, peat plugs was 5·7 log CFU per g and it significantly increased to 6·8 log CFU per g at time of harvest.…”

Microbiological and Nutritional Analysis of Lettuce Crops Grown on the International Space Station

“…Bacterial counts from the roots in the VEG-03A grow out were significantly lower than the other two grow outs and lower than the corresponding ground control (Figure 3) although these counts were in a range of 10 6 -10 8 per gram which is typical of lettuce root tissue (Riser et al, 1984;Adesina et al, 2009).…”

Ecosystems in vegetable foods

“…The numbers of these bacteria on whole heads and on lettuce prepared for institutional use were 1.6 x lo5 and 4.2 x 105/g respectively. On freshly harvested, hydroponically grown lettuce (34 samples) the count of aerobic bacteria on the entire head was between 3.8 x lo4 and 2.3 x 108/g with a mean of 2.2 x 107/g (Riser et al 1984). Most of the Gramnegative rods isolated from the lettuce were Enterobacter cloacae, Citrobacter freundii and E. agglomerans.…”