Date of Award

Spring 5-10-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Plant, Soil, and Environmental Sciences

Advisor

Jennifer J. Perry

Second Committee Member

David Handley

Third Committee Member

Robson M. Machado

Abstract

The hydroponic system is an increasing sector for horticultural production. It is used mostly for fruit and vegetable production. Lettuce (Lactuca sativa) is one of the most cultivated types of produce in the hydroponic system. It runs on nutrient solution and in some cases substrates. Water serves as the backbone for hydroponic production, mainly utilized for nutrient solution preparation. Substrates are sometimes added to provide support for plants root systems. Selection of substrate depends on the type of crop and the availability of the substrate. A good substrate should be able to balance the oxygen - water ratio around the root system and have a high water retention ability. Peat moss is an organic substrate mostly used by growers due to its sustainability and additional ability to retain nutrients on its surface. The hydroponic system is classified as open or closed system depending on the nutrient solution usage. The closed system reuses spent nutrient solution and is economical with less water wastage. There is, however, a high rate of pathogen build-up in this system.

The assurance of food security, food safety, and high yield has made the hydroponic system a widely accepted mode of production many vegetable horticultural commercial growers. Due to less to no contact of growing media to edible portions, the system is believed to provide a relatively safe, healthy, and clean product. However, the isolation of pathogens such as Salmonella, Eshericheria coli O157:H7, Listeria monocytogenes, and Campylobacter spp. from hydroponically grown crops has created awareness about the potential risk of foodborne illnesses from this system. Research is geared toward screening of source of irrigation water and other potential sources of contamination in the hydroponic production. However, little is known about the possible source of contamination in the hydroponic system.

The objectives of this study were to: (i) identify possible sources of contamination in the hydroponic system; (ii) evaluate the efficacy of behavior modification and/or sanitization in the reduction of microbial count on harvested produce throughout expected shelf life; and (iii) evaluate the microbial load on different peat moss substrates as well as heat-treated peat moss substrates.

Water, leaf, root, and substrate samples were collected from an actively growing, closed hydroponic system. Water samples included ‘water outlet’, ‘water inlet’, tap water and ‘water reservoir’. The leaf samples consisted of onsite leaf and harvested leaf while the substrates were onsite substrate and fresh substrate. Substrate used in this study was of peat moss origin. Samples were enumerated for aerobic plate count (APC), coliform bacteria (CB), and yeast and mold (YM). Detection of Listeria was carried out and none was detected on any of the samples. Enumerated count for all microbes was highest in the onsite substrate samples. Interestingly, onsite lettuce leaves had the lowest count for all counts. The harvested leaves were relatively higher in APC and YM count compare to the onsite leaves. The time of contact of the other samples with the onsite substrate significantly increased the microbial count on these samples, raising the possibility of the substrate being the source of contamination.

Reduction in the microbial load on the substrate was carried out by combining sanitizers, storage time, and packaging method. Sanitizers consisted of chlorine (Cl-200 ppm), peroxyacetic acid (PAA-80 ppm), and sterile distilled water (SDW). Microbiological and sensory quality measures were carried out on harvested substrate (plug), roots, and leaves. The harvested lettuce maintained its appearance and color after sanitizer application. Storage time and sanitizer significantly reduced APC and yeast count. PAA was most effective against APC and YM while chlorine was effective against CB. Sensory quality measurement indicated that dipping the harvested lettuce substrate in a solution before packaging aided in maintaining the lettuce color and fresh appealing look.

Other peat moss substrates and heat-treated substrates were examined for microbial populations. A difference in microbial load was found on substrates due to difference in rate of decomposition, chemical, and physical properties.

Overall, this research shows that substrate is a possible source of contamination in the hydroponic closed system. This research demonstrates that sanitizer wash could effectively help reduce microbial load on lettuce leaves and different compositions of substrates influence their ability to host microbes.

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