Date of Graduation

7-2026

Document Type

Thesis

Degree Name

Master of Science in Food Science (MS)

Degree Level

Graduate

Department

Food Science

Advisor/Mentor

Gibson, Kristen

Committee Member

Acuff, Jennifer

Second Committee Member

Dickson, Ryan

Keywords

soilless substrates, critical control point, mitigating food safety risks, hydronponic leaf lettuce production

Abstract

The effect of hydroponic soilless substrates and microbial biocontrol agents on the persistence of foodborne pathogens in indoor lettuce production is variable depending on substrate type, pathogen species, and biocontrol formulation. This study addressed key gaps by first characterizing the ability of different substrates to support Salmonella and Listeria monocytogenes survival and then evaluating the ability of pine biochar and Bacillus subtilis to suppress pathogen persistence in selected substrates under controlled conditions. In the first experiment, the persistence of Salmonella and L. monocytogenes was evaluated in four soilless substrates: rockwool (RW), low density foam (LDF), peat moss (PM) and coco coir (CC), with and without lettuce plants over 14 days. Substrates in sub-irrigated nutrient solution were inoculated with pathogen cocktails at ~6 log CFU/mL on day 0, and sampled on days 0, 3, 5, 7, 10, and 14 for enumeration on selective media. The effects of substrate type, planting status, bacteria type, and their interactions were tested with a generalized linear mixed model with negative binomial distribution with temperature, pH and electrical conductivity of the nutrient solution as covariates. Pathogen concentrations (log10 CFU/substrate) decreased significantly over time, but substrates varied significantly: CC supported the highest and most sustained populations compared with RW, LDF, and PM, and Salmonella and L. monocytogenes showed different persistence profiles. Notably, interactions of substrate type × bacteria type and substrate type × planting status were significant, suggesting that the physical properties of the substrate and the presence of plants modulated pathogen survival dynamics in indoor hydroponic lettuce systems during germination. The second study focused on whether two biocontrol agents (softwood pine biochar (20% v/v) and aqueous B. subtilis suspension (1% v/v)) could reduce the persistence of Salmonella and L. monocytogenes in RW and PM for 14 days when used independently. On day 0, pre‐treated (biochar or B. subtilis) substrates in nutrient solution were inoculated with ~6 log CFU/substrate of pathogen cocktails, and duplicate samples were taken on days 0, 3, 5, 7, 10, and 14 for enumeration. A similar mixed-effects modeling approach was used to assess the effects of biocontrol treatment, substrate type, bacteria type, and their interactions with covariates of solution temperature, pH, and electrical conductivity. Rockwool consistently supported higher pathogen concentrations relative to PM, and B. subtilis‐treated RW, the pathogen (i.e., Salmonella) peaked at ∼6.3 log CFU by day 3 and declined to ∼4.6 log CFU by day 14, whereas biochar and control treatments in RW declined to ∼1.9 log CFU by day 14. Two-way interactions were significant for substrate type and bacteria type; substrate type and biocontrol agent; and bacteria type and biocontrol agent, indicating that the responses of pathogens to biochar and B. subtilis were highly dependent on the substrate type and pathogen type. Together, these studies demonstrate that pathogen persistence during the initial two weeks of hydroponic lettuce production is likely influenced by substrate choice and biocontrol strategy, and that biocontrol efficacy can vary greatly across substrate–pathogen combinations, underscoring the need to tailor interventions to system configurations.

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