Date of Graduation

7-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Food Science (PhD)

Degree Level

Graduate

Department

Food Science

Advisor/Mentor

Kristen E. Gibson

Committee Member

Ryan W. Dickson

Second Committee Member

Jung Ae Lee-Bartlett

Third Committee Member

Luke R. Howard

Fourth Committee Member

Steven C. Ricke

Keywords

Leafy greens, Lettuces, Microgreens, Norovirus, Pre-harvest, Tulane virus

Abstract

Since mostly consumed raw, foodborne pathogen contamination of leafy greens has led to a large number of foodborne disease outbreaks and illnesses each year in the United States. Human noroviruses (hNoV) are the most common viral pathogen transmitted by leafy greens. In this dissertation, the persistence of the hNoV surrogate Tulane virus (TV) on pre-harvest lettuce and microgreens was investigated. Lettuces are the most studied leafy green model, while previous hNoV research has mainly focused on the post-harvest stage of production. Here, pre-harvest hydroponically grown lettuce were used to determine TV persistence on leafy greens. After inoculation on leaves at 40 days age, TV reached over 4 log PFU/leaf reductions over the subsequent 4 days of observation. On day 45, TV was still detected on leaves, indicating that the pre-harvest viral contamination may last to post-harvest stages including consumption. Meanwhile, microgreens are a group of novel salad greens whose color, texture, flavor and nutritional values have attracted more consumers in recent years. Currently, the understanding of viral risks in microgreen cultivation systems is limited. This dissertation used sunflower (SF) and pea shoots (PS) as model microgreens to study the virus transmission from two types of soil-free cultivation matrix (SFCM)—biostrate and peat. Without the presence of plants, TV survived over 10 days in SFCM with only 2.08 and 1.76 log PFU/tray reduction in biostrate and peat, respectively. However, when TV were inoculated in SFCM on day 0 before sowing seeds, no virus was detected in harvested microgreen edible tissues on day 10, regardless of the plant variety and SFCM type. Notably, there were significantly lower virus concentrations in the planted SFCM compared to the unplanted control areas. Later, the virus transfer from SFCM to microgreen was further investigated when inoculated with TV at day 7 of plant age. On day 10, there were minor reductions in virus concentrations in SFCM, but in microgreen edible tissue, TV was still not detected. In addition, another study was carried out to characterize virus persistence on microgreen leaves surfaces. A significantly higher virus persistence was observed on PS than SF. From plant age of 7 to 10 days TV reduced on average over 4.5 log PFU/plant (n=2) on SF, while the reduction was only 2.52 on average (n=2) for PS, indicating a plant variety-dependent virus persistence on microgreens. The findings in this dissertation provides insights on virus transmission during pre-harvest production stage of two types of leafy greens—head lettuce and microgreens. This information will help to develop more effective virus prevention and control strategies within leafy green production systems.

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