Final report for GNE24-331
Project Information
This project explored UV-B and cold exposure as a means to elicit secondary metabolites in microgreens and investigated the outcomes as an approach to improve microgreen quality and food safety. We found that preharvest UV-B exposure improved the phenolic and glucosinolate levels, fresh weight retention, and to some extent the microbial safety of radish microgreens during refrigerated storage. Additionally, in separate experiments we found that microgreens exposed to periods of cold altered several compounds associated with plant stress responses and nutritional quality. At harvest, cold-exposed microgreens had higher concentrations of chlorophyll, carotenoids, and anthocyanins than control plants. These differences indicate that the cold exposure influenced the physiological and chemical composition of the crop. However, there were no clear reductions post-harvest in pathogen persistence from cold-exposed microgreens.
The goal of this project was to investigate the efficacy of regulated abiotic stress as an elicitor of plant secondary metabolites on plants grown under controlled environments to improve extranutritional properties, food safety and post-harvest quality (shelf-life). This goal was explored by using a series of experiments where microgreens were grown either with UV-B or cold exposure and the nutritional quality was monitored post-harvest throughout storage. Once microgreens are harvested, they rapidly exude nutrient-rich fluid, wilt and can quickly decay, potentially favouring enteric pathogen persistence (Turner et al., 2020). This project addressed the following objectives:
The objectives of this project were:
- Improve nutrition and quality: We assessed a) the effect of exposure to UV-B and cold on secondary metabolite accumulation in microgreens of leafy vegetables and b) the retention of nutritional quality post-harvest throughout storage.
Utilizing controlled environments, seedlings were exposed to abiotic factors such as cold temperature and UV irradiation to improve microgreen phytochemical profiles and quality post-harvest. Secondary metabolites which are typically indicative of improved nutritional value such as total phenolics, total flavonoids, carotenoids, anthocyanins and glucosinolates (Brassica spp. only) were measured. The change in major phytochemical groups were measured over a one-week period post-harvest during refrigerated storage.
- Improved food safety: We evaluated the effect of phytochemical manipulation via abiotic stress application on foodborne pathogen association with microgreens.
This objective had two sub-objectives:
2.1 Measure the persistence of E. coli O157:H7 in the phyllosphere following seed inoculations: Radish seeds were inoculated with E. coli O157:H7 and grown under UV-B. The persistence of E. coli O157:H7 in the phyllosphere was assessed at harvest and throughout refrigerated storage.
2.2 Determine the persistence of foodborne pathogens following epiphytic inoculations: Radish microgreens grown under cold conditions were harvested and separately inoculated with E. coli O157:H7, Listeria monocytogenes and Salmonella. The persistence of each pathogen in the phyllosphere was assessed at harvest and throughout refrigerated storage.
- Integrated food safety and sustainability: We assessed the food safety implications of two growth media commonly used in microgreen production.
The effect of substrate type on the persistence of E. coli O157:H7 was evaluated. To determine the persistence of E. coli O157:H7 in soil and jute substrates separate inoculation experiments were performed and counts of E. coli O157:H7, yeast and mould were collected over a two-week period.
The purpose of this project was to investigate methods to synergistically improve extranutritional properties, food safety and post-harvest quality of microgreens.
Microgreens are delicate young seedlings of edible plants and include the shoots, two cotyledons and small emerging true leaves which appear 7-14 days post-germination. Due to their short production time and suitability for controlled environment production, microgreens offer growers an opportunity to produce a high-value crop quickly. However, their short shelf life and potential for contamination with foodborne pathogens present challenges maintaining product quality and safety.
Although abiotic stressors such as UV-B radiation and cold exposure can alter the accumulation of secondary metabolites and other quality-related compounds in plants, it remains unclear whether these changes influence the ability of foodborne pathogens to colonize and persist on microgreens. In particular, limited information is available on how preharvest abiotic stress, substrate type, and pathogen introduction through contaminated seed interact to affect microgreen quality, food safety, and post-harvest persistence.
There is an opportunity to advance current knowledge by exploring the interaction between elicitation of secondary metabolites via abiotic stress in microgreens and the subsequent effect on foodborne pathogens when seeds are grown on various substrates. I hypothesized that microgreens grown under abiotic stress would produce seedlings with enhanced nutritional and functional properties, with reduced persistence of foodborne pathogens in the phyllosphere and exhibit improved post-harvest quality.
The outcomes of this project help inform current and future microgreen growers about best management practices to boost nutrition and minimize contamination risk of their microgreen crop. Further, the outreach portion of this project has provided growers the confidence to make decisions about their microgreen production methods to produce the best quality and safest product possible.
Research
Growing Microgreens
Basil, cilantro, and radish microgreens were grown from seed using a soil-based growing substrate. Seeds were planted according to the recommendations provided for each variety. After germination, microgreens were grown under controlled conditions with consistent light, temperature, and humidity.
Microgreens were harvested approximately 10–14 days after planting, when the first true leaves had emerged. Harvested plants were either immediately evaluated for foodborne pathogens and plant compounds or packaged and stored for post-harvest quality and food safety evaluations.
Abiotic Stress Treatments
Two preharvest treatments were evaluated to determine whether environmental stress could improve microgreen quality and influence foodborne pathogen persistence: UV-B exposure and cold exposure.
For the UV-B treatment, microgreens were exposed to UV-B light for 5 h each day over five consecutive days. Control plants were grown under the same conditions without UV-B exposure.
For the cold treatment, microgreens were exposed to cold temperatures (4°C) during the nighttime period for approximately three weeks. Control plants remained at optimal growing temperature (16°C at night and 21°C during the day). Following treatment, microgreens were harvested and evaluated for quality, plant compounds, and foodborne pathogen persistence.
Foodborne Pathogen Experiments
Experiments were conducted using Escherichia coli O157:H7, Salmonella enterica Enteritidis, and Listeria monocytogenes to determine whether the preharvest treatments affected pathogen persistence on microgreens.
For some experiments, seeds were intentionally inoculated with foodborne pathogens before planting. This approach was used to represent contamination that could be introduced through contaminated seed and carried into the growing crop. In other experiments, E. coli O157:H7 was introduced directly onto the growing substrate, including both soil-based media and jute fiber.
After inoculation, microgreens were grown and sampled during the experiment. Samples were tested to determine how many viable bacteria remained on the plants or growing substrate over time.
Plant Quality and Phytochemical Analysis
Plant samples were collected at harvest and processed for analysis of compounds associated with plant quality and potential health benefits. These included phenolic compounds, flavonoids, chlorophyll, carotenoids, anthocyanins, and glucosinolates. Proline, a compound associated with plant responses to environmental stress, was also measured in cold-treated microgreens.
Plant tissue was dried, ground, and extracted before analysis. Laboratory tests were then used to estimate the concentration of these compounds and determine whether UV-B or cold exposure altered the chemical composition of the microgreens.
Post-Harvest Evaluation
Following harvest, selected microgreen treatments were packaged and stored under refrigerated conditions. Samples were evaluated during storage to determine whether preharvest treatments affected product quality and the persistence of foodborne pathogens after harvest.
UV-exposure of vegetables and herbs
Elicitation trials were conducted using cilantro, basil and radish microgreens to assess the efficacy of UV-B as an elicitor of secondary metabolites. Cilantro and basil showed no significant changes in measured metabolites following cumulative UV-B doses of 1,300 and 3,200 mJ cm-2 respectively. In contrast, radish microgreens exposed to 3,400 mJ cm-2 showed significant increase in all measured phenolic compounds and total glucosinolates compared to non-exposed controls, without a reduction in fresh weight. Based on these findings, radish microgreens were selected for continued research to assess the impacts of pre-harvest abiotic stress on food-safety outcomes as described below.
Radish seed E. coli O157:H7 inoculation and pre-harvest UV exposure
Preharvest UV-B exposure enhanced the functional quality of radish microgreens without reducing crop yield. UV-B-treated microgreens had higher levels of all measured phenolic metabolites compared with untreated controls. These improvements were also generally maintained during refrigerated storage, although the differences became smaller over time.
Despite these changes in plant compounds, UV-B exposure did not reduce the persistence of E. coli O157:H7 on microgreens grown from contaminated seed. E. coli O157:H7 populations generally declined during refrigerated storage, but this decline was related primarily to storage time rather than UV-B treatment. In some cases UV-B-treated microgreens had similar or slighter higher levels of E. coli O157:H7 than controls.
These results demonstrate that UV-B exposure can be used to improve the functional quality of radish microgreens, without compromising yield. However, UV-B should not be relied upon as a microbial food safety control for seed-associated E. coli O157:H7. The results reinforce the importance of maintaining established food safety practices while considering UV-B as a potential tool for improving crop nutritional quality and adding value to microgreen production.
Radish microgreen cold exposure and epiphytic persistence of foodborne pathogens
Pre-harvest cold exposure did not affect fresh weight of radish microgreens. Cold exposure did, however, influence several physiological and quality-related characteristics.
At harvest, cold-exposed microgreens had higher concentrations of chlorophyll, carotenoids, and anthocyanins than controls. These differences indicated that the cold treatment altered plant pigment accumulation without reducing yield. However, chlorophyll and carotenoid concentrations declined during storage in the cold-exposed plants, and differences between treatments diminished. Proline concentrations were also slightly higher at harvest following cold exposure but declined during storage, resulting in slightly lower concentrations than controls after 8 d.
Phenolic compounds showed a different response. Although quercetin equivalents were initially higher in controls, quercetin concentrations increased during storage in cold-exposed microgreens and were slightly higher than controls at the end of storage. Chlorogenic acid equivalents similarly increased during storage in cold-exposed plants. Catechin and gallic acid were largely unaffected by treatment. These results suggest that pre-harvest cold exposure can influence not only the concentration of plant compounds at harvest but also how some compounds change during storage.
Glucosinolate concentrations were higher in controls at harvest, but concentrations increased substantially during storage in both treatments. Thus, the initial treatment difference was largely eliminated during storage.
Importantly, the cold treatment did not reduce persistence of the foodborne pathogens evaluated. E. coli O157:H7 declined slightly during storage in both treatments, but the reductions were small and did not differ between cold-exposed and control microgreens. Listeria and Salmonella also showed little difference in recovery between treatments. Although pathogen levels generally declined during storage, pre-harvest cold exposure did not provide a measurable food-safety benefit under the conditions tested.
Overall, overnight cold exposure modified several quality-related characteristics of radish microgreens while maintaining fresh yield. The effects varied among compounds and changed during storage, suggesting that pre-harvest temperature conditions can influence microgreen quality beyond harvest. However, the treatment should not be considered a food-safety intervention, as it did not meaningfully reduce persistence of the foodborne pathogens evaluated.
Inoculations of substrates
Recovery of E. coli O157:H7, mould, and yeast differed between jute and soil and changed substantially during the 14-day incubation period. The experiment included both a distilled water (DI) control and an E. coli O157:H7 inoculation treatment, allowing changes in mould and yeast to be evaluated in the presence and absence of the pathogen. On jute, mould and yeast were detectable on the first day following both treatments and generally increased through day 7 before declining slightly by day 14. In the soil substrate, mould and yeast populations increased substantially during the experiment, regardless of whether E. coli O157:H7 was introduced. Similar patterns observed in both treatments suggest that the increase in naturally occurring mould and yeast was primarily associated with the soil substrate and growing conditions rather than the presence of E. coli O157:H7. These results suggest that the growing environment may have had a greater influence on the native microbial community than the introduced pathogen.
In contrast, E. coli O157:H7 was readily recovered from jute on days 1 and 7 but was too numerous to count (TNTC) by day 14. In soil, E. coli O157:H7 was below the limit of detection on days 1 and 7 but was TNTC by day 14. The lack of E. coli O157:H7 recovery from soil during the first 7 days coincided with a rapid increase in native mould and yeast populations. This pattern raised the possibility that interactions with the native soil microbial community, including competition or antagonism, contributed to the early suppression of E. coli O157:H7 recovery. However, because these interactions were not directly measured, competition cannot be established from these data alone. The subsequent recovery of E. coli O157:H7 at TNTC levels on day 14 suggests that the pathogen was not eliminated, and that conditions later became more favorable for its recovery or growth.
Together, these studies demonstrate that preharvest environmental treatments can influence the phytochemical quality of radish microgreens without substantially affecting fresh weight. UV-B exposure consistently increased phenolic compounds and glucosinolates, while cold exposure increased several compounds associated with plant stress responses some of which would also improve nutritional potential, including proline, chlorophyll, carotenoids, anthocyanins, and selected phenolics. Some of these bioactive compounds were retained or became more pronounced during refrigerated storage.
However, improvements in phytochemical quality did not translate into consistent reductions in E. coli O157:H7 persistence. In the UV-B and cold-exposure studies, storage conditions and time generally had greater effects on pathogen populations than the preharvest treatments. The substrate incubation study further demonstrated that pathogen recovery was influenced by the matrix environment. E. coli O157:H7 was recovered from jute throughout incubation but was below the detection limit in soil during the first 7 d before reaching TNTS levels by the end of the experiment. At the same time, native mould and yeast populations increased substantially in soil in both uninoculated and E. coli O157:H7. Further research is needed to determine the role native microbes may play in the suppression of E. coli O157:H7 in the early stages of contamination.
Overall, these findings indicated that UV-B and preharvest cold exposure have potential as sustainable approaches for enhancing the functional food quality of microgreens, but do not serve to improve food safety and should not be considered substitutes for established food-safety practices. The results also highlight the importance of considering substrate, native microbial communities, and post-harvest environmental conditions when evaluating persistence in microgreen production systems.
Education & outreach activities and participation summary
Participation summary:
On June 15, we published one newsletter through the University of Maryland Food Safety Group which highlighted our work with UV-B as an elicitor of secondary metabolites, the article can be found here. The newsletter was delivered to the full subscription of 761 recipients of which 365 (47.96%) read the article.
A second newsletter will be published which highlights our research using cold temperatures as a elicitor of phenolic compounds.
In June of 2026 we presented a poster at the ASM meeting titled: Preharvest UV-B exposure bolsters radish microgreen quality during
storage but does not affect Escherichia coli O157:H7 persistence.
We plan to submit a poster to the next ASM meeting to disseminate the work further.
Project Outcomes
Project outcomes and Knowledge Gained
- Preharvest environmental treatments can enhance microgreen quality. UV-B and cold exposure increased several phytochemicals associated with nutritional and functional quality, including phenolics, glucosinolates, proline, chlorophyll, carotenoids, and anthocyanins.
- Phytochemical responses can persist during storage. Some treatment-induced changes were maintained or became more pronounced during refrigerated storage, indicating that preharvest conditions can influence postharvest quality.
- Preharvest treatments did not consistently improve microbial food safety. UV-B and cold exposure did not measurably reduce persistence of the foodborne pathogens evaluated. E. coli O157:H7 declined slightly during storage following cold exposure, but reductions were small and similar to controls. Listeria and Salmonella also showed limited treatment-associated differences and generally persisted during storage.
- Substrate strongly influenced pathogen persistence. E. coli O157:H7 recovery differed between jute and soil, with the pathogen temporarily below the limit of detection in soil but was readily recovered from jute.
- Native microbial communities may influence pathogen survival. Soil supported rapid increases in native mould and yeast populations in both inoculated and uninoculated treatments, suggesting that interactions with background microbial communities may affect E. coli O157:H7 persistence and recovery.
- Post-harvest conditions remain critical for food safety. Time, storage conditions, substrate, and the surrounding microbial community can substantially influence pathogen populations, reinforcing the need for established sanitation and food-safety practices even when preharvest treatments improve microgreen quality.
Please see above.