A recent study evaluated the effectiveness of post-harvest ultraviolet-C (UV-C) treatment for reducing key foodborne pathogens on microgreens, finding that while the intervention achieved measurable reductions, it did not completely eliminate microbial food safety risks.

Microgreens are considered particularly vulnerable to microbial contamination because pathogens can transfer from contaminated seeds, irrigation water, or soilless substrates directly to the edible portion of the plant. The authors noted that post-harvest sanitizer treatments are generally not preferred due to the fragility of microgreens, prompting investigation into alternative interventions such as UV-C.

Published in MDPI’s Foods, the study was conducted by researchers from Muş Alparslan University and Atatürk University in Türkiye, as well as Colorado State University.

Study Design

Researchers first inoculated nutrient-soaked agricultural perlite with Salmonella enterica, Shiga toxin-producing Escherichia coli O157:H7 (STEC), and Listeria monocytogenes at concentrations of 10⁵ to 10⁶ colony forming units per gram (CFU/g). Sunflower and radish microgreens were grown in the contaminated soilless substrate to evaluate pathogen transfer and subsequent inactivation.

Following harvest, microgreens were subjected to UV-C treatment applied either unidirectionally or bidirectionally at distances of 10, 20, and 30 centimeters (cm), and exposure times ranging between five and 120 seconds. UV-C doses ranged from 0.03 to 2.07 kilojoules per square meter (kJ/m²).

Pathogen Reduction and Treatment Efficacy

The study found that UV-C treatment reduced pathogen populations under most conditions. The highest level of microbial inhibition was achieved with bidirectional exposure at 10 cm for 120 seconds, resulting in reductions of up to 3.1 log CFU/g for S. enterica, 3.0 log CFU/g for STEC, and 2.0 log CFU/g for L. monocytogenes.

The authors reported a strong inverse relationship between UV-C application distance and efficacy, with significantly lower reductions observed as distance increased. Bidirectional treatment consistently provided greater reductions than unidirectional exposure, likely due to improved surface coverage and reduced shadowing effects.

Among the tested pathogens, Salmonella was identified as the most sensitive to UV-C irradiation, while L. monocytogenes exhibited the greatest resistance. The researchers attributed these differences, in part, to structural characteristics such as cell wall thickness and DNA repair capacity.

Influence of Microgreen Type and Surface Characteristics

Differences in treatment efficacy were also observed between microgreen types. Sunflower microgreens showed slightly greater pathogen reductions compared to radish microgreens under identical conditions. The authors suggested that broader and smoother cotyledons in sunflower microgreens may facilitate more uniform UV-C penetration, whereas the more complex surface structure of radish microgreens may shield microorganisms from exposure.

Regrowth During Refrigerated Storage

Despite initial reductions, the study found that UV-C treatment did not provide a lasting antimicrobial effect. During 14 days of refrigerated storage at 4 °C, pathogen populations increased by 0.3–1.7 log CFU/g.

The authors suggested that sublethal injury and mild tissue damage caused by UV-C treatment may release nutrients, creating a favorable environment for surviving microorganisms to recover and proliferate. Similar increases were observed for total mesophilic aerobic bacteria and yeast and mold populations, indicating that microbial growth can continue under refrigeration.

Implications for Food Safety

The findings demonstrate that UV-C treatment can significantly reduce pathogen populations on microgreens as a post-harvest intervention. However, the authors emphasized that UV-C alone cannot fully address microbial food safety concerns associated with these products.

They concluded that while UV-C may serve as a useful mitigation strategy in both commercial and small-scale production, it should be used in combination with other preventive measures, including the use of safe inputs and strict hygiene practices. Further research was recommended to evaluate combined preservation approaches that could improve long-term microbiological control on microgreens.