Characterization of key aroma compounds in microgreens and mature plants of hydroponic fennel (Foeniculum vulgare Mill.)

Authors: LIU, JINGSI - Virginia Tech; LI, SONG - Virginia Tech; O'KEEFE, SEAN - Virginia Tech; HURLEY, KEN - Virginia Tech; RUTTO, LABAN - Virginia Tech; Eriksen, Renee; YIN, YUN - Virginia Tech

Submitted to: Food Research International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2024
Publication Date: 11/7/2024
Citation: Liu, J., Li, S., O'Keefe, S., Hurley, K., Rutto, L., Eriksen, R.L., Yin, Y. 2024. Characterization of key aroma compounds in microgreens and mature plants of hydroponic fennel (Foeniculum vulgare Mill.). Food Research International. 197. Article 115229. https://doi.org/10.1016/j.foodres.2024.115229.
DOI: https://doi.org/10.1016/j.foodres.2024.115229

Interpretive Summary: Fennel is a popular culinary herb with unique aromas and flavors. It can be harvested approximately 3 weeks post germination when it has only one leaf for microgreens, or it can be harvested between 8-10 weeks post germination for mature leaf fennel. This study identified and compared aroma compounds in microgreen and mature leaf fennel grown hydroponically. Using a method that incorporates standard gas chromatography-mass spectrometry techniques with specially trained volunteers to describe scents, this study identified 32 aroma-active compounds in fennel microgreens and 28 aroma-active compounds in mature fennel leaves. Statistical analysis showed a clear distinction in aroma contents through different growth stages. Fennel microgreens contained the highest total concentrations of aroma chemicals and nutritional compounds that have benefits for human health. This study provides baseline information for understanding the aroma and nutritional evolution in fennel under hydroponic production from microgreen to mature plants.

Technical Abstract: Fennel is a popular culinary herb known for its unique flavor. It can be harvested approximately 3 weeks post germination when it has only one true-leaf for microgreens, or it can be harvested between 8-10 weeks post germination for mature leaf fennel. In this study, we identified key aroma-active compounds in fresh fennel (Foeniculum vulgare Mill.) at 3 weeks or microgreen stage, as well as at 8, 9, and 10 weeks post germination or harvest maturity stage.. Fennel was cultivated hydroponically with soilless substrates. Upon harvesting, samples were ground with liquid nitrogen, and headspace solid phase microextraction (SPME) gas chromatography-mass spectrometry-olfactometry (GC-MS-O) was used for volatile analysis. Thirty-two and 28 key aroma-active compounds were identified in fennel microgreens and mature leaves, respectively. Phenylpropenes, especially (E)-anethole, were the predominant aroma-active compound in all samples. Quantification results showed that fennel microgreens contained a significantly higher amount of monoterpenes. Principal component analysis (PCA) of identified volatiles indicated a distinctive difference in the overall aroma profile between microgreens and mature leaf. The changes in aroma contents over different growth stages revealed the underlying volatile biosynthesis discrepancy. This study provided baseline information for understanding the aroma evolution from microgreen to mature fennel herbs.