Why Does Houttuynia cordata Smell Like Fish?

Damage a leaf of Houttuynia cordata, known in Japan as dokudami, and a distinctive odor spreads into the air. A study using aroma reconstitution and component-omission tests has confirmed that a compound called 3-oxododecanal plays an important role in its fish-like character.

This 2025 paper is a follow-up that goes beyond identifying compounds to ask which ones truly contribute to the plant’s characteristic aroma. Comparing fresh rhizomes and leaves, the researchers examined concentrations, the levels people can detect, and the odors produced when the compounds are mixed.

From finding candidates to testing their contributions

In earlier work, the team extracted aroma compounds from Houttuynia cordata rhizomes and leaves and looked for those that remained detectable after dilution. One promising candidate was 3-oxododecanal, with an odor reminiscent of metal, soap, and fish.

The present study quantified 25 major compounds, including that candidate. The researchers used fresh rhizomes and leaves harvested from potted plants and measured concentrations using methods including stable-isotope-based quantitation. Some compounds present at high concentrations were measured by a different analytical method, and the team checked that extraction was complete.

For several compounds, trained assessors also determined the threshold: the lowest concentration at which an odor could be detected in water. Dividing a compound’s concentration by its threshold gives its odor activity value, or OAV. A higher value makes the compound a more important candidate contributor to aroma. That number alone, however, does not determine the strength of a mixture’s odor or how characteristic it is of the plant.

The fish-like compound did not have the highest OAV

The concentration of 3-oxododecanal was 2.48 g per kilogram of fresh rhizomes and 1.34 g per kilogram of fresh leaves. Its OAVs were 27,000 and 15,000, respectively.

Myrcene, whose odor resembles geranium leaves, had even higher OAVs: 93,000 in rhizomes and 120,000 in leaves. Although 3-oxododecanal was more abundant, myrcene can be detected at a lower concentration, giving it a higher OAV.

Even so, the authors considered 3-oxododecanal a likely key to the plant’s characteristic aroma because it matched the fish-like note. Testing that distinction required moving beyond the numbers for individual compounds to the actual odor of their mixtures.

Rhizomes and leaves differ in molecular handedness as well as quantity

The researchers also examined differences between plant parts. For example, rhizomes contained more limonene than leaves, whereas leaves contained more (3Z)-hexenal, one of the compounds with a green, grassy odor.

Some compounds also occur in mirror-image forms, like left and right hands. The team analyzed the proportions of these enantiomers. For α-pinene, one form accounted for 94% in rhizomes, while leaves had an almost even split of 46% to 54%.

The two forms of α-pinene have very different odor thresholds. The researchers therefore calculated their contributions separately, considering not just the total amount of α-pinene but how much of each form was present. Molecular shape is another piece of information needed to understand the aromas of different plant parts.

Mixing the compounds recreated the character of the fresh plant

The team mixed compounds with OAVs of at least 1 at their measured concentrations, creating reconstitution models with 23 components for rhizomes and 22 for leaves. The base contained water, acid, small amounts of fat, and other ingredients, with acidity adjusted to match the respective plant samples.

When trained assessors directly compared the aromas of the fresh plant material and the models, the patterns of odor-note intensities closely matched. The models also reproduced the difference between plant parts, with a stronger green, grassy impression in leaves than in rhizomes. This supports the analyzed compounds and concentrations as a useful combination for explaining the original aromas.

Next, the researchers progressively omitted compounds with lower OAVs. In the discrimination test, there was no significant difference between models retaining only five major compounds, different for each plant part, and the complete models containing 23 or 22 compounds. A small number of compounds may therefore capture the main characteristics, but this does not prove that the five-component and complete models were identical.

Leaving out the fish-like compound made a detectable difference

The decisive step was to remove one more compound at a time from those five-component models and compare the resulting four-component models with the complete models. Assessors selected the different sample from a set of three to test the effect of each omission.

When 3-oxododecanal was absent, 16 of 19 assessors correctly detected the difference for rhizomes, as did 18 of 20 for leaves. Both results were significantly above chance, supporting an important contribution from this compound to the characteristic aroma.

For the leaf model, omitting myrcene also produced a significant result, with 14 of 20 assessors detecting the difference. Omitting myrcene from the rhizome model, by contrast, produced no significant difference. Thus, while the central contributor to the fish-like character was shared by both plant parts, myrcene was also important to the overall leaf aroma. OAV rankings do not necessarily match a compound’s role in a characteristic odor.

Does damage create the compound, or release it?

Another question was whether 3-oxododecanal is newly formed through enzymatic reactions after damage or is already present in the tissue.

This experiment used rhizomes. The researchers compared immediate extraction of rhizomes harvested without cutting, under conditions that suppress enzyme activity, with the same treatment after cutting rhizomes into thin strips within five minutes. The 3-oxododecanal concentration was not higher in the cut samples, and the difference fell within the variability of the measurements. Terpenes measured at the same time for comparison showed a similar pattern.

Under the short-term conditions tested in these rhizomes, the results support release of a pre-existing compound rather than substantial synthesis after injury. The same experiment was not performed on damaged leaves or on reactions over longer periods, so those possibilities need to be distinguished.

Summary

3-Oxododecanal was important to the characteristic fish-like aroma of Houttuynia cordata rhizomes and leaves. The researchers established this step by step: measuring concentrations and thresholds, recreating the aroma, and showing that omitting this compound produced a detectable difference. Myrcene was also important in leaves, and the study identified differences between plant parts in both concentrations and enantiomeric proportions.

The rhizome-damage experiment also supported the explanation that the fish-like compound is already present and is released by mechanical disturbance. Understanding an aroma requires more than finding the most abundant compound or the highest OAV. It also means using the human nose to test which compounds carry the characteristic odor within a mixture.

The paper reports that author Zhenli Xu received support from the China Scholarship Council (CSC). The authors declared no conflicts of interest.

Reference

Xu, Z., Oellig, C., Vetter, W., Steinhaus, M., & Frank, S. (2025). Key Odorant Identification Confirms 3-Oxododecanal as the Most Important Contributor to the Characteristic Aroma of Fresh Rhizomes and Leaves of Houttuynia cordata. Foods, 14(18), 3147. Original research article