If a green-leaf-smelling compound is added to vanilla, will the result simply smell greener? Mixing odors can produce changes that are difficult to predict from each ingredient alone. A 2022 study examined how a green-smelling compound enhanced the rum-like impression of vanilla.
The researchers wanted to know how this change appeared in the responses of olfactory receptors, which receive odor molecules. Starting with expert smelling experiments, they used cultured cells expressing human receptors to examine both the strength and time course of responses to mixed odors.
Smelling components alone and against a vanilla background
First, the researchers separated an aroma concentrate obtained from Madagascar vanilla beans into individual components by gas chromatography. Two flavorists smelled the compounds as they emerged from the instrument and assessed their odor qualities.
Next, they smelled the components in the same way while continuously supplying vanilla bean aroma to the sniffing port. Called AROMATCH, this method examines not just a component’s isolated odor but also its effect when added to a background aroma.
Six components were found to enhance the rum-like impression. One was 3-octen-2-one: although described as green on its own, it strengthened the rum-like character of vanilla. Phenylethyl alcohol, which smells rose-like alone, also enhanced the rum-like impression in vanilla. An ingredient’s odor label does not by itself determine its role in a mixture.
3-Octen-2-one is also an aroma compound detected in shiitake mushrooms. Its chemical name may sound unfamiliar, but it occurs in familiar foods too.
In the vanilla study, combining this compound with vanilla aroma led two flavorists to perceive a stronger rum-like impression. A component judged green on its own took on a different role in combination. The intriguing point is that the impression of a single ingredient cannot fully predict the smell of a mixture.
Testing 388 receptors with an eight-component vanilla aroma
Because natural vanilla beans contain many components, the next experiments used a model vanilla aroma made from eight compounds, including vanillin and guaiacol. The flavorists judged that adding 3-octen-2-one to this model strengthened its rum-like character in a way similar to adding it to the natural bean aroma.
In the cell experiments, 388 types of human olfactory receptor were expressed in separate cultured cells. The researchers applied the model vanilla aroma, 3-octen-2-one alone, or their mixture and followed intracellular calcium changes as fluorescence for 350 seconds. Measurements were repeated five times. This was neither a smelling experiment with 388 people nor a preference test with five participants.
The method’s distinctive feature is its ability to follow responses over time as well as measure their magnitude. It allows comparison of which receptors respond, how strongly, and how their responses continue after an odor is applied.
Adding a compound strengthened some responses and eliminated another
Two receptors showed particularly striking changes. OR1D2 responded weakly to vanilla aroma and to 3-octen-2-one individually, but its response to the mixture exceeded the sum of those separate responses.
OR5K1, by contrast, responded to vanilla aroma but no longer showed a detectable response in the assay when 3-octen-2-one was mixed in. Rather than uniformly strengthening every receptor, mixing changed the response pattern. No receptor newly responsive only to the mixture was found.
The timing also changed. With vanilla aroma alone, three receptors continued responding until the end of measurement. With the mixture, eight receptors, excluding the suppressed OR5K1, still responded at the end. Mixing may therefore change not only whether a response occurs and how large it is, but also how it persists.
Did the study establish what causes the rum-like impression?
Chemical analysis of the mixed samples did not detect the formation of new odor molecules. The authors therefore focused on how mixing changes actions at the receptors. Possibilities include one molecule modifying receptor function to enhance responses to another component, or interfering with binding and weakening a response.
However, they did not directly establish which molecule bound at which site. They also proposed that multiple intracellular signaling pathways might explain responses with two peaks, but the mechanism remains unresolved.
Nor did they prove that enhancement of OR1D2 or suppression of OR5K1 directly causes the rum-like impression. Sensory assessment involved two experts, while the receptor experiments stimulated cultured cells. How people perceive mixed odors also involves subsequent neural and brain processing. Longer-lasting cell responses cannot be translated into a claim that perfume lasts longer on skin.
The authors included researchers affiliated with Soda Aromatic, Osaka University, and Komi Hakko. The paper states that it received no specific grant and that there were no conflicts of interest to declare.
Summary
The study showed that one compound that changes the impression of vanilla also changes the pattern and duration of receptor responses. Understanding odor mixtures requires not only knowing how each ingredient smells alone but also considering which responses are enhanced and which are suppressed when they come together.
The authors suggest that these time-dependent changes might also relate to the development and lingering character of perceived aromas. That correspondence remains hypothetical, but it points toward studying aspects of odor change that intensity alone cannot capture. A next step is to combine the eight vanilla components and the impression-modifying compounds individually to establish which molecular pairs cause enhancement or suppression at each receptor.
References
Yasunaga, M., Takai, E., Hattori, S., Tatematsu, K., & Kuroda, S.(2022). Effects of 3-octen-2-one on human olfactory receptor responses to vanilla flavor. Bioscience, Biotechnology, and Biochemistry, 86(11), 1562–1569. DOI: 10.1093/bbb/zbac147 / Full original article.
Table 3 of receptor responses incorporates the correction published in November 2022.
© The Author(s) 2022. Published by Oxford University Press on behalf of Japan Society for Bioscience, Biotechnology, and Agrochemistry. Original article reuse conditions.
On the shiitake compound discussed in the sidebar: Li, W., Chen, W.-C., Wang, J.-B., et al. (2021). Effects of enzymatic reaction on the generation of key aroma volatiles in shiitake mushroom at different cultivation substrates. Food Science & Nutrition, 9(4), 2247–2256. DOI: 10.1002/fsn3.2198 / Full original article.
