You are smelling grapes, yet the aroma brings lychee to mind. Can that impression be explained by a single lychee-like compound?
A 2024 study of the white grape variety Muscaris showed that a combination of rose oxide and geraniol contributes to its lychee-like aroma. The clue came from reconstructing the aroma from its components and then leaving some out. The discovery lay in the fact that removing one lychee-like compound did not weaken the aroma as much as expected.
Comparing a new grape’s aroma with its parent variety
Muscaris was bred to combine resistance to fungal diseases with a rich aroma. Its maternal parent is Solaris and its paternal parent is Gelber Muskateller, a white Muscat grape known for floral and fruity aromas.
The researchers had three aims: to identify compounds contributing to Muscaris aroma, examine similarities and differences from its paternal variety Muskateller, and determine what produces its distinctive lychee-like note. The work also provides a foundation for breeding disease-resistant varieties and studying how fruit aromas transfer into wine.
The samples were grapes of both varieties harvested in September 2022 at a research institution in Freiburg, Germany. Both had a clear lychee-like aroma. Rather than investigating an aroma that had suddenly appeared only in Muscaris, the study examined a trait connected to its parent variety.
Separating compounds by instrument and finding odorants by nose
First, volatile compounds were extracted from the fruit and separated by gas chromatography. People sniffed the compounds as they emerged, helping identify the fractions that actually had an odor. The extracts were also progressively diluted to determine how far each odor remained detectable.
This revealed 39 odorants in Muscaris and 35 in Muskateller. Thirty-five were shared by both, indicating broadly similar compositions. Of the 39 Muscaris odorants, 38 were structurally identified.
Geraniol, which smells floral and rose-like, remained detectable at high dilutions in both varieties. Other compounds included grassy hexanal, citrus- and flower-like linalool, and cucumber-like odorants. This does not mean that the whole fruit smells simply of cucumber or grass. Rather, separating grape aroma reveals elements that can be described in those terms.
Quantifying 20 potentially important compounds showed that 16 exceeded their odor detection thresholds in water. Dividing a compound’s concentration by this threshold gives its odor activity value, or OAV. This helps identify not only abundant compounds but also those present in small amounts to which the nose is particularly sensitive.
Geraniol concentrations were approximately 1,160 µg/kg in Muscaris and 442 µg/kg in Muskateller. In contrast, the predominant linalool enantiomer was more abundant in Muskateller. Even within similar aromas, the proportions differed.
Reconstructing grape aroma with 16 compounds
Finding the compounds alone does not establish whether they explain the fruit’s aroma. The researchers therefore mixed 16 compounds at their measured concentrations in the fruit, using an aqueous solution containing tartaric acid and adjusted to the fruit’s pH.
Fourteen trained assessors rated nine odor attributes, including grassy, floral, lychee-like, and green apple-like, on a scale from 0 to 3. The models for both varieties had a harmonious combination of grassy, floral, lychee-like, and green apple-like notes, including the lychee character of particular interest in Muscaris.
Sample storage conditions matter here. Because identifying the compounds took time, much of the quantification used frozen and thawed grapes. Some readily changing green-leaf compounds were also measured in fresh fruit. Since fresh grapes and the models could not be compared directly at the same time, the researchers separately prepared models matching the concentrations in frozen and thawed grapes and confirmed a good correspondence with those grapes’ aromas.
Removing one compound versus removing two
Rose oxide was the leading candidate for the lychee note. The study focused on its (2S,4R) form, one of several stereoisomers, which has a floral, lychee-like odor. The researchers distinguished not just the molecule’s name but its three-dimensional form.
However, removing this rose oxide alone from the 16-compound model did not significantly lower lychee-like ratings. When rose-like geraniol was removed as well, the ratings fell. On the 0–3 scale, the decrease was 0.6 for Muscaris and 0.7 for Muskateller, both statistically significant.
Removing geraniol alone also produced no significant decrease. In these mixtures, the lychee character was relatively well maintained when either compound was removed separately, but weakened when both were omitted.
Geraniol on its own is usually described as floral or rose-like rather than lychee-like. Yet within the mixture it contributed to lychee character. A compound’s role in a mixture cannot be inferred solely from the odor label applied to it in isolation.
A role within a combination, not an aroma made from just two compounds
These results identify rose oxide and geraniol as an important combination contributing to grape lychee aroma. However, the reconstitution model contained 14 other compounds. The study did not test whether mixing only these two could reproduce the entire fruit aroma.
Nor does the absence of a significant difference after removing one compound prove it is unnecessary. It means that no clear decrease was detected with this panel size, these concentrations, and these mixtures. The study also did not measure how interactions occur at olfactory receptors.
The fruit came from one location in one year. Transfer into wine and which aromas people prefer remain questions for future work. Even so, the study’s value lies in connecting a bred variety’s aroma to its parent through chemical analysis, reconstruction, and compound omission.
Summary
Muscaris and its parent Muskateller shared many aroma compounds but differed in concentrations of geraniol and other substances. Removing both rose oxide and geraniol from a 16-compound model weakened the lychee-like aroma of both varieties.
The lychee character of grapes cannot be captured by the name of a single compound. A substance that smells rose-like on its own can help support fruity character when mixed with other compounds. This study tested that role by actually rebuilding the aroma.
One author received support from the China Scholarship Council (CSC). The authors declared no competing financial interest.
Reference
Wang X, Frank S, Steinhaus M. (2024). Molecular Background of the Lychee Aroma of Vitis vinifera L. ‘Muscaris’. Journal of Agricultural and Food Chemistry, 72, 1674–1682. Original paper / Open full text
