Adding an aromatic oil to water can change its character rather than simply dilute it. A 2026 study examined how adding coffee oil to water shifted its aroma from a sweet, toasty impression toward a more prominent roasted note.
Besides analyzing the compounds, the researchers removed selected components and smelled what remained. This showed that 1-furfurylpyrrole and 4-vinylguaiacol contributed to the roasted aroma.
Oil pressed from beans, not brewed coffee
The samples were made from roasted Brazilian coffee beans. Oil was extracted by pressing and then centrifugally filtered. The aim was to establish how the oil’s aroma changed when added to water and investigate which compounds were involved.
The researchers compared oil alone with oil added to pure water and thoroughly stirred. During analysis, the oil in the water-containing sample floated on top. This differs in both sample and conditions from adding water to an ordinary cup of brewed coffee and comparing the taste.
Aroma was collected from the headspace above samples at 40°C and analyzed by gas chromatography–mass spectrometry. Three experimenters each carried out sensory evaluations three times for each sample.
Water changed the composition of the released aroma
The oil alone smelled sweet and toasty, whereas the sample with water had a roasted aroma. Analysis detected fewer components in the water-containing sample, with particularly smaller signals for earlier-eluting compounds such as furfural.
Peak heights cannot be interpreted directly as concentration ratios between the conditions. Because the water-containing sample gave a smaller signal, the oil-only condition used 1 µL of oil and collected 1 L of gas, whereas the water-containing condition used 2 µL of oil plus 10 mL of water and collected a total of 5 L of gas. Both the amount of oil and the collection volume differed.
Even with the larger collection volume, the earlier-eluting compounds were less prominent in the water-containing sample, confirming a change in the composition of the volatile aroma. What matters is not only the total amount of aroma but which components become relatively more prominent.
Removing components to test their contribution to the roasted note
The largest instrumental peaks are not necessarily the most important contributors to perceived aroma. The researchers therefore used omission analysis: creating aromas without selected components and comparing them.
They instrumentally separated the aroma released by the water-containing sample into four fractions: low-boiling components, a fraction containing pyridines and pyrazines, a fraction containing furans, and a fraction dominated by high-boiling components. They omitted each fraction in turn, collected the remainder in a bag, and evaluated its aroma.
Removing the predominantly high-boiling fraction weakened the roasted impression and restored sweet, toasty and nutty notes closer to those of the original coffee oil. Removing the other fractions left the roasted impression intact, highlighting the importance of this high-boiling fraction.
Closer examination showed that removing 1-furfurylpyrrole and 4-vinylguaiacol weakened the roasted aroma and shifted it toward a sweet, toasty character. However, removing the entire high-boiling fraction weakened the roasted note further. The two compounds therefore do not explain the whole effect; other components appear to contribute as well.
Compounds that enter the water and those that remain prominent in the aroma
The authors propose that hydrophilic aroma compounds move into the water and become less readily released into the air, allowing the roasted impression of the remaining compounds to emerge more clearly. Adding water does not roast the oil again; rather, it changes how existing compounds are released and combined.
Details of the mechanism still require confirmation. The discussion states both that volatilization of the two compounds increased and that hydrophobic compounds, including these two, showed no major change in volatilization. The data supporting the latter statement are not presented. Because sample amounts and collection volumes also differed, the study cannot establish how much the release of these two compounds increased.
The clearer findings are that adding water changed the aroma character and that removing the two compounds or the high-boiling fraction weakened the roasted note. Evaluation involved three experimenters, and no statistical tests of sensory differences are reported. Consumer preference and the enjoyment of a beverage remain separate questions for future work.
Summary
Adding coffee oil to water shifted its aroma from a sweet, toasty impression toward a composition in which roasted notes stood out. Omission experiments indicated that 1-furfurylpyrrole, 4-vinylguaiacol, and other compounds in the high-boiling fraction contributed to this impression.
Adding an oil to water does not necessarily weaken every part of its aroma equally. Because compounds partition differently between oil and water, the features that stand out in the overall aroma can change. This study examined that idea using coffee oil.
At the time of the study, two authors were affiliated with Suntory Global Innovation Center and one with the Institute of Aroma Science. The acknowledgments credit Shimadzu for assistance with operating analytical equipment. No separate funding section was found, and the authors declared no matters relating to conflicts of interest to disclose.
References
Tsuchiya T, Ishii H, Kita J. Analysis of Volatile Aroma Compounds Released upon Addition of Coffee Oil to Water. Journal of Japan Association on Odor Environment. 2026;57(2):133–136. doi:10.2171/jao.2025.15
