Can a faint aroma change perceived sweetness even when the amount of sugar stays the same? In a 2007 study, adding ethyl butyrate at concentrations set below the detection threshold to sugar water raised sweetness ratings compared with sugar water without the odorant. Participants did not report noticing an aroma during the evaluation.
However, the other odorant tested, maltol, did not produce the same consistent sweetness enhancement. This small experiment distinguished between clearly perceptible aroma and concentrations that were difficult to detect, and examined differences between odorants.
First, selecting odorants likely to change sweetness
The aim was not simply to confirm that sweet-smelling aromas enhance sweetness. It was to find out whether an odorant could change the sweetness of sugar water at a concentration lower than could be detected. As a preparatory step, the researchers compared six odorants at perceptible concentrations.
The first experiment involved nine people aged 18–25 with no sensory-evaluation experience. Benzaldehyde, ethyl butyrate, furaneol, vanillin, maltol, and isoamyl acetate were each added at three concentrations to water containing 5 g/L sugar. Including sugar water without odorants, participants ranked the odor smelled from the cup, aroma perceived in the mouth, and sweetness.
All six odorants changed odor and aroma intensity, but their effects on sweetness differed. Fruity-smelling ethyl butyrate enhanced sweetness most consistently. Furaneol, isoamyl acetate, and benzaldehyde also showed sweetness-enhancing effects, whereas the enhancement from vanillin and maltol was not significant.
The researchers therefore selected ethyl butyrate, which had a large effect on sweetness, and maltol, which had a small effect, for the next experiment. The design did not treat odorants as interchangeable merely because they smelled sweet.
Diluting below the threshold of the most sensitive participant
The second experiment involved a different group of nine people aged 18–25. This avoided the possibility that experiencing odorants together with sugar in the first experiment would influence later perception.
First, each person’s detection threshold was determined by increasing the concentration in a task that asked them to select the different sample from two water-only samples and one odorant-containing sample. The lowest threshold among the nine people was then used as the reference, and diluted further to 1/16, 1/32, 1/64, 1/128, and 1/256 for the five main-test concentrations. Concentrations were not adjusted to the same proportion of each individual’s threshold: everyone evaluated the same concentrations.
In the sweetness test, liquid containing a fixed sugar concentration of 15 g/L was continuously delivered to the mouth through a thin tube. The flow switched between liquid with and without odorant without stopping, reducing cues from changing cups and evaporation of the aroma. Each condition lasted 18 seconds, and participants rated sweetness on an 11-point scale. Changes were calculated by subtracting the rating of the immediately preceding odorant-free solution from the odorant-containing solution’s rating. Each odorant test was performed twice.
Ethyl butyrate increased sweetness at all five concentrations
At all five low concentrations tested, ethyl butyrate significantly increased sweetness ratings relative to the immediately preceding sugar water. However, there was no proportional relationship in which higher concentrations produced greater sweetness. This differed from the findings at clearly perceptible concentrations.
Maltol enhanced sweetness at the lowest concentration but reduced it at the highest. The three intermediate concentrations produced no significant change. Although the paper’s abstract summarizes maltol as not enhancing sweetness, the detailed results include changes in opposite directions depending on concentration.
Individual differences were also present. Seven of the nine participants more often reported greater sweetness enhancement with ethyl butyrate than with maltol. Of the remaining two, one responded strongly to both odorants and the other responded weakly to both. The authors suggest that threshold differences alone are unlikely to explain these patterns.
Finally, when sugar water without odorants was continuously delivered through the same apparatus, sweetness did not change significantly. This did not support the explanation that the continuous-flow tasting procedure alone caused the same enhancement.
How was the absence of noticeable aroma checked?
Participants were asked to record sensations other than sweetness, but none reported perceiving aroma during the main test. The authors also report that a supplementary odor-discrimination test involving 24 people met their specified similarity criterion.
However, the original paper contains inconsistent concentration values. The values described as the highest subthreshold concentrations in the supplementary test do not match those calculated from the main experiment’s thresholds and dilution factors. For maltol, the dilution scheme in the methods also differs from the concentration range in the figure. These discrepancies warrant caution about reproducing the exact concentrations and about how far the supplementary test verifies the main experiment. The appropriate interpretation is that sweetness ratings changed without participants reporting aroma awareness under conditions the authors had set as subthreshold.
Associations with familiar food aromas may play a role
The authors suggest that experiencing ethyl butyrate’s fruity aroma together with sweetness in everyday eating may contribute to the perceptual change. They also discuss the possibility that maltol and vanillin engage associations with sweetness when combined with the textures and aromas of more complex foods.
The study did not test this mechanism by manipulating food experience. Each experiment involved only nine people, and to avoid mixing odorants in the apparatus, ethyl butyrate was evaluated before maltol; the order of concentrations was also fixed. The authors themselves identify randomized presentation order and concentrations tailored to each individual’s threshold as improvements for future work.
Summary
In this experiment, ethyl butyrate at concentrations set below the detection threshold raised sweetness ratings for sugar water without changing its sugar content. Enhancement appeared at all five concentrations, but did not increase proportionally with concentration. Maltol produced a mixture of enhancement, reduction, and no change.
The lesson is that sweetness is not determined by sugar alone: it can vary with odorant type even when people do not report clearly perceiving an aroma. How much sugar could actually be removed from real foods remains a question for future research.
At the time of the research, all authors were affiliated with the Nestlé Research Center. No separate research-funding section or conflict-of-interest statement was found in the original paper.
Reference
Labbe D, Rytz A, Morgenegg C, Ali S, Martin N. Subthreshold Olfactory Stimulation Can Enhance Sweetness. Chemical Senses. 2007;32(3):205–214. doi:10.1093/chemse/bjl040
