The same aroma compound can be described differently when added to water and when added to wine. A 1999 study added two compounds associated with Sauvignon blanc aroma to water and two types of white wine, examining how concentration and combinations changed the impression.
One compound was described as grassy or green-pepper-like, and the other as guava, sulfur-like, or cat urine. These descriptions were not fixed labels attached one-to-one to a substance: they changed with the medium, the amount added, and the presence of the other compound.
- Can two key compounds explain a wine’s characteristic aroma?
- Adding tiny quantities and describing the aroma with eight assessors
- The pyrazine was dusty in water, but grassy or green-pepper-like in wine
- MMP’s guava and cat-urine notes also depended on the medium
- Combining the compounds did more than add their separate descriptions
- Some characteristics could not be reproduced with two compounds
- Summary
Can two key compounds explain a wine’s characteristic aroma?
The compounds examined were 2-methoxy-3-isobutylpyrazine and 4-mercapto-4-methylpentan-2-one, abbreviated in the original paper as ibMP and MMP. MMP, also called 4MMP, is a sulfur-containing aroma compound.
The researchers aimed to investigate how Sauvignon blanc aroma changed when these compounds were added separately or together. Rather than simply smelling each compound in isolation, they wanted to examine its effects against the background of other wine constituents.
They used three sample bases: deionized water, a neutral non-Sauvignon blanc white wine, and a neutral Sauvignon blanc. Both wines were locally produced from the 1998 vintage and were described mainly as fruity before additions. Neutral did not mean odorless.
Adding tiny quantities and describing the aroma with eight assessors
ibMP was added at increasing concentrations from 2 to 30 ng/L, and MMP from 0.2 to 20 ng/L. A nanogram is one-billionth of a gram. The amounts, selected from earlier research, were within ranges that can occur in wine. In addition to samples with either compound alone, combinations with different ratios of the two were prepared. Samples were stored under nitrogen in darkness at 18°C for one day before evaluation.
The eight assessors had experience in wine sensory evaluation. Each wrote descriptions on a blank form, then discussed them to reach shared descriptions. Familiar terms such as grassy, green pepper, guava, and cat urine were introduced beforehand, and foliage from a conifer whose odor can resemble cat urine was also demonstrated.
Samples were presented in ascending order of added concentration. The paper does not report blinded or randomized comparisons, or statistical tests of the frequency of descriptions. The following results should therefore be read as descriptions agreed upon by this panel rather than scores on a trained rating scale.
The pyrazine was dusty in water, but grassy or green-pepper-like in wine
When 2 ng/L of ibMP was added to water, it was described as dusty; at 4 ng/L, a grassy impression joined it. With further increases, descriptions included green beans, herbaceous notes, and green pepper.
When the same 2 ng/L was added to the two white wines, only the original fruity wine aroma was described. Grass and green-pepper characteristics appeared as additions increased. In Sauvignon blanc, 4–8 ng/L was described as grassy and fruity, whereas 16 ng/L and above was described as green pepper and grassy.
The dusty impression found only in water at low additions was not described in wine. This provides a concrete example of how a different background mixture can change which characteristics of the same molecule stand out.
MMP’s guava and cat-urine notes also depended on the medium
Adding 1 ng/L of MMP to water produced descriptions of guava, cat urine, and sulfur-like odor. At 5 ng/L and above, cat urine and conifer-like descriptions persisted. In the non-Sauvignon blanc white wine, by contrast, only the original wine and fruity aroma was described up to 1 ng/L, with guava, fruitiness, and cat urine appearing at 5 ng/L.
However, MMP added alone to Sauvignon blanc was described mainly as sulfur-like and fruity. Conifer-like notes appeared at higher additions, but this differed from the guava and cat-urine descriptions recorded for water and the other white wine in the same table.
Summarizing the study as showing that low MMP always means guava and high MMP always means cat urine would therefore miss an important point. Alongside concentration, the composition of the base liquid influenced perception.
Combining the compounds did more than add their separate descriptions
Samples containing both compounds produced still other descriptions depending on their combination. For example, Sauvignon blanc with 16 ng/L ibMP and 10 ng/L MMP was described as green pepper, grassy, and fruity. The sulfur-like description recorded for the same amount of MMP added alone was not listed for this combination.
In the non-Sauvignon blanc white wine, the same combination was described as grassy, green pepper, and guava. Even with identical additions of the two compounds, changing the wine did not yield identical aroma descriptions.
The authors considered MMP combined with ibMP to contribute to wine complexity and discussed possible synergistic interactions between compounds. Here, synergy was an interpretation of differences in the observed descriptions, not the result of a formal test showing that combined intensity exceeded a prediction based on individual intensities.
Some characteristics could not be reproduced with two compounds
Descriptions associated with Sauvignon blanc, including asparagus, gooseberry, tomato leaf, passion fruit, and fig, were not obtained for these samples. The authors called for further investigation of their relationships with other pyrazines and sulfur-containing compounds.
They also acknowledged that even the neutral Sauvignon blanc might already have contained trace amounts of ibMP. The table reports added concentrations, not guaranteed total concentrations in the wine. These observations came from two wines and a small consensus panel, so they should not be used as a formulation chart for predicting the flavor of all wines.
Summary
ibMP and MMP were associated with characteristics such as grass and green pepper, or guava and sulfur-like notes. Their descriptions changed not only with concentration, but with water versus wine, the wine used, and the presence of the other compound. In Sauvignon blanc, for example, sulfur-like descriptions for MMP alone sometimes changed to descriptions including grass and fruit when ibMP was added.
Meanwhile, some characteristic varietal notes remained absent with these two compounds alone. The study shows why understanding an aroma compound means looking beyond a molecule’s name and a single odor label to its concentration and the other compounds mixed with it.
At the time of the study, both authors were affiliated with South Africa’s ARC-Fruit, Vine and Wine Research Institute. No separate funding or conflict-of-interest statement was found in the paper.
Reference
Marais, J., & Swart, E. (1999). Sensory Impact of 2-Methoxy-3-Isobutylpyrazine and 4-Mercapto-4-Methylpentan-2-One Added to a Neutral Sauvignon blanc Wine. South African Journal of Enology and Viticulture, 20(2), 77–79. Original paper
