An aroma compound described as catty on its own enhanced a fruit-like impression in coffee. A 2020 study compared ground Arabica and Robusta beans and used addition experiments to demonstrate the importance of combinations of aromas.
The researchers also investigated clues to how this compound forms during roasting. The study connects not only which molecules are present, but also how they are perceived among other aromas and how they arise.
Comparing the aromas of two ground coffees
The samples were Colombian Supremo Arabica and Vietnamese Robusta G1. The researchers selected beans processed by the same washed method and examined their aroma immediately after the roasted beans were ground.
Six researchers involved in coffee-flavor development at Ogawa & Company evaluated the aroma above 2.5 g of ground beans three times each. Rather than taste during drinking, they compared the intensities of the aroma notes Sour, Floral, Fruity, Green, Catty, Smoky, Nutty, and Roasty. Here, Sour refers to an aroma suggestive of sourness.
The Colombian sample was significantly stronger in Sour, Floral, and Fruity notes and significantly weaker in Smoky, Nutty, and Roasty notes than the Vietnamese sample. The researchers next sought candidate compounds behind these differences through chemical analysis.
Compounds more abundant in Arabica do not necessarily smell floral or fruity
Aromas collected from the air surrounding the ground beans were diluted in fourfold steps for aroma extract dilution analysis, or AEDA, in which people smell the separated compounds. Using the dilution steps at which at least two of three assessors could detect an odor, the researchers identified 15 odor events with high flavor dilution, or FD, values in at least one sample and identified 13 of the compounds involved.
The main candidates were common to both samples: their balance differed more than their presence or absence. Compounds more abundant in the Arabica sample included 3-mercapto-3-methylbutyl formate, or MMBF, and 2-isobutyl-3-methoxypyrazine, or IBMP.
Yet MMBF has a Catty aroma and IBMP a Green aroma. These descriptions do not directly match the stronger floral and fruity impressions of the Arabica sample. The authors proposed that, when mixed with other coffee compounds, these substances might play roles different from their individual aromas.
Adding MMBF increased fruitiness and reduced smokiness
The Colombian ground beans contained 200 ppb MMBF. The researchers therefore added either 200 or 400 ppb MMBF to the Vietnamese ground beans and compared their aroma with a control receiving the same amount of solvent alone.
At an addition of 200 ppb, no significant difference from the control was reported for any of the aroma notes tested. At 400 ppb, Sour and Fruity notes increased significantly, and the ratings shown in the figure also indicated a significant decrease in Smoky notes. No significant changes were found in Floral, Catty, or the other remaining notes.
In other words, under these conditions, adding MMBF did not simply add a catty smell. Within the coffee aroma, the change appeared as a stronger impression of sourness and fruitiness and a weaker smoky impression.
A green aroma also depends on its background
The researchers also conducted an addition experiment with IBMP. Its original concentration was 70 ppb in the Colombian sample and 6 ppb in the Vietnamese sample. The Vietnamese sample supplemented with 70 ppb was perceived as significantly greener than either the unmodified Colombian sample or the Colombian sample supplemented with another 70 ppb.
Even with more IBMP present, the Green note did not become stronger in the Colombian aroma. The authors suggested that interactions with other aroma compounds suppressed the green impression.
This experiment cannot identify which compounds interacted or at which stage of sensory processing. Nevertheless, the comparison shows that the quantity of a single compound is not enough to explain an aroma’s intensity or character.
Investigating the ingredients behind an aroma formed during roasting
Another aim was to explore why the Arabica sample contained more MMBF. Earlier research had proposed a pathway in which prenol gives rise to a sulfur-containing substance called MMB, which then reacts with formic acid to form MMBF.
The researchers therefore measured prenol, MMB, and MMBF in green beans and in beans at different roast levels. These compounds were hardly detectable in green beans but appeared after roasting. Prenol was more abundant in the Arabica sample at both roast levels examined; MMB and MMBF were more abundant in Arabica under the darker-roast condition.
Analysis of free sugars in the green beans further showed that most were sucrose, which was also more abundant in Arabica. Combined with earlier findings that heating sugars can produce formic acid, this led the authors to suggest that differences in the amounts of materials involved in formation could contribute to the difference in MMBF.
The study did not directly trace the entire process from hydrogen sulfide and formic acid formation through to MMBF, and the precursor of prenol remains unknown. The comparison also involved limited origins and samples; more beans would need to be examined before these ratios could be treated as representative of all Arabica and Robusta coffees.
Summary
In this comparison of ground beans, the Arabica sample had stronger fruity, floral, and sour-associated aromas and contained more MMBF, which smells catty on its own. Adding 400 ppb to the Robusta sample increased fruitiness and the Sour impression while reducing smokiness. The perception of IBMP’s green aroma also varied with the coffee background.
Differences in related compounds after roasting and in green-bean sugars also provided clues to how MMBF forms. Coffee’s distinctive character emerges not simply from the names of individual compounds, but from the conditions in which they form and their roles when mixed.
All five authors were listed as affiliated with Ogawa & Company at publication. No separate research-funding or conflict-of-interest declaration section could be found.
Reference
Baba, R., Yamato, R., Higuchi, K., Matsuda, T., & Kumazawa, K. (2020). Characterization of the Unique Aroma of Arabica Coffee and Its Formation. Nippon Shokuhin Kagaku Kogaku Kaishi, 67(4), 134–140. doi:10.3136/nskkk.67.134 (Full original paper)
