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The Art Behind the Science: Deconstructing the aroma of coffee

Scientists are trained to communicate in an objective way; factual, disciplined and void of emotion, because we are collaborating with our peers throughout history, on an unfinished novel where every research project and journal article aims to add a single sentence to the chapter. To live in these storylines scientists draw heavily on their imaginations, often coming across as eccentric. It is in this informal creative process where science parallels art.

Written by Anja Rahn
Posted in Research on March 15, 2022

The storyline behind coffee aroma has been a narrative that I have been reflecting on for quite some time. Coffee aroma is disputably the most important component within coffee. In 1969 Mozell [1] demonstrated that without the volatile component, relying solely on taste, coffee could not be identified, emphasising the importance of coffee volatiles. Understanding coffee’s iconic quality therefore hinges on understanding coffee aroma and the underlying chemistry. Aroma chemistry is complex, but it is no more complex than a musical composition or art hanging in a gallery. This article aims to reveal the elegance of coffee’s complex aroma chemistry by drawing parallels with the broader art community and aims to be part of a broader group of articles deconstructing topics around coffee’s aroma science.

Work of Art

Unlike art hanging in a gallery, where everyone can see the painting, or music where the composition can be heard, recorded and transcribed, methods had to be developed to observe and appreciate aroma. Initially these methods- gas chromatography (GC) coupled to a mass spectrometer (MS) or flame ionized detector (FID)- could detect volatiles above a coffee product but not discriminate between their presence versus their sensory relevance. This means that while there are thousands of compounds that have been detected in coffee products only a few contribute to the aroma [2,3]. This is like knowing that a piece of music is composed of notes or that painting involves colour, but not knowing which ones. To differentiate between compounds that are present in coffee versus those that are perceivable and odour active, GC-Olfactometry (O) was developed, where the detector is a person’s nose. This methodology trimmed the extensive list of compounds [4-8] revealing simultaneously that there are compounds perceptible in coffee that could not be detected. This reduces the possible notes, or colour palette, to those used in the composition.

Scientific instruments, like human senses, have ranges within which they work well. These boundaries are empirically characterised in scientific instruments using terms, e.g. Limit of Detection (LOD) and Limit of Quantitation (LOQ), and in human senses as ranges, e.g. humans auditory (20 Hz – 20 kHz) and visual (380-720 nm) acuity. Smell’s range remains undefined as the biological mechanism continues to be unknown, hampering the development of artificial sensors as well as standardising the assessment in humans.

Even though we can see a wide range of colours and hear many sounds in a work of art or music, it is really only a selection of these that are needed to identify an object, e.g. a tree, in a painting, or a melody in a piece of music, like Beethoven’s fifth symphony. This is also the case in coffee aroma: there are many odour-active compounds in coffee, but only a handful are needed for us to recognize something as coffee. To identify which aromas are required for coffee’s iconic aroma profile, omission studies are carried out. These studies recreate the coffee aroma with all odour-active compounds (the control) and then systematically leave groups of compounds out, observing whether these new mixtures are different from the control. This is comparable to using visual or acoustic software to remove colours or notes from a piece and seeing whether it remains recognizable.

It is insufficient to just identify the core components of a melody, painting or aroma, as many works of art contain the same notes, colours or compounds. Odour-active compounds found in coffee are also in products with completely different aroma profiles, e.g. nuts and beef. What differentiates one distinct piece of art from another is how much of each component is used, and their arrangement. For example one could paint a brilliant sunny day with the same colour palette used to paint a dark room illuminated by a candle. Any great work of art is then reduced to, pressing the right keys at the right time, to paraphrase an adage from Johann Sebastian Bach.

Artists

Bach’s phrase not only emphasises the notes and their arrangement but also conveys the importance of the interpretation. This interpretation is introduced by the musician or artist and ranges from changing nuances within the piece to changing the medium entirely, e.g. digital to orchestra or oil painting to mosaic.

In coffee the most frequently encountered interpreters are roasters. Roasters are really like conductors of an orchestra where each instrumental section is a reaction pathway. It’s the job of the conductor not only to present the piece of music but to ensure that there is harmony when two sections play at once. Consider flautists playing with the percussion section. The role of the conductor is to ensure that there is harmony between sections and that the drums do not overpower the flautists’ contribution. This is not unlike roasting where balance is sought between the delicate aromas generated by the Maillard reaction and advanced reactions that contribute to the roasty aromas and brew body.

In order for roasters to determine if the aroma profile is as desired, they must be capable of assessing coffee quality. Sensory quality of coffee is initially determined by green coffee traders. Trading houses are essentially auction houses and have the responsibility of determining coffee quality to establish a fair market price. Buying expensive beans therefore is not unlike hiring an expensive musician or artist where the expectation is assured quality. Conversely, a lower grade of bean does not necessarily equate to a lower quality product: there are roasters who leverage the properties present, delivering a higher quality product. Leveraging a perceived lower grade of bean remains possible, as quality is subjective and partially evaluated post-roasting, thereby containing an inherent bias of the trading house roaster. This point remains controversial as the price of green coffee may be seen as indicative of farmer wages, which is an issue worth solving. Furthermore, it should also be mentioned that roasting defective beans, e.g. sours or underripe, is akin to conducting an orchestra of musicians with broken instruments, inadvisable if the aim is to deliver a harmonious product. While the inherent bias of the roaster is currently unavoidable, efforts by the Coffee Quality Institute (CQI) to unify vocabulary, standards and protocols through their Arabica(Q) and Robusta(R) Grading certifications should be lauded. Evaluating coffee in this manner is similar to taking a class trip to an art gallery or concert and teaching them metrics with which to consciously appreciate the art before them. Imparting tools with which to communicate this appreciation, including associating the aroma to other products [9].

Relating aroma to different products is a learned association that most people acquire passively; you learn what an orange is and then how it smells once you peel and eat it. Some chefs actively cultivate a recognition for each ingredient, allowing them to recreate recipes after tasting the dish. Aroma chemists are similar to chefs in this regard, but instead of dissecting foods into their ingredient components, aroma chemists separate aromas into their chemical components and associated pathways. This is similar to recognizing instruments by their timbre in a musical piece or the ability to identify colours in a work of art. Not unlike the colour green, coffee’s aroma is a mixture that has its own identity, for instance instead of thinking, ‘blue and yellow’ every time we see grass we think, ‘green’. Identifying the core chemical components through chemical analysis, as discussed earlier, is comparable to defining green as a mixture of blue and yellow, suggesting to an aroma chemist which reaction pathways are critical to this mixture.

Admittedly understanding the mixture of reaction pathways responsible for coffee’s iconic aroma is not the objective of many. Flavour houses can easily recreate coffee aroma in any number of variations for the greater market space based on current knowledge. Recreating an aroma from a chemical recipe is much like music without the timbre of instruments, it is the preference of some, but not of others.

Classifying coffee’s core chemical constituents by their chemical structures (functional groups) is comparable to classifying sounds into their orchestral sections by their timbre; nutty pyrazines (woodwind), savoury thiols (brass), fruity-sweet carbonyls/furanones (strings) and earthy phenols (percussion). Narrowing down the possible aroma precursors, may be likened to identifying which orchestral section a timbre belongs to. Thiols and pyrazines are well defined examples of Maillard reaction products; interaction between proteins and carbohydrates, with the incorporation of either sulphur or nitrogen into the aroma’s molecular structure. The Maillard reaction lowers the temperatures at which carbohydrates degrade, generating, in addition to thiols and pyrazines, carbonyls and furanones, through by-passing caramelization.

Phenols are not generated through the Maillard reaction, but rather from degradation of chlorogenic acids and carotenoids. These pathways are considered “direct” as the precursors degrade in a similar manner independently, in model mixtures, as they would during roasting of the actual product, allowing for a level of predictability to be established between green and roasted coffee composition. This is similar to listening to music, recognizing a drum, finding the drum was part of the orchestra and that when the drum is played the timbre is reproduced.

The absence of the same level of predictability within coffee’s Maillard reaction chemistry may be responsible for the critical knowledge gap currently existing between green and roasted coffee quality. Green coffee behaving like the embryo of music in the mind of the composer. Such a disruption in understanding ignites the imagination of an aroma chemist, much like an ethereal piece of art or music would evoke emotion and provoke thought. The Maillard reactions contribution to coffee’s aroma will be revealed in subsequent articles in this series.

Author

Anja Rahn is an award winning researcher with a Ph.D. in Food Chemistry. Specializing in process chemistry, she taught Food Chemistry and Food Analytics at ETH Zurich, before starting her career in coffee as Aroma & Flavour lead at ZHAW's Coffee Excellence Centre.

After becoming an Q & R Grader and leading a number of successful industry projects, she joined JDE-Peet's to lead their Aroma and Flavour research as well as support their sustainability initiatives. Anja is a Senior Scientist at Wageningen Food Safety Research, specializing in Process-Induced Contaminants, she aims to remain an active part of the coffee community.

References

1. Mozell M.M., Smith B.P., Smith P.E., Sullivan R.L. Jr., and Swender P. (1969). Nasal chemoreception in flavor identification. Arch Otolaryngol. 90: 367-373.

2. Dart S.K., and Nursten H.E. (1985) Volatile Components. In: Clarke R.J., Macrae R. (eds) Coffee. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-4948-5_7

3. Mahmud, MMC, Shellie, RA, and Keast, R. (2020). Unravelling the relationship between aroma compounds and consumer acceptance: Coffee as an example. Compr Rev Food Sci Food Saf. 19: 2380– 2420. https://doi.org/10.1111/1541-4337.12595

4. Semmelroch P. and Grosch W. (1995). Analysis of Roasted Coffee Powders and Brews by Gas Chromatography-Olfactometry of Headspace Samples. Lebensm.-Wiss. u.-Technol. 28: 310-313.

5. Mayer F., Czerny M. and Grosch W. (2000). Sensory study of the character impact aroma compounds of a coffee beverage. Eur. Food Res. Technol. 211: 272-276.

6. Czerny M., Mayer F. and Grosch W. (1999). Sensory Study on the Character Impact Odorants of Roasted Arabica Coffee. J. Agric. Food Chem. 47: 695-699.

7. Semmelroch P., Laskawy G., Blank I. and Grosch W. (1995). Determination of Potent Odourants in Roasted Coffee by Isotope Dilution Assays. Flavour Fragr. J. 10: 1-7.

8. Semmelroch, P. and Grosch W. (1996). Studies on Character Impact Odorants of Coffee Brews. J. Agric. Food Chem. 44: 537-543.

9. World Coffee Research (2017). Sensory Lexicon. World Coffee Research. 20170622_WCR_Sensory_Lexicon_2-0.pdf (worldcoffeeresearch.org)

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