Unveiling the Secrets: How Coffee Produces Its Distinctive Odor and Flavor
We drink coffee to savor its unique flavor notes and rich aroma. When we enjoy a pour-over or relish an espresso, we might evoke memories of chocolate, caramel, almonds, fruits, and flowers. But what is happening on a molecular level to create these sensations?
Where Coffee Aroma Comes From
Green coffee beans lack much aroma. It is only through roasting that we transform the beans into the rich and complex odors we recognize. What we perceive as flavor is often essentially aroma, with mouthfeel and basic tastes like sweet, salty, bitter, and sour detected by the tongue. These tastes combine with the aroma sensed by olfactory receptors in the nose to form flavor.
The complexity of coffee aromas depends on the chemical composition of compounds in the green beans. These precursors of volatile compounds vary in concentration based on several factors, including variety, weather conditions at the origin, level of maturation, and processing choices.
Which Compounds Create Which Aroma
The aroma precursors in coffee are various forms of carbohydrates, proteins, and acids. Throughout the roasting process, particularly during the Maillard reaction, these compounds transform into volatile aromatics. Different compounds create various sensory characteristics during roasting, and the degree of roast can further impact how we perceive the aroma.
2-Methylpyridine creates roasted notes, while pyrazines can produce nuttiness like 2,3-Dimethylpyrazine or a burnt smell like 2-Ethyl-3-methylpyrazine.
Advanced roast phases lead to caramelization of sugars, producing furans and furanones, such as 4-Hydroxy-2,5-Dimethyl-3(2H)-Furanone, which offers caramel notes.
The breakdown of amino acids in aldehydes during Strecker Degradation also creates aromatic volatile compounds. 3-Methylbutanal, for example, imparts fruity and sweet aromas.
Ketones, with aromas ranging from fruity to musty, are the result of auto-oxidation of fatty acids such as propanone. Ketones like β-damascenone have the distinct odor of fruit, with buttery flavors created by smaller ketones such as 2,3-Butanedione.
Unpleasant aromas are also produced during roasting. Phenols such as guaiacol, produced by the breakdown of free phenolic acids during extended roasts, impart an ashy and smoky aroma.
How We Perceive Coffee Aromas
Understanding the roasting process and the formation of volatile compounds is crucial, but it is equally important to note that aromas are perceived in different ways. While over 900 aromatic volatile compounds are produced during roasting, only around 30 of them significantly contribute to the aroma and flavor we perceive in a cup of coffee. Our ability to detect most compounds is limited, and achieving a balanced sensory experience involves precise roasting and brewing techniques.
When we smell coffee grounds, molecules freely interact with our olfactory receptors. However, when the grounds are added to water to make a cup of coffee, different volatile compounds are extracted. The polarity of molecules drives the extraction of aromatics in the beverage. Different polarities of compounds, such as 2,3-Butanedione (highly polar) and β-damascenone (less polar), extract at different rates, resulting in a varying aromatic profile throughout the brewing process.
Interactions between compounds can also alter perception. For instance, 2,3-Butanedione, known for its buttery aroma, can increase the perception of sweetness in coffee. Fabiana Carvalho, a neuroscientist who investigates neural processes of memory, sensation, and perception, notes the phenomenon of increasing or decreasing taste intensity between congruent compounds. Her research demonstrates that adding a strawberry odorant to a sucrose solution can make participants perceive it as sweeter.
By understanding the chemistry and perception of coffee aromas, we can make more informed decisions about brewing techniques. This knowledge also explains why different levels of extraction can produce such distinct sensory experiences in a single cup of coffee.
Coffee brewed in a French Press being poured into a cup. Credit: Nik MacMillan
A barista visits a coffee farmer in Brazil. Credit: Raissa Castor
Coffee roaster in Brazil. Credit: Raissa Castor
Cupping coffee. Credit: Raissa Castor
Raw coffee beans. Credit: Raissa Castor
Roasted coffee beans. Credit: Raissa Castor