Aromatics are perceived by the olfactory lobe at the top of the nasal passage. Two passageways lead to the olfactory lobe: the nostrils of the nose, referred to as the orthonasal channel; and a passage leading from the mouth and throat area, called the retronasal channel. The olfactory lobe itself has numerous cilia (small, hair-like organs) in a layer of mucous. When a water-soluble aromatic reaches this organ, it is dissolved in the mucous, where the cilia sense the chemical and a signal is sent to the brain. When one becomes aware of the stimulus, one can classify the type and intensity of the smell experienced.
In brewed coffee, the perception of aroma is the result of the aromatics originally present in the ground coffee and what is dissolved into liquid during the brewing process, some of which might not be available from dry ground coffee. Brewed coffee aroma differs significantly from the fragrance of dry ground coffee because a shift in concentrations occurs. Some of the sulphurous, or roasty, aromas that are prominent in ground coffee are not as dominant in brewed coffee. Conversely, some of the aromas that are barely perceptible in ground coffee are key odorants of brewed coffee. The volatility and stability of the compounds also contribute to aroma, such as the fresh-fruit; long-chain aldehyde aromas are delicate and subject to rapid oxidation, while some earthy odorants are highly stable once extracted.
Different aromas have different strengths and their perception may vary according to concentration. One of the most important aromas of coffee, diacetyl, is pleasantly buttery at low concentrations found in coffee, but perceived as burnt and sulpherous, or even chlorine-like, in high concentrations. Vanilla aroma is almost always present to some degree in coffee, but is rarely perceived due to the presence of stronger aromas.
Most aromatic constituents can be found in the oils of brewed coffee. The number of aromatics that escape into the atmosphere depends on temperature, the properties of the aromatic, and interaction with the gases above the liquid surface. Acetic acid (conventionally known as vinegar) is especially active and can be readily perceived in any liquid in which it is present. More volatiles are released from a liquid surface than a solid surface and are transmitted by movement of available gas. When cuppers break the surface of a crust of coffee, the carbon dioxide under the grounds releases aromatics into the atmosphere.
It is possible to detect more than 1,200 aromatic compounds in coffee, making aromas the most variable component of the coffee. These aromas are based on numerous variables, such as the origin of the green bean, the species and variety of tree and the degree of roast. Because of their great number, aromas are often classified according to how they are perceived (such as sweet or earthy) by the olfactory lobe. Together, the various descriptors create an aroma profile for the particular coffee.
Certain acids also contribute to brewed coffee aroma, and include those acids responsible for the unpleasant fermented aroma and flavor. An acid is considered an aroma if perceived by the olfactory lobe. The most important aromatic acids include lactic acid (responsible for the sour taste and smell of yogurt, sour cream and buttermilk) and acetic acid (vinegar). Water with high pH or total alkalinity will directly affect these aromas.
The aroma profile of a cup of brewed coffee is affected by grind, water, heat and time. These variables affect the amount of aromatic compounds being extracted and in what quantities. The perception and enjoyment of the aroma profile depends upon the balance of these many constituents.
