Coffee aroma is responsible for all coffee flavor attributes other than the mouth-feel and sweet, salt, bitter, and sour taste attributes that are perceived by the tongue. Therefore, it might be said that coffee aroma is the most important attribute to specialty coffee. Even instant coffee has the components responsible for stimulation of our taste buds. The difference, however, is that instant coffee lacks most of the aromatic volatile compounds causing a dramatic decrease in the overall coffee flavor.
Perception of Coffee Aroma
Coffee aroma is perceived by two different mechanisms. It can either be sensed nasally via smelling the coffee through the nose or retronasally. Retronasal perception occurs when the coffee is either present in the mouth or has been swallowed and aromatic volatile compounds drift upward into the nasal passage.
The number of aromatic compounds found in coffee increases every year. Today the number is well over 800, and as our analytical methods become more precise, more will be uncovered. Yet, the perception of coffee aroma is dependent upon both the concentration of the compound and its odor threshold. With that said, understanding coffee aroma is not as difficult as understanding how over 800 coffee elements interact with the olfactory epithelium. It is probable that a relatively small group of compounds that share both a high concentration and a low odor threshold make up the fragrance we know as coffee aroma. This article will discuss the recent research that has narrowed in on these aroma impact compounds.
Illy listed the following chemical processes that affect the development of volatile compounds in coffee (112):
1) Millard or non-enzymatic browning reaction between nitrogen containing substances, amino acids, proteins, as well as trigonelline, serotonin, and carbohydrates, hydroxy-acids and phenols on the other.
2) Strecker degradation.
3) Degradation of individual amino acids, particularly, sulfur amino acids, hydroxy amino acids, and pro line.
4) Degradation of trigonelline.
5) Degradation of sugar.
6) Degradation of phenolic acids, particularly the quinic acid moiety.
7) Minor lipid degradation.
8) Interaction between intermediate decomposition products.
In a review article published by Clarke, he asserts that various research groups have identified 150 aliphatic compounds including 56 carbonyl compounds and 9 sulfur containing compounds; 20 alicyclic compounds, including 10 ketones; 60 aromatic benzenoid compounds, including 16 phenols; 300 heterocyclic compounds, including 74 furans, 10 hydrofurans, 37 pyrroles, 9 pyridines, 2 quinolines, 70 pyrazines, 10 quinoxalines, 3 indoles, 23 thiophens, 3 thiophenones, 28 thiazoles, and 28 oxazoles (34).
The pyrazines are the second most abundant class of aromatic compounds and contribute to the roasted, walnut, cereal, cracker, or toast-like flavors in coffee. Along with thiazoles, the pyrazines have the lowest odor threshold and therefore significantly contribute to the coffee aroma. Next, the pyrroles are responsible for some of the sweet, caramel-like, and mushroom-like aromas in coffee. Conversely, the thiophens are known to have a meaty aroma and are thought to be produced from Maillard reactions between sulfur containing amino acids and sugars. Thiazoles have an even smaller presence in the overall aroma and are said to be formed via sugar degradation.