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Coffee Aroma

The aroma of a coffee is responsible for all flavour attributes other than the mouthfeel and sweet, salt, bitter, and sour taste attributes that are perceived by the tongue. Therefore, it might be said that the 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 flavour.

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 is well over 800 and as analytical methods become more precise more will be uncovered. Yet, the perception of aroma is dependent upon both the concentration of the compound and its odour threshold. It is probable that a relatively small group of compounds that share both a high concentration and a low odour threshold make up the fragrance we know as coffee aroma.

The aroma of coffee is for a large part determined by the roasting of the beans. The 4 main reactions during the roasting are:

  1. Maillard reaction; a reaction between nitrogen containing substances (amino acids, proteins, as well as trigonelline and serotonine) and carbohydrates (sugars).
  2. Degradation of individual amino acids, particularly, sulphur amino acids, hydroxy-amino acids, and proline.
  3. Degradation of sugar resulting in caramel-like substances.
  4. Degradation of phenolic acids, particularly the quinic acid moiety.

Other reactions involve lipid degradation and hundreds of interactions between intermediate decomposition products.

Table 1 shows some of the compounds that are likely to be the most influential in coffee aroma. Furans are found to be the most predominant group of compounds amongst the coffee aromatics. They typically have caramel-like odours since they result from the pyrolysis of sugars (‘burnt caramel'). Furans also produce key aroma notes when secondary reactions take place with sulphur containing compounds.

Table 1 . Some important compounds in coffee aroma

 

Volatile

Conc. (mg/L)

Aroma Description

Structure

(E)-ß-Damascenone

1.95x10 -1

honey-like, fruity



2-Furfurylthiol

1.08

roasty (coffee)

 

3-Methyl-2-buten-1-thiol

8.20x10-3

amine-like

 

2-Isobutyl-3-methoxypyrazine

8.30x10-2

earthy


5-Ethyl-4-hydroxy- 2-methyl-3(2H)-furanone

17.3

 



Guaiacol

4.20

phenolic, spicy



2,3-Butanedione (diacetyl)

50.8

buttery



2,3-Pentanedione

39.6

buttery



Methional

2.40x10-1

potato-like, sweet



2-Isopropyl-3-methoxypyrazine

3.30x10-3

earthy, roasty



Vanillin

4.80

vanilla



4-Hydroxy-2,5-dimethyl- 3(2H)-furanone (Furaneol)

1.09x102

caramel-like



2-Ethyl-3,5-dimethylpyrazine

3.30x10-1

earthy, roasty



3-Hydroxy-4,5-dimethyl- 2(5H)-furanone (Sotolon)

1.47

seasoning-like



4-Ethylguaiacol

1.63

spicy



5-Ethyl-3-hydroxy-4-methyl- 2(5H)-furanone (Abhexon)

1.60x10-1

seasoning-like



The pyrazines contribute to the roasted, walnut, cereal, cracker, or toast-like flavours in coffee. Along with thiazoles, the pyrazines have the lowest odour 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 sulphur containing amino acids and sugars.

Sources

  • http://www.coffeeresearch.org/science/aromamain.htm
  • Clarke, R. J. The Flavour of Coffee. In Dev. Food Science. 3 B. 1986. 1-47.
  • Blank, I.; Sen, A.; and Grosch, W. 14th ASIC Colloq. San Francisco . 1991. 117-129.
  • Grosch, W. 16th ASIC Colloq. Kyoto. 1995. 147-156.
  • Illy, A. and Viani, Rinantonio. Espresso Coffee: The Chemistry of Quality.

 



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