Staling of roasted coffee

Nicoli M.C., Innocente N., Pittia P. and Lerici C.R. (1993) Staling of roasted coffee volatile release and oxidation reactions during storage. 15th Int. Colloq. Chem. Coffee (Montpellier, 6-11.6.1993) (ASIC, 1993), 2, 557-66. 

The flavor and aroma qualities of the common coffee beverages are dependent on the source of the coffee bean used, soil and climatic conditions, as well as the duration and temperature of roasting. The determination of the quality of roasted coffee is mainly by tasting of the brew by professional tasters. Such processes are qualitative only. Chemical analysis of coffee quality is based on headspace analysis of the aroma components of roasted or brewed coffee. For instance, drastic decreases in methylfuran and methylethylketone in coffee aroma have been observed in roasted and ground coffee within a few days, which may be correlated to coffee staleness. A... 

As these examples indicate, the characteristic flavor of a food, fruit, etc., usually derives from a complex mixture of components. In a few cases, one unique sulfur compound is a character-impact compound, a material recognized as having the same organoleptic character as the material itself. Although some 670 compounds, of which more than 100 are sulfur-containing, have been identified in roast coffee, one material, furfurylmercaptan (2-furylmethanethiol) is considered to be a character-impact compound.43,44 The threshold level for detection of 2-furylmethanethiol in water is 0.005 ppb, and at levels of 0.01-0.5 ppb, it has the very characteristic aroma of freshly roasted coffee. However, as in many other cases, there is a concentration effect. At levels from 1-10 ppb the aroma is that of staled coffee with a sulfury note .43 Hence, 2-furylmethanethiol has a two headed property - at low concentrations it is a character impact compound and at higher levels it is an off-flavor component. 

A careful organoleptic evaluation of the a,p-unsaturated aldehyde (17) which was detected in various food products such as carrot root oil (72), tomato (74), beef (342) and cranberry (17) has shown that this aldehyde has some remarkable flavor properties. Above the threshold concentration of about 0.1 ppb (72), for example at 0.4 to 2 ppb in water, ( )-2-nonenal (17) possesses a woody character (470). Above 8 ppb the sensory impression turns into a fatty one which becomes unpleasant above 30 ppb. Finally, an aqueous solution of 1,000 ppb of aldehyde (17) has a strong taste of cucumber. The fresh-brew woody note of roasted and ground coffee as well as the woody effect in bell peppers is due to the presence of this compound (470). Addition of (jE)-2-nonenal (17) to cranberry juice at a level of 1 ppb causes a considerable reduction in the normal astringent character of the juice (470) without any change of the original odor impression. An antagonistic effect of (17) is observed on the flavor of (Z)-3-hexenal (5) (375). Above 2 flavor units (E) 2 nonenal (17) causes the stale flavor of spoiled beers (381). 

Furfurylthiol is the primary character impact compound for the aroma of roasted Arabica coffee (45). It has a threshold of 5 ppt and smells like freshly brewed coffee at concentrations between 0.01 and 0.5 ppb (46). At higher concentrations it exhibits a stale coffee, sulfury note. Other potent odorants in roasted coffee include 5-methylfurfurylthiol (0.05 ppb threshold), which smells meaty at 0.5-1 ppb, and changes character to a sulfury mercaptan note at higher levels (46). Furfuryl methyl disulfide has a sweet mocha coffee aroma (15). A key aromatic that markedly contributes to coffee aroma is 3-mercapto-3-methylbutyl formate. The pure compound has a blackcurrant-like, catty note, however, in the context of brewed coffee, it contributes roast coffee likeness (47,48). 

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