Aroma compounds in roasted coffee

Phenols. See also Phenol Phenolic entries achiral derivatizing agents, 6 96t alkylation, 2 196-197, 212-214 aroma chemicals, 3 243-246 aroma compounds in roasted coffee, 7 256ts... 

There is very little published on the interaction of either taste or aroma components with minor components in foods. Earlier in this chapter (Section 6.3.2), some discussion was presented on the interaction of high potency sweeteners with aroma. There is substantial interest in this particular interaction since the industry would like to know why diet products do not taste like the full sugar versions and vice versa. There are few other flavonfood interactions that have such a strong economic link to support this type of research activity. One other area that has received some attention is the interaction of melanoidins with aroma compounds in roasted coffee. This is again driven by economic considerations. A brief discussion of this interaction and pH effects follows. 

Holscher W. (1996) Comparison of some aroma impact compounds in roasted coffee and coffee surrogates. R. Soc Chem., Spec. Pub . 197, 239-44. 

Holscher W. and Steinhart H. (1992c) New sulfur-containing aroma-impact compounds in roasted coffee. 14th Int. Colloq. Chem. Coffee (Sait Francisco, 14-19.7.1991) (ASIC, 1992), 130-6. 

Significant aliphatic sulfur compounds are methional, 3-methyl-but-2-ene-1-thiol, 3-mercapto-3-methylbutan-l-ol (8-124), its ester 3-mercapto-3-methylbutyl formate, methanethiol and dimethyltrisulfide. 3-Mercapto-3-methyl-l-ol also occurs in passion fruit and blackcurrant, and as a putative cat pheromone in cat urine, where it is formed as a degradation product of amino acid L-felinine (see Section 2.2.1.2.2). Of more than 70 known pyrazines, the most important compounds in roasted coffee are isopropylpyrazine, 2-isobutyl-3-methoxypyrazine, 2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, 2,6-dimethyl-3-vinylpyrazine and 2-ethyl-6-methyl-3-vinylpyrazine. Pyridine and its alkyl derivatives and bicyclic pyridines have a negative impact on the quality of coffee aroma. Important aromatic... 

Several of the lower molecular weight aliphatic compounds, in a mixture, are part of the roasted coffee aroma. A nine-compound mixture with roasted coffee aroma contained isopentane, n-hexane, acetaldehyde, dimethyl sulfide, propanal, isobutanal, isopentanal, methanol, and 2-methylfuran.20 In addition, the freshness of aroma and taste has been correlated with 2-methylpropanal and diacetyl. When the concentration of these falls off, so does the taste.21 Other aliphatic compounds that are steadily lost from ground roasted coffee, unless it is vacuum packaged, include methyl formate, methyl acetate, methyl thioacetate, and acetone.22 The concentrations in roast coffee for four compounds whose contribution to the fresh flavor have long been known are dimethyl sulfide (4 ppm), methyl formate (12 ppm), isobutanal (20 ppm), and diacetyl (40 ppm). The taste thresholds are 0.1, 0.5, 0.5, and 1.0 ppm, respectively, in the brew made with 5 g coffee per 100 ml water.15... 

Almost all the heterocyclic compounds listed are volatile and have been recognized as present in roasted coffee or its aroma, suggesting that almost all are thermal transformation products rather than compounds present in the green coffee bean. 

Holscher, W., Steinhart, H., Formation pathways for primary roasted coffee aroma compounds, in ACS Symposium Series 543, Thermally Generated Flavors, 1994, 206. (CA120 105189t)... 

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. 

Furans and reductones are major components in roasted coffee as shown in Figure 3. Arabicas possess higher amounts of furanaldehydes and Furaneol than Robustas, when roasted under comparable conditions. The aldehydes and reductones are Strecker-active components and further transformed into typical aroma and flavor compounds as demonstrated in model experiments. 

Perhaps the most important compounds identified in the roasted sesame oils are 2-furfurylthiol and guaiacol. Using aroma extract dilution analysis method, these two compounds have been characterized by Schieberle (92) to be the most odor-active compounds in roasted sesame seeds. 2-Furfurylthiol, having an intense coffee-like odor, increased from 16 ppb in roasted oil processed at 160°C for 30 min to 158 ppb in the oil processed at 200°C for 30 min (Table 12). Guaiacol has a burnt and smoky odor with an extremely low-odor threshold of 0.02 ppt in... 

The knowledge that not all of the volatiles (e.g. more than 800 in roasted coffee) [5] that occur in a food contribute to its aroma was the rationale for changing the methodology of analysis. Since 1984, when the procedure for charm analysis was published [4], techniques have been developed that focus on the identification of compounds contributing to the aroma with higher OAV. 

Smith (1963a) and Feldman et al. (1969) underlined the importance of non-volatile compounds to the flavor of coffee. The comparison between the composition of green and of roasted coffee showed an important decrease in the content of proteins, chlorogenic acid and sucrose on roasting. Fractionation and analysis of the aroma precursors in green coffee have also been studied by Russwurm (1970) who considers that the non-volatile constituents of green coffee that may be involved in flavor formation are carbohydrates, proteins, peptides and free amino acids, polyamines and tryptamines, lipids, phenolic acids, trigonelline and various non-volatile acids. 

Quantification of aroma-impact components by isotope dilution assays (IDA) was introduced in food flavor research by Schieberle and Grosch (1987), when trying to take into account losses of analytes due to isolation procedures. The labeled compounds have to be synthesized, the suitable fragments have to be chosen, and calibration has to be effected. A quantitative determination of ppb levels of 3-damascenone (Section 5,D.38) in foods, particularly in roasted coffee (powder and brew), was developed by Sen et al. (1991a). Semmelroch et al. (1995) quantified the potent odorants in roasted coffee by IDA. Hawthorne et al. (1992) directly determined caffeine concentration in coffee beverages with reproducibility of about 5 % using solid-phase microextraction combined with IDA. Blank et al. (1999) applied this combined method to potent coffee odorants and found it to be a rapid and accurate quantification method. They also concluded that the efficiency of IDA could be improved by optimizing the MS conditions. 

To date, approximately 850 compounds have been identified in roasted coffee. The components most frequently identified in the aroma (Figure 4.2) are furans (16.1 %), pyrazines (11.8%), pyrroles (1 1.0%,) and ketones (10.5%,). Let us however remind ourselves that the most abundant families are not necessarily those which contain the most characteristic or powerful components. 

Figure 4.3 compares the number of compounds identified in green and in roasted coffee beans. Green coffee contains a larger number of identified alcohols (B) and nearly the same number of identified aldehydes (C) and esters (F) than roasted beans. On the contrary, the latter are richer in furans (I), pyrazines (O), ketones (D) and phenols (H). Thiophenes (J), oxazoles (L) and thiazoles (M) have only been identified in roasted coffee. The roasting effect is also revealed by the increased number of pyrroles (K) and sulfur-containing compounds. The distribution of aroma volatiles is shown in Figure 4.4. 

Identified in coffee aroma by Vitzthum and Werkhoff (1974a,b), the concentration being of the order of 0.1 ppb. Tressl (1989) found 0.1-0.2 ppm in roasted coffees (see remark in Section 5.1, sulfur compounds). It has been found in a heated cysteine/glucose model system (Sheldon et al, 1986). 

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