Release of Odorants from Roasted Coffee - ACS Symposium Series

Chapter 35

Release of Odorants from Roasted Coffee W . Grosch and F . Mayer

Downloaded by GEORGETOWN UNIV on August 20, 2015 | http://pubs.acs.org Publication Date: September 7, 2000 | doi: 10.1021/bk-2000-0763.ch035

Deutsche Forschungsanstalt für Lebensmittelchemie, Lichtenbergstraβe 4, D-85748 Garching, Germany

A n apparatus designed for quantitative headspace analysis was used for the determination of the release of 22 potent odorants from roasted Arabica coffee. Within 5 min of grinding, 32 % of methanethiol present in the coffee sample as well as 15-20 % of acetaldehyde, methylpropanal, 2- and 3-methylbutanal were lost by volatilization. Within 30 min 20-30 % of 2-furfurylthiol, methional, vanillin and 2-isobutyl-3-methoxypyrazine, approximately 10 % of four alkylpyrazines and only 1 % of three furanones were lost from ground coffee. In contrast, after a storage period of 15 min, the losses of these odorants amounted only to 2-12 % in whole beans. The different evaporation rates of the odorants caused changes in the odor profile of the coffee sample. A synthetic mixture of 21 odorants in the concentrations found in the headspace was prepared. Sensory assessment of the aroma from this mixture compared well with that of the authentic coffee sample.

One goal of flavor research is the identification and quantification of volatile compounds which are perceived by the human nose in the air above a food. Two techniques, static and dynamic headspace sampling, have been proposed to solve this problem (7-3). The static method accurately provides the odorant composition, but the samples are too small to quantify odorants that are present at low concentrations. A dynamic method in which volatile compounds are swept by an inert gas into a trap containing a porous polymer which, more or less adsorbs the organic constituents, might be an alternative. However, the disadvantages of this technique are the strong dependence of the yields of the odorants on the velocity of the carrier gas (4) and on the selectivity of the adsorption and desorption processes (5, 6). As it is difficult to control these parameters accurately, the results of quantitative measurement may be inaccurate. As shown in a study on the flavor of French bread (baguette) the results of quantification are independent of the conditions of dynamic headspace sampling when stable isotopomers of the odorants are used as internal standards (7). After

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© 2000 American Chemical Society

In Flavor Release; Roberts, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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injection of known amounts of these compounds into the headspace, they were collected by the dynamic procedure together with the odorants to be analyzed. In the present study, the new method was applied to medium roasted Arabica coffee from Colombia. The headspace concentrations of the volatiles which contribute significantly to the aroma of roasted coffee (reviewed in references 8-10) were quantified. A s the aroma of freshly ground coffee changed rapidly when the sample was stored in the presence of air, the effect of this condition on the release of odorants was measured. Furthermore, the aroma of roasted coffee was reproduced on the basis of the results of headspace analysis and assessed sensorially.

Experimental Section Coffee. Coffee beans (Coffea arabica) that originated in Colombia were medium roasted, stored under vacuum at -35°C and ground just before analysis (9). Chemicals. Pure samples of the compounds in the tables and of the labeled internal standards were obtained as previously reported (9, 10). Headspace Sampling and Quantification. The apparatus used for headspace sampling (Figure 1) was made of glass as detailed by Zehentbauer and Grosch (7). The inside surface was deactivated by treatment with dichlorosilane (7). The coffee sample (0.1-10 g) was placed into tube no. 8 which was then sealed. Valves nos. 3 and 4 were opened and a solution containing known amounts of the odorants in diethyl ether was injected onto the glass finger no. 10 which was heated up to 80°C. These odorants were labeled with stable isotopes to differentiate them from the analytes. The release of odorants was determined for a period of 15 min or 30 min (see Tables for details) during which time the coffee sample remained in the apparatus. To equilibrate the odorants and the labeled internal standards in the apparatus (volume 6.84 L ) , the piston (no. 5) was pushed and pulled during this period. Subsequently, the valves nos. 3 and 4 were closed and nos. 1 and 2 were opened. After opening valve no. 1, the gaseous nitrogen pressed the headspace collected in tube no. 6 into the Tenax trap no. 7 by moving the piston. For analysis of the highly volatile odorants 1-7 (Table I) the Tenax trap no. 7 (Figure 1) was put in the desorption heating block of a gas chromatograph (7). The analytes and their labeled internal standards were desorbed and then analyzed by mass chromatography (10). The less volatile odorants 8-22 (Table I) were extracted from the Tenax trap no. 7 using diethyl ether and then quantified by multidimensional gas chromatography (10, 11).

Analysis of the Coffee Sample. The odorant content of samples was quantified by stable isotope dilution assays (10). Odor Profile Analysis. After removal of the Tenax trap (no. 7 in Figure 1) the apparatus for headspace sampling was used as an olfactometer (7). With the exception of 2-ethenyl-3-ethyl-5-methylpyrazine, 21 odorants in the headspace concentrations listed in Table I were dissolved in pentane. To compensate for the lack of 2-ethenyl-3-ethyl-5-methylpyrazine, the amount of pyrazine no. 15 (Table I) was



Figure 1. Apparatus for quantitative headspace analysis (7) 1-4, Valves; 5, piston; 6, tube; 7, Tenax trap; 8, tube containing the coffee sample; 9, septum; 10, glass finger


433 Table I. Concentrations (pg/g ) of Odorants in the Ground Coffee Sample and in the Headspace after 30 min Equilibration. a


No.

Odorant

1 Methanethiol 2 Acetaldehyde 3 Methylpropanal 4 2-Methylbutanal 5 3-Methylbutanal 6 2,3-Butanedione 7 2,3-Pentanedione 8 (E)-B-Damascenone 9 2-Furfurylthiol 10 Methional 11 Guaiacol 12 4-Ethylguaiacol 13 4-Vinylguaiacol 14 Vanillin 15 2-Ethyl-3,5-dimethylpyrazine 16 2,3-Diethyl-5-methylpyrazine 17 2-Ethenyl-3,5-dimethylpyrazine 18 2-Ethenyl-3-ethyl-5-methylpyrazine 19 2-Isobutyl-3-methoxypyrazine 20 4-Hydroxy-2,5-dimethyl-3(2H)-furanone 21 2-Ethyl-4-hydroxy-5-methyl-3(2H)furanone 22 3-Hydroxy-4,5-dimethyl-2(5H)-furanone a

Concentration in coffee

Release

4.4 118 24.2 25.8 16.5 48.8 35.3 0.26 1.65 0.25 3.42 1.78 45.1 4.05 0.4 0.1 0.053 0.015 0.117 144 15.9

2.9 54 6.0 8.2 4.4 9.1 9.0 0.030 0.38 0.070 0.60 0.15 2.2 0.79 0.050 0.013 0.0035 0.0020 0.024 2.0 0.20

1.85

0.020

% 66 45 25 32 27 19 25 12 23 29 18 8.4 4.9 20 12 13 6.6 13 21 1.4 1.3 1.1

Ground coffee (0.1-10 g) was placed into the apparatus (Figure 1).

correspondingly increased. The pentane solution was injected onto the glass finger no. 10 which had been heated up to 80°C. To distribute the odorants evenly in the apparatus, the piston (no. 5) was pushed and pulled for 10 min. Valves nos. 3 and 4 were closed and no. 2 was opened. The piston was pushed slowly to produce a constant stream of odorants which were smelled by an assessor at the outlet of valve no. 2. As the solvent pentane was odorless, it did not disturb the sensory evaluation of the odorants. To prepare the reference sample, the coffee sample (2.5 g) was placed into the apparatus and after 30 min the assessor at the outlet of valve no. 2 smelled the headspace. Altogether, six assessors compared the synthetic mixture of the odorants with the original coffee sample.


434


Results and Discussion

Detection of Odorants O f the 28 volatiles screened in dilution experiments as potent odorants of roasted coffee (see review 8), the compounds listed in Table I were detected by gas chromatography/mass spectrometry. In this experiment headspace samples were drawn from 10 g of ground coffee which was placed into the apparatus (Figure 1) for 30 min. Only 2-methy 1-3-furanthiol, 3-mercapto-3-methylbutyl formate, 3-methyl-2butenthiol and 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone were not detected in the headspace by instrumental analysis. Furthermore, propanal and dimethyltrisulfide were not analyzed. Release of Odorants In Table I the release of 22 odorants during a period of 30 min is contrasted with their total concentrations in the coffee sample. Methanethiol and acetaldehyde showed the greatest reductions in concentration with 66 % of the former and 45 % of the latter lost in 30 min. The losses of 2-furfurylthiol, methional, vanillin and 2isobutyl-3-methoxypyrazine (20 % to 30 % in 30 min) were comparable with that of the malty smelling Street aldehydes methylpropanal, 2- and 3-methylbutanal. The four alkylpyrazines were liberated at a slower rate; approximately 10 % of the amounts present in the coffee sample were lost in 30 min. The lowest evaporation rates were found for the three furanones of which only 1 % disappeared. Sensory Experiment A sensory experiment was performed to examine whether it was possible to imitate the aroma of roasted coffee on the basis of headspace data. The odorants in concentrations equal to those shown in Table I were dissolved in pentane. A s not enough 2-ethenyl-3-ethyl-5-methylpyrazine was available, the concentration of 2ethyl-3,5-dimethylpyrazine was correspondingly increased in the aroma model, as both compounds have very similar aroma quality and odor thresholds (12). The results in Table II indicate that the aroma profile of the synthetic mixture matched the aroma profile of the coffee sample very closely. The highest differences in intensity which amounted only to 0.2 points for the sweetish/caramel and the roasty, sulfurous notes, were not significant. The good agreement in the aromas was underlined by the high score of 2.6 given by the assessors for the similarity of the overall odor of the imitate with that of the real coffee sample. In a previous sensory experiment, the odorants found in medium roasted coffee (provenance Colombia) were dissolved in an oil/water mixture (1:20 w/w). In contrast to the headspace imitate M this model only reached a score of 2.3 (P). The lower score is mainly caused by the base, which differs from the solid coffee matrix in the binding of the odorants. This conclusion is supported here by the experiment with the headspace aroma model. The higher similarity of this model with the aroma of ground roasted coffee was achieved by elimination of the matrix. Furthermore, the


435 Table II. Odor Profiles of Ground Coffee and of the Corresponding Headspace Aroma Model M. Values for odor intensity and similarity range from 0 (absent or no similarity) to 3 (strong or identical). Attribute

ΰ

Μ

Coffee" Intensity


Sweetish/caramel-like Earthy Roasty/sulfiirous Smoky Similarity

2.4 1.4 2.3 1.3 2.6

2.2 1.5 2.5 1.2

a

The coffee was equilibrated for 30 min in the apparatus and then smelled. " Mixture of the 21 odorants which were found in the headspace (cf. Table I).

Table III. Sensory Changes in the Odor Profile of Roasted Coffee with Storage Time. Values (0 no odor; 3 strong odor) are the mean of six assessors. 0

Attribute 0 Sweetish/caramel-like Earthy Roasty/sulfiirous Smoky

1.5 1.5 2.4 1.2

Storage time in air (h) 0.25 1 Intensity 0.8 0.6 1.7 1.7 2.3 2.3 1.6 1.8

3 0.3 2.1 2.2 2.1

a

After grinding the coffee sample (2.5 g) was stored in air for the times given in the table. Then it was placed into the apparatus (Figure 1). After equilibration (15 min) the odor profile was evaluated by the assessors.

concentrations of the odorants in the headspace are much lower than in the matrix. Hence, chemical reactions of the odorants are very unlikely. Change in the Odor Profde As detailed in Table III the odor profile of the roasted coffee sample changed rapidly after grinding. The intensity of the sweetish/caramel-like odor quality decreased distinctly within 15 min and continuously during the next 3 h. The earthy and the smoky notes increased slowly whereas the roasty/sulfiirous note remained nearly constant. To get an insight into the changes in the concentrations of the odorants during storage of roasted coffee in the presence of air, the following experiment was undertaken. The ground coffee sample was stored at room temperature and, after different periods of time, the sample was placed into the apparatus (Figure 1) and then the highly volatile odorants which were released within 15 min were determined.


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Table IV. Headspace Concentrations (pg/g) of Highly Volatile Odorants after Storage of Coffee at Room Temperature


Odorant Methanethiol Acetaldehyde 2,3-Butanedione 2,3-Pentanedione Methylpropanal 2-Methylbutanal 3-Methylbutanal

Storage time (h) in air after grinding 1 3 0 0.25 n.d. n.d. 1.9 1.9 n.d. n.d. 45.7 40.0 2.1 2.6 7.5 3.5 3.3 7.4 3.7 3.7 1.0 1.2 4.9 1.7 1.8 1.6 7.0 2.3 1.1 1.0 1.4 3.6

n.d.: not determined

The results in Table IV. indicate that the release of methanethiol was not affected by a storage period of 15 min and also the relatively high amount, which evaporated from acetaldehyde, remained nearly constant. This was in contrast to the diones and to the malty smelling Strecker aldehydes methylpropanal, 2- and 3-methylbutanal. The release of the latter compounds decreased to one third and that of butanedione and pentanedione by 50 %. In the case of the diones and Strecker aldehydes, the release within 15 min was determined in coffee samples which had been stored for up to 3 h. The results in Table IV indicate that the amounts liberated decreased further, although the differences become much smaller with respect to the amounts found after the first 15 min of storage. In the experiments listed in Table V the release of 7 highly volatile odorants was determined at different times and related to their total concentrations in the coffee sample. According to the results in Table V , methanethiol was the most volatile odorant of which 32 % evaporated during the first 5 min. Acetaldehyde evaporated somewhat slower, but after 1 h its loss (64 %) was nearly as high as that of methanethiol (70 %). The released amounts of the two diones agreed although their concentrations in the coffee sample were different. During the first 5 min, 15 % to 19 % of the Strecker aldehydes were lost. Then, as also found for the other odorants, the rate of loss slowed down. After 1 h the concentration of the Strecker aldehydes had decreased to one third. Whole versus Ground Beans In the final experiment (Table VI), the release of the highly volatile odorants from whole beans as well as from ground beans during a period of 15 min was compared. As expected the increase in surface area, caused by the grinding process, strongly enhanced the amount of odorants, which were lost from the coffee sample. The largest increase was found for 2,3-butanedione. The amount released in the gas phase was 8-times higher in ground beans than in whole beans. The smallest


437 Table V. Concentration (pg/g ) of the Highly Volatile Odorants in the Headspace above Ground Roasted Coffee with Time at Room Temperature. Percentage Figures are the Amounts Expressed as a Percentage of the Original Sample Content


Odorant

Methanethiol Acetaldehyde 2,3-Butanedione 2,3-Pentanedione Methylpropanal 2-Methylbutanal 3-Methylbutanal

Amount of volatile in headspace with time 5 min 15 min 30 min 60 min 1.4 23.8 4.6 4.7 3.6 5.0 2.7

32 20 9.4 13 15 19 16

1.9 45.7 7.5 7.4 4.9 7.0 3.6

43 39 15 21 20 27 22

2.9 53.5 9.1 9.0 6.0 8.2 4.4

66 45 19 25 25 32 27

3.1 75.1 10.2 9.8 6.6 9.4 5.2

70 64 21 28 27 36 32

Table VI. Headspace Concentrations of Highly Volatile Odorants after 15 minutes for Roasted Whole and Ground Beans. Figures represent the actual headspace concentration (pg/g) and the Percentage Figures are the Amounts Expressed as a Percentage of the Original Sample Content Odorant Methanethiol Acetaldehyde 2,3-Butanedione 2,3-Pentanedione Methylpropanal 2-Methylbutanal 3-Methylbutanal

Whole beans 0.5 14.1 0.9 1.1 0.9 1.1 0.6

% 11 12 1.8 3.1 3.7 4.3 3.6

Ground beans % 1.9 43 45.7 39 7.5 15 7.4 21 20 4.9 7.0 27 22 3.6

difference was found for acetaldehyde, as the amount was only 3-times higher than in ground beans.

Conclusions The results confirm that the new analytical method is sufficient for an accurate quantification of the odorants occurring in the headspace of foods. The rapid change in the odor profile of ground roasted coffee is caused by large differences in the evaporation rates of potent odorants. The aroma of roasted coffee can be reproduced on the basis of the results of headspace analysis.


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References


1.

Schaefer, J. In: Isolation, Separation and Identification of Volatile Compounds in Aroma Research; Maarse, H.; Belz, R.; Eds.; Akademie-Verlag, Berlin, 1981, pp. 37-49. 2. Parliment, T.H. In: Biogeneration of Aromas; Parliment, T.H.; Croteau, R.; Eds.; A C S Symposium Series 317, American Chemical Society, Washington, D C , 1987, pp. 34-52. 3. Risch, S.J.; Reineccius, G . A . In: Thermal Generation of Aromas; Parliment, T.H.; McGorrin, R.J.; Ho, C.-T.; Eds.; A C S Symposium Series 409, American Chemical Society, Washington, DC, pp. 42-50. 4. Werkhoff, P.; Bretschneider, W.; Herrmann, H.-J.; Schreiber, K . Lab. Prax. 1989, 426-430. 5. Jennings, W.; Filsoof, M. J. Agric. Food Chem. 1977, 25, 440-445. 6. Schaefer, J. In: Flavour'81; Schreier, P.; Ed.; de Gruyter, Berlin, 1981, pp. 301313. 7. Zehentbauer, G.; Grosch, W. Z. Lebensm. Unters. Forsch. 1997, 25, 262-267. 8. Grosch, W. Nahrung 1998, 42, 344-349. 9. Czerny, M.; Mayer, F.; Grosch, W. J. Agric. Food Chem. 1999, 47, 695-699. 10. Mayer, F.; Czerny, M.; Grosch, W. Euro Food Res. Technol. 1999, 209, 242250. 11. Reiners, J.; Grosch, W. J. Agric. Food Chem. 1998, 46, 2754-2763. 12. Wagner, R.; Czerny, M.; Bielohradsky, J.; Grosch, W. Ζ. Lebensm. Unters. Forsch. 1999, 208, 308-316.


Release of Odorants from Roasted Coffee - ACS Symposium Series

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