Model Study on Changes in Key Aroma Compounds of Dornfelder

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Chapter 8

Model Study on Changes in Key Aroma Compounds of Dornfelder Red Wine Induced by Treatment with Toasted French Oak Chips (Q. robur) Stephanie Frank, Thomas Koppmann, and Peter Schieberle* Deutsche Forschungsanstalt für Lebensmittelchemie, Lise-Meitner-Strasse 34, 85354 Freising, Germany *E-mail [email protected]. Phone +49 8161 71 2932. Fax +49 8161 71 2970.

A young Dornfelder wine was treated for 15 days with French oak chips, and changes in the overall aroma were evaluated by aroma profiling, the application of an aroma extract dilution analysis followed by quantitative measurements based on stable isotope dilution assays. In particular trans- and cis- oak lactone, 4-ethyl-2-methoxyphenol and 2, 6-dimethoxyphenol were not detectable in the untreated wine, while, e.g., vanillin or 2-methoxyphenol were increased. Among the 17 odorants quantitated, vanillin, but also 2,6-dimethoxyphenol, iso-eugenol and both oak lactones increased significantly in the oak treated wine. A sensory experiment, in which the untreated young wine was administered with 13 odorants in the concentrations present in the oaked wine revealed a good agreement of both aroma profiles.

Among color and taste, aroma is an important quality attribute of wine, and thus, in numerous previous studies over 800 volatiles have been considered to contribute to the aroma of different red and white wines (1). It is well-accepted in the literature that especially storage in oak barrels has a significant influence on the overall wine aroma (barrique-type). But, storage in oak barrels is a time© 2015 American Chemical Society In Advances in Wine Research; Ebeler, Susan B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


consuming and expensive process, and thus, alternative aging systems have been proposed. One alternative is the use of oak chips leading to the desired barriquetype flavor in a faster and cheaper way, and since this practice was approved and allowed by the European Community in 2006, attempts were done to increase knowledge on the differences between aroma compounds present in barrel vs. oak chip aged wines. The many studies on the influence of barrel aging on wine aroma were previously summarized in a review by Garde-Cerdán and Ancín-Azpilicueta (2) also taking storage with oak chips into consideration. However, it becomes clear from a literature survey that only a few studies characterized the odor-active volatiles among the tremendous set of odorless compounds, for example in a study by Díaz-Maroto et al. (3), who compared odor-active compounds in untoasted and toasted oak wood by GC-Olfactometry or Culleré et al. (4), who reported on the odorants in six different types of wood. However, up to now, no systematic investigation on the key aroma compounds in raw and toasted oak chips has been performed using the Sensomics approach (5), except in our recent publication (6) in which the key aroma compounds in hydro-alcoholic extracts of oak chips of different toasting degree were compared. In total 39 odorants could be identified, and in the heavily toasted oak chips 14 aroma compounds, particularly vanillin, trans-isoeugenol, and 2-methoxyphenol showed very high odor activity values (ratio of concentration to odor threshold). The same concept was previously used by us to characterize the key aroma compounds in a Dornfelder red wine (7), and a total of 27 key odorants was quantitated by means of stable isotope dilution assays. Knowing the key aroma compounds in oak chips as well as in an untreated young Dornfelder red wine, the aim of the present study was to characterize the influence of a treatment with oak chips on the aroma compounds of this wine variety. For this purpose, a young, non-aged Dornfelder wine was stored for 15 days in the presence of French oak chips, and changes in the key odorants were followed by aroma extract dilution analyses and quantitative measurements.

Experimental Part Wine and Oak Chips A young Dornfelder red wine (vintage 2012; not stored in a barrel) was obtained from a winegrower in Saulheim (Rheinhessen, Germany), and was directly used after fermentation for two weeks. Heavily toasted French oak chips (Quercus robur) were purchased from a German wine distributor.

Wine Treatment Oak chips were ground and soaked in the wine (40 g per liter of wine). The sample was shaken daily and kept for 15 days at room temperature in the dark. 124 In Advances in Wine Research; Ebeler, Susan B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Aroma Profile Analysis Aroma profile analyses of the wine samples were performed by a trained sensory panel consisting of at least 15 panelists as previously described (6). The following aroma descriptors, represented by the compound given in parentheses, were chosen for sensory evaluation, and their intensities were ranked on a seven-point scale from 0 (not perceivable) to 3 (strongly perceivable): flowery, honey-like (2-phenylethanol), smoky (2-methoxyphenol), clove-like (4-allyl-2-methoxyphenol), malty (3-methyl-1-butanol), vanilla-like (vanillin), vinegar-like (acetic acid), cooked apple-like (β-damascenone), coconut-like (cis-oak lactone), fruity (ethyl 3-methylbutanoate), and woody (powdered oak chips). The judgments of each panelist were averaged.

Aroma Extract Dilution Analysis and Quantitation by Stable Isotope Dilution Assays Aroma extract dilution analyses on a distillate isolated by SAFE distillation from the two wine samples, and quantitation of the key odorants by stable isotope dilution assays were carried out as previously described (6). In the AEDA experiments, the undiluted sample was evaluated by four panellists to eliminate potential gaps in detecting odor-active areas. The detailed FD-factors were then determined by one panellist. The quantitation was done in three different work-ups of the same sample.

Spiking Experiment The young Dornfelder wine without added oak chips was spiked with the following 13 odorants: 4-allyl-2-methoxyphenol, 2,6-dimethoxyphenol, 4-ethyl-2-methoxyphenol, 3-ethylphenol, cis-isoeugenol, trans-isoeugenol, 2-methoxy-phenol, 4-methyl-2-methoxyphenol, cis-oak lactone, trans-oak lactone, 4-propyl-2-methoxyphenol, sotolon, and vanillin. The amounts of the odorants were adjusted to get identical concentrations to those in the wine stored with oak chips.

Results Aroma Profile Analysis First of all aroma profile analyses of both wine samples were performed, and distinct differences in the aroma profiles between the wine stored without chips and with chips were observed (Figure 1). The aroma qualities smoky, clove-like, vanilla-like, coconut-like, and woody were rated higher in the wine stored with oak chips, while smaller changes were noticed for flowery, malty, and cooked applelike odors. On the other hand, in particular the intensity of the odor impression “fruity” decreased in the oak stored wine. 125 In Advances in Wine Research; Ebeler, Susan B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 1. Aroma profile analysis of the young Dornfelder wine stored without (black) and with oak chips (gray).

To elucidate the reasons for these differences on a molecular basis, first a comparative aroma extract dilution analysis ( cAEDA) was performed.

Comparative Aroma Extract Dilution Analysis The cAEDA showed clear differences in the FD-factors for some compounds listed in Table 1. A few odorants were only present in the oak treated wine, e.g., trans- and cis-oak lactone, 4-ethyl-2-methoxyphenol, and 2, 6-dimethoxyphenol. Further aroma compounds such as vanillin, 4-ally-2-methoxyphenol, and 2-methoxyphenol were increased by the chip treatment and thus, showed higher FD-factors in the wine stored with oak chips. On the other hand, in particular β-damascenone was lowered by the oak chip treatment.

Quantitative Measurements To confirm the results indicated by the aroma extract dilution analyses, 17 odorants, namely 4-allyl-2-methoxyphenol, β-damascenone, 2,6-dimethoxyphenol, 4-ethyl-2-methoxyphenol, 2-methoxyphenol, cis-oak lactone, trans-oak lactone, sotolone, and vanillin) and the further oak related odorants (3-ethylphenol, 4-ethylphenol, cis-isoeugenol, trans-isoeugenol, 126 In Advances in Wine Research; Ebeler, Susan B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

4-methyl-2-methoxyphenol, 3-methylphenol, 4-methylphenol, and 4-propyl-2-methoxyphenol) were quantitated in both wines by means of stable isotope dilution assays (Table 2).


Table 1. FD-Factors of Selected Odorants in Dornfelder Young Wine Stored without and with Oak Chips aroma compound

FD-factor without oak chips

with oak chips

4-allyl-2-methoxyphenol

128

512

β-damascenone

512

64

2,6-dimethoxyphenol

< 1

128

4-ethyl-2-methoxyphenol

< 1

32

2-methoxyphenol

32

256

cis-oak lactone

< 1

32

trans-oak lactone

< 1

32

sotolon

1024

4096

vanillin

1024

8192

The concentrations of most compounds were significantly increased in the wine treated with oak chips, in particular vanillin, 2, 6-dimethoxyphenol, cis-oak lactone, 4-ethyl-2-methoxyphenol, trans-isoeugenol, 4-methyl-2-methoxyphenol, cis-isoeugenol, and 4-propyl-2-methoxyphenol. 2-Methoxyphenol, trans-oak lactone, 4-allyl-2-methoxyphenol, sotolon, and 3-ethylphenol showed a moderate increase in their concentrations, while 4-methylphenol and 3-methylphenol remained nearly constant. The next step was to confirm that all oak related odorants do contribute to the change in the overall aroma observed for the oaked red wine

Spiking Experiment For the spiking experiment, the young Dornfelder wine was spiked with 13 odorants showing an increase concentration in the wine treated with oak chips. The added amounts of odorants were chosen in the way that the concentrations in the resulting wine were the same as in the Dornfelder stored with oak chips. To see the change in the overall aroma, aroma profile analyses were performed (Figure 2). 127 In Advances in Wine Research; Ebeler, Susan B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 2. Concentrations of 17 Odorants in the Young Dornfelder Red Wine Stored without and with Oak Chips. Selection Was Done in Particular for Compounds Showing Increased/Decreased FD Factors during the AEDA


aroma compound

concna [µg/L] in Dornfelder wine stored without

with oak chips

vanillin

50.0

701.0

2,6-dimethoxyphenol

2.95

104.0

cis-oak lactone

n.d.b

27.9

2-methoxyphenol

1.80

16.8

trans-oak lactone

n.d.b

6.88

4-ethyl-2-methoxyphenol

0.04

3.06

4-allyl-2-methoxyphenol

5.72

16.5

sotolon

0.99

2.59

β-damascenone

2.14

2.33

trans-isoeugenol

0.40

26.0

4-methyl-2-methoxyphenol

0.23

16.0

cis-isoeugenol

0.11

3.00

4-propyl-2-methoxyphenol

0.02

1.00

3-ethylphenol

0.66

3.15

4-methylphenol

0.47

0.96

3-methylphenol

0.44

0.85

4-ethylphenol

0.26

0.28

a calculated as the mean value of at least three different workups; standard deviation ≤ 20 % b not detected.

The aroma profiles of the spiked wine and the wine stored with oak chips stored wine were in quite good agreement. Almost all odor impressions were rated very similar in both samples except the smoky and woody odors, which were perceived more intensely in the spiked wine. The compounds causing these odor attributes are currently under investigation, because these are also present in Whisky and Cognac stored in oak barrels (unpublished results). The results, however, indicate that compounds, such as such as furfural, 5-methylfurfural, and syringaldehyde, which were proposed as important odorants in oak stored wines in the literature, do not contribute to the overall aroma of the spiked Dornfelder wine. On the other hand, cis-isoeugenol and trans-isoeugenol, which to our knowledge have not been discussed in the literature as being relevant for the oak aroma of wine, clearly contributed to the overall aroma. The data confirm our previous results on the influence of a smaller set of odorants in generating a barrique-type aroma (8). 128 In Advances in Wine Research; Ebeler, Susan B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 2. Aroma profile analysis of the spiked Dornfelder wine (grey) and the young wine stored with oak chips (black).

Conclusion The application of the Sensomics concept on a Dornfelder red wine spiked with oak chips with the respective non-spiked wine of the same batch clearly revealed 13 odorants which were transferred from the oak into the red wine during a 15 days storage. Although all odorants, except the two eugenol isomers, have previously been reported as volatiles in oak wood, this is the first study confirming the contribution of the identified compounds to the aroma differences by sensory experiments. The data are a good basis for further studies on the influence of the signature of the considered set of key odorants on the aroma profiles of different red wines with a pronounced barrique-type aroma.

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130 In Advances in Wine Research; Ebeler, Susan B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Model Study on Changes in Key Aroma Compounds of Dornfelder

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