For today’s post, we will set out to examine a paper published in March of this year in the Journal of Agriculture and Food Chemistry regarding the aroma of wine and possible viticulture techniques for altering aroma of varietal wines.
It has been well-documented in the literature (with one specific article reviewed here on this blog), that application of smoke during veraison alters the chemical profile of the grapes, must, and more importantly the finished wine, which results in changes in the aroma and flavor of the finished wine. The aroma of a wine comes in two forms; one as volatile compounds, and the other as odorless precursors, which over time will undergo a chemical change, become volatile, and eventually released. These volatile compounds change, depending upon the varietal and terroir effects (soil, climate, environment, etc). For example, research has shown that vineyards planted adjacent to eucalyptus forests result in finished wines with aromas of eucalyptus. Also, studies have shown that certain the applications of certain fungicides can influence the aroma of a finished wine. Finally, as we’ve already discussed once in a post on this blog, application of smoke to grape vines (during veraison) results in changes in the aroma of the finished wine after bottle ageing.
One way in which winemakers purposely alter the aromatic profile of a wine is to ferment and age in oak barrels. Wines that are in contact with oak barrels will often exhibit aromas of wood, spices, toasted/smoke, and so forth, which are caused by chemical compounds such as oak lactones, eugenol, vanillin, and guaiacol, etc. Since it has been shown that wine absorbs these chemical compounds which alter the flavor of the finished wine, and application of specific substances (smoke, vicinity to eucalyptus forests, fungicide, etc) to grape vines alters the flavor of finished wines, it is possible that by applying an oak extract containing compounds known for causing these “oaked” flavors, the aroma of the resulting wine would be likened to one that had been fermented and aged in oak barrels. This could have significant implications for viticulture and wine-making practices, whereby if successful, could negate the need for purchasing costly oak barrels.
The overall goal of the study reviewed today was to determine if application of an oak extract to Verdejo wines would transfer the oak flavor to the finished wine, in lieu of oak barrel fermentation or ageing, or the use of oak chips in stainless steel tanks.
The oak extract used in this study was a food-grade extract used in spirits and fruit juices, which was made by macerating French-toasted oak chips from natural seasoning for at least 18 months in demineralized water at 100o for 32 hours.
The grape vines used were Verdejo varietals, grown in the La Mancha region of Spain (Albacete province, southeast Spain).
The oak extract was applied to the grapes during veraison, where skin permeability is at its greatest. Extracts were applied in 100% concentrations (without dilution), 25% concentrations (diluted with water at 25%), a control (no oak extract), and a standard chemical solution of eugenol plus guaiacol.
For each of these four treatments, an entire row of 188 plants was used, with two rows left untouched between treatments to avoid possible contamination. A total of 752 plants were used over the entire experiment. 230mL of each treatment was sprayed over the leaves of the grape vines in each treatment row. Grape sampling occurred a few hours before harvest, alternatively picking grapes from both north and south-facing clusters. Grapes from all of the clusters (of each treatment) were destemmed and mixed together for analysis.
When the Baumé/titratable acidity ratios were between 2.5 and 3, grapes were harvested and subsequently processed. Must was fermented (without skins) in stainless steel tanks. Enological parameters were measured for each treatment, to determine if extract treatments change the natural characteristics of the grapes themselves. Degrees Baumé, reducing sugars, titratable acidity, volatile acidity, pH, alcohol, and yeast assimilation nitrogen were all measured. Volatile components of both the grapes and the finished wine were measured.
For the sensory analysis, a panel of eight judges (three females and five males; between the ages of 25 and 50 years old), analyzed the wines for sensory characteristics. Wines were analyzed right at the end of alcoholic fermentation, and also after 6 months of ageing. The sensory analysis detailed 17 attributes grouped by a visual phase (sight), an olfactory phase (smell), and a gustatory phase (taste/mouthfeel). The wines produced from the treatment of grapes with eugenol plus guaiacol did not undergo sensory analysis, because the chemicals are not food-grade, and thus may have caused health risks toward the expert judges.
Grape Enological Parameters
- All enological parameters of the grapes were equal across all treatment types
Oak Extract Volatiles
- The concentration of trans-oak lactone in the extract was 2x greater than cis-oak lactone.
o These compounds produce the “wood”, “toasted” and “coconut” aromas. In American oak barrels, the cis-oak lactone concentration is higher than the trans-oak lactone, and in French oak barrels, the two are nearly the same.
- Furanic compounds were among the most abundant in the oak extract, and is often found in wood that is subject to high-temperature treatments (i.e. toasting of barrels).
- 4-vinyl-guaiacol was found to be in the normal range, however, 4-ethylguaiacol, guaiacol, and eugenol (“smokey” character) were found in significantly higher concentrations.
- Vanillin, acetovanillone, and methyl vanillate were found in very low concentrations.
- Oak lactones were not present in the grapes.
- Concentrations of furfural, guaiacol, and 4-vinylguaiacol were similar in the control grapes compared to those treated with oak extract.
- Grapes treated with oak extract contained significantly lower concentrations of 4-ethylguaiacol, and the three vanillin derivatives described above.
- Grapes treated with eugenol plus guaiacol did not exhibit an increase in these compounds when compared with the control.
o It is important to recall that volatiles take two forms, a volatile component and an odorless precursor. It is the latter than may change after fermentation and ageing, whereby when measured initially in the grapes, difference may not yet be seen.
Wine Enological Parameters
- Wine from all treatments showed no significant differences in the enological parameters described in the methods.
Wine Volatile Parameters
- Neither of the two oak lactones (cis or trans) were found in the control wines, therefore these compounds were present only because of oak extract applications.
- The highest concentrations of oak lactones were found in the 25% oak extract treatment wine, but without exceeding the perception threshold, even after 6 months of ageing.
- Furfural compounds (bitter almond aroma) were found to be in low concentrations, and below the perception threshold.
- There was an increase in 5-methylfurfural in the oak extract treated wines, and after 6 months, the untreated control wines increased these concentrations to match that of the oak extract wines.
- After 6 months in the control wine, there was an increase of eugenol, which shows that it is released from an odorless precursor.
- The wines treated with oak extract contained significantly more eugenol than the control wines, and after 6 months, the concentration increased to exceed the perception threshold.
- 6-methoxyeugenol (spicy aroma) increased significantly after 6 months in the oak extract treated wines, with greater concentrations with the 25% treatment level, which exceeded the perception threshold.
- Guaiacol was found to be higher in the oak extract treatments, though not exceeding the perception threshold.
- After alcohol fermentation, the vanillin compounds were all found to be relatively low across all treatments. After 6 months, there was a significant increase in acetovanillone and methyl vanillate, especially in the 25% treatment; however, none were higher than the perception threshold.
o It is likely these volatiles are released with ageing.
- After 6 months of ageing, there was a significant increase in eugenol, which guaiacol remained relatively constant in all treatments.
o It is likely that the eugenol precursors change to volatile components over time, and do so at a faster rate than guaiacol.
o 6 months after fermentation, there was a significant increase in 6-methoxyeugenol, which suggests that somewhere along the line there is the formation of a precursor to it as a result of the wine making process, since this compound was not found in the grapes alone.
- At the end of alcoholic fermentation, wines of all treatments showed the same “young Verdejo wine” characteristics, with the only significant differences coming from the 25% concentration oak extract-treated wines regarding the “mouthfeel” portion of the analysis.
- After 6 months:
- The control wines displayed more “yellow tones”, whereas the oak extract-treated wines shown more “green tones”.
o With Verdejo, “green tones” are more desirable, thus this change indicates an improvement in color with oak extract treatments.
- With oak extract-treated wines (especially the 25% concentration), there was a significant increase in “wood” character in the olfactory portion of the analysis.
- “Mouthfeel” and “astringency” characteristics increased significantly with oak extract-treated wines (especially the 25% concentration).
- Wines treated with the 25% oak extract concentration also exhibited the highest “persistence” and “balance” attributes scores.
Based on this extensive analysis, it is clear that over time, the chemical composition of the grapes change when the wines created from them are aged. This analysis showed some compounds were not present in the grapes themselves, but due to the wine making process, were present in the finished and more so in the aged wines.
The most significant result ties in both the chemical analysis and the sensory analysis, in that those grapes that were initially treated with oak extract in the field at veraison, resulted in finished wines that exhibited qualities of being fermented and aged in oak barrels, even though they were processed in stainless steel tanks.
It is clear from the analysis that many of the volatile compounds are present as precursors in wines immediately after alcohol fermentation, and are released at least 6 months after ageing, therefore increasing the “oaky” character of the wine over time. The results of this study showed from the sensory analysis that wines produced from oak extract-treated vines do not show much oak character when first finished with fermentation, but when given the chance to age, they display significantly more oak character over time.
This research has strong implications in viticulture and enology, and provides a potential new mechanism for imparting oak flavor into wines without spending the funds on expensive oak barrels. This technological advance could have profound implications, and represents a novel way to produce a wine in a different style from a viticulture point of view.
I’d love to hear what you all think! Please feel free to comment below!
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!