According to many studies, the aromatic composition of grape berries may provide information for predicting the quality of the finished wine. Aromatic compounds in the grape berry come from metabolic processes, mainly in the forms of terpenes, norisoprenoids, benzenes, and C6 alcohols. Volatile compounds, particularly those in the free form, are directly involved with the aroma of the finished wine. Other compounds, such as bound glycosides, are odorless but can contribute to the aroma of a finished wine when they are transformed into volatile compounds by the process of hydrolysis.
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Where these volatile compounds are located in the grape is not equal, in that many more of them are located within the skins of the berry. The concentrations of specific volatile compounds are different between varietals as well, and may vary greatly depending upon a variety of external factors, such as climate, terroir, viticulture practices, and winemaking practices. Even within the same grape vine, different clusters of grapes may vary in their concentrations of volatile compounds. One study even found that there are significant differences in the volatile composition of grapes located on the shoulder of a single cluster, and grapes located on the tip of that same cluster. Finally, the stage of ripening plays one of the biggest roles in the aromatic characteristic of grapes, since free and bound compounds are being accumulated and altered throughout the maturation process.
The goal of the study presented today was to examine in detail the volatile composition of Brancellao berries (V. vinifera), a grape native to Galicia, NW Spain, that is also known as Serradelo and Alvarelhao (in Portugal). The authors also examined grapes from two different positions within the grape cluster; the shoulder and the tip. The results of this study could have important implications for winemaking practices and Brancellao wine quality.
Methods
Brancellao grapes were harvested in 2009 from an experimental vineyard in Salcedo, Pontevedra (NW Spain). The vineyard area was 1.4ha, and the soil was sandy loam (70.1% sand, 16.1% silt, 13.8% clay) with 7.3% organic matter. Varietal identity was confirmed using genetic microsatellite protocols. Vines were planted in 1993, and all were subjected to identical pruning and cultivation practices.
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| Figure 1 from Noguerol-Pato et al, 2012 |
At veraison, some clusters were dropped, leaving the same number of clusters on each vine. Three sampling sessions were performed between September and October of 2009, with 10 day intervals in between each sampling. The first session occurred on 1/3 of the vines, taking care to separate grapes depending upon if they were located on the shoulder of the cluster or on the tip (see Figure 1 for these locations). For each sampling period, about 3kg of grapes were collected. The second and third sessions occurred using the same exact methods; however, the grapes were sampled on different grape vines than the session prior.
For each sample, probable alcohol degree and total acidity were measured, as well as volatile compound composition. The samples were separated not only by location on the cluster, but were also separated by skins and flesh. For all samples, odor activity values (OAV) were calculated. These values identify the contribution of each volatile compound to grape aroma. OAV’s were calculated by dividing the total concentration of the volatile compound by its’ odor threshold value.
Results
Individual Compounds
- There were significant differences between grapes at the tip and at the shoulder in regards to probable alcohol degree at the first sampling session.
o No significant differences were found in the 3rdsampling.
- Total acidity of must showed significant differences in the 3rd sampling.
o Highest levels were found in the shoulders.
- Free monoterpenes in berry flesh increased 40% in tips and in shoulders in the 3rd sampling when compared with the 1st sampling.
o This increase was mainly due to a 60% increase in geraniol.
o Increases were relatively similar between tip and shoulder grapes, however, in absolute terms, the tips should higher levels of monoterpenes in the 3rd sampling compared with the shoulders.
- Free monoterpenes were associated with the grape skins.
o These compounds were higher in the 3rdsampling for both cluster positions.
§ For tips, this increase was due to geranic acid.
§ For shoulders, there were no significant increases were found for any monoterpene, though geranic acid tended to increase, leading to 39% increase of monoterpenes by the 3rdsampling.
- Bound monoterpenes were higher than free monoterpenes in the flesh of the berries at all times.
o Linalool increased the most in all cluster locations and times.
- Free norisoprenoids were reduced the 3rdsampling for both tips and shoulders, as well as in both the skins and the flesh of the berries.
- There were no changes in bound norisoprenoids.
- Free aldehydes were found mainly in the skins.
o The two compounds found in highest concentrations were hexanal and trans-2-hexenal.
o In berry flesh, these compounds decreased during ripening in the tips and increased during ripening in the shoulders.
o In skins, there were no significant differences between tips and shoulders.
o At the 3rd sampling, grapes at the shoulders had higher levels of aldehydes than the tips, except for trans-2-hexenal in the skins.
- For bound aldehydes, the only significant differences were found in the berries from the tips; hexanal in the flesh decreased 55% and trans-2-hexenal in the skins increased 116%.
- Highest levels of free C6 alcohols were for hexanol and trans-2-hexenol.
o In flesh, these compounds decreased by 33% in the tips, and 47% in the shoulders.
o In skin, hexanol decreased in the shoulders whereas trans-2-hexenol decreased in the tips.
o Berries from the tips showed highest levels of C6alcohols in the flesh, whereas berries in the shoulder showed highest levels of these compounds in the skin.
- For free aromatic alcohols, there appeared to be a transfer from the skin to the flesh throughout the sampling process.
o This is likely due to bound aromatic alcohols decreasing in the skin and increasing in the flesh in free form.
- Lower free volatile phenol levels were found in flesh compared to the skin.
o In flesh, these compounds decreased in the 3rdsampling, but increased in the skin in both cluster positions.
- Bound volatile phenols were found mostly in the flesh of berries.
o In tips, concentrations increased throughout the sampling process, whereas in the shoulders, levels remained relatively constant (except for vanillin, which increased by 80%).
- For Brancellao grapes, it appears as though grapes located at the tip of the cluster are ahead in developing aromatic compounds than grapes located at the shoulder.
General Chemical Classes
- In the 3rd sampling, monoterpenes were dominant in the flesh, and their free forms were dominant in the skins.
o Berries from the tips had higher levels of these compounds in the flesh, whereas berries from the shoulders had higher levels of these compounds in the skins.
o Tips had higher concentrations of monoterpenes in general, though mostly in the bound form.
- Norisoprenoids were higher in the skins than in the flesh for either position.
o In the skins, these compounds were mostly in the free form, whereas in the flesh, most of these compounds were in the bound form.
o Both positions had similar levels of norisoprenoids, though the tips showed higher levels of the bound form.
- Aldehydes were mostly concentrated in the skin in the free form, with no significant differences between tips and shoulders.
o There were more free aldehydes in flesh of the shoulder berries compared with the tips, where they were mainly in the bound form.
- C6 alcohols were mainly present in free form.
o In the tips, these compounds were found mostly in the flesh, whereas in the shoulders, these compounds were found in the skins (4x higher than in the flesh).
o Generally speaking, grapes from the tips had higher concentrations of C6 alcohols than grapes from the shoulders.
- Aromatic alcohols were present mainly in the bound form.
o These alcohols were rarely present in the skin, and were found mostly in the flesh.
o These compounds were found in higher levels in the tips than in the shoulders.
- Volatile phenols showed highest levels in the bound form in the flesh, particularly in the tips whereas the skins showed higher levels of the free volatile fraction.
o In general, grapes from the tips had higher levels of volatile phenols, mainly in the bound form.
- For miscellaneous compounds, pantolactone was the highest contributor.
o This compound appeared only in the flesh of berries, with higher levels occurring in the shoulders.
o Free pantolactone was similar for both tips and shoulders, whereas bound pantolactone was contained more in the shoulders.
- In general, concentrations of free volatiles were similar for both tip and shoulder berries, with the main differences occurring with the bound forms.
OAV’s / Odor Descriptors
- OAV’s higher than 1 contributed to wine aroma.
- The terpene with the highest OAV value was linalool (orange flowers).
- Aldehydes with major sensory impacts were hexanal and trans-2-hexenal (grass).
- Eugenol (clove), 4-vinylguaiacol (phenolics), and vanillin (vanilla) all displayed high OAV values.
- The main odor contributors found in the berries were floral and herbaceous tones.
o Berries from the shoulders were richer in floral and herbaceous notes than those from the tips.
o Spicy notes were found at higher levels in the tips.
o In general, linalool (orange flowers) was the most active floral odor in the tips, whereas β-ionone (violet) was the most active floral odor in the shoulders.
Conclusions
The results of this study showed that there was very little variability between shoulders and tips in regards to probable alcohol degree and total acidity, though more importantly, significant differences were found in the aromatic composition of the grapes in the different positions.
The authors suggest that these results may be useful for wineries that are considering separating grapes from the shoulders and tips, in order to make different styles of wine with different aromatic characteristics and quality. Since there were significant differences between grapes from the shoulders and grapes from the tips, wineries would be able to combine various amounts of shoulder grapes and tip grapes at different times in order to produce a finished wine with a specific desired aromatic profile.
Also, since it was found that aromatic alcohols were found mainly in the bound form, the use of glycolytic enzymes during the maceration process could be employed in order to free the bound aromatic alcohols and increase the desired floral notes produced by the berries.
Overall, I thought this was a very fascinating experiment were interesting results that could have direct applications for improving wine quality (at least in regards to Brancellao wines). One thing I would have liked to see in this experiment was a sensory analysis comparing what they predicted would occur with the aromatic profile based on the detailed chemistry analysis with an actual sensory analysis of finished wines created from different combinations of shoulder grapes and tip grapes under different winemaking protocols.
What do you all think about this topic? Please feel free to comment below (no HTML tags, please).
Source: Noguerol-Pato, R., González-Barreiro, C., Cancho-Grande, B., Santiago, J.L., Martínez, M.C., Simal-Gándara, J. 2012. Aroma potential of Brancellao grapes from different cluster positions. Food Chemistry 132: 112-124.
DOI: 10.1016/j.foodchem.2011.10.042
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!
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