Dr. Aude Watrelot
This new research winemaking article focuses on another group of pigments found in grapes, the flavonols. This group of polyphenols is often forgot but is very important in the protection of berries during their development and ripening. It is also very important for the stabilization of red wine color.
This article will provide an overview of this group of polyphenol in terms of chemistry and the importance in viticulture and winemaking.
What are flavonols?
- Chemical structure:
Flavonols are a type of polyphenol from the flavonoid group that are colorless and yellow pigments found in grape skins. Similar to other flavonoids such as flavanols and anthocyanidins, the chemical backbone of flavonols are composed of 2 benzene cycles (A and B) on the left and right side of the central oxygenated cycle (C). Depending on the type and number of substitutions of the B-ring, the structure changes. Examples include kaempferol with one hydroxyl (OH) group, quercetin with 2 hydroxyl groups and myricetin with 3 hydroxyl groups (Figure 1A).
In grapes, those flavonols are present in glycoside form, meaning that they have a sugar moiety on the backbone structure such as the 3-O-rhamnosylquercetin, which is a quercetin with a deoxy-rhamnose, as shown in Figure 1B.
During winemaking, the sugar moiety of flavonol is hydrolyzed and the aglycone form (Figure 1A) is found in wines.
Figure 1. Chemical structure of a flavonol (A) and of a glycoside form of a flavonol (B).
- Biosynthesis in grapes:
Grape flavonols as the glycoside form (Figure 1B) are biosynthesized during the berry development, reaching the highest concentration at flowering. The concentration of flavonols tends to decrease with the increase of the berry size. Flavonols act as “sunscreen” for berries, meaning that they protect the berries from the UV radiation during development. The quantity and quality of light and temperature on the berries influences the concentration of flavonols. It has been observed that high levels of UV exposure led to increased flavonol levels in Cabernet sauvignon (Blancquaert et al., 2019). The main flavonols found in this grape variety were quercetin-3-O-glucoside and quercetin-3-O-glucuronide (Blancquaert et al., 2019; Mattivi et al., 2006).
Why are flavonols important in wine?
During the wine making process, flavonols as the glycoside form such as quercetin-3-O-glucoside, are extracted from grape skins and hydrolyzed during alcoholic fermentation. This leads to a cleavage of the glycoside group and release of the flavonol unit, such as quercetin (Figure 2).
Figure 2. Hydrolysis of quercetin-3-O-glucoside into quercetin during alcoholic fermentation of grape skins.
Therefore, flavonols are found in higher concentration in red wines because alcoholic fermentation occurs on grape skins, unlike in white wines. The average content is around 100 mg/L of flavonols in red wine compared to about 1-3 mg/L in white wines. In interspecific hybrids, quercetin was not detected in Marquette, Corot noir, and Marechal Foch musts. It was only detected at about 0.3 and 0.5 mg/L in Corot noir and Marechal Foch wines, respectively after malolactic fermentation (Manns et al., 2013). Because they are found in grape skins and in red wines, flavonols are able to react with anthocyanins through co-pigmentation, as explained in the newsletter article of August 2020. Therefore, flavonols have a role in co-pigmentation and red wine color stabilization.
If you have any suggestions or questions about the research/winemaking topic, feel free to contact me at firstname.lastname@example.org.
References to learn more:
Blancquaert, E. H., Oberholster, A., Ricardo-da-Silva, J. M., & Deloire, A. J. (2019). Grape Flavonoid Evolution and Composition Under Altered Light and Temperature Conditions in Cabernet Sauvignon (Vitis vinifera L.). Frontiers in Plant Science, 10. https://doi.org/10.3389/fpls.2019.01062
Manns, D. C., Lenerz, C. T. M. C., & Mansfield, A. K. (2013). Impact of Processing Parameters on the Phenolic Profile of Wines Produced from Hybrid Red Grapes Maréchal Foch, Corot noir, and Marquette. Journal of Food Science, 78(5), C696–C702. https://doi.org/10.1111/1750-3841.12108
Mattivi, F., Guzzon, R., Vrhovsek, U., Stefanini, M., & Velasco, R. (2006). Metabolite Profiling of Grape: Flavonols and Anthocyanins. Journal of Agricultural and Food Chemistry, 54(20), 7692–7702. https://doi.org/10.1021/jf061538c