15
Flavonols

15.1 Introduction

In nature, flavonols are perhaps the most diverse of the non‐polymeric flavonoids. Although the number of flavonol aglycones is limited (only six are known in grapes and wines), they possess a tremendous diversity of glycosides, both in terms of the position and type of sugar substituent. The principal aglycones in grape include quercetin, myricetin, laricitrin, and kaempferol with generally lesser levels of isorhamnetin and syringetin (Figure 15.1), although there are varieties with substantial amounts of the latter compounds. Additionally, because of their photoprotective effect in unripe fruit, their total concentrations show one of the strongest and most consistent increases in response to fruit sunlight exposure of any grape compound.

Figure 15.1 Flavonol aglycones found in wine

15.2 Concentrations of flavonols

Flavonols occur in a very wide range of vegetable food sources. This class of compounds is always present in glycosidic form in plants including grape berries, where it is found in grape skin. The major glycosides in grapes include the 3‐O‐glucosides as well as the 3‐O‐glucuronides, and these appear in the wine [1].

Flavonol concentrations in wines are dependent on skin extraction, so white wines have much less than reds. The hydrolysis of the glycosides during wine storage (Chapter 25) complicates studies of typical glycoside and aglycone concentrations found in wine and the amount of aglycone increases with time; however, there are a number of survey reports with an adequate number of wines to provide a good sense of typical values (Table 15.1).

Table 15.1 Levels of flavonols in red and white wines. Data from References [1] and [2]

	Red wine		White wine
	Range	Mean	Range	Mean
Quercetin‐3‐galactoside	ND–6	2	ND	ND
Quercetin‐3‐glucoside	ND–14	2	ND	ND
Quercetin‐3‐glucuronide	ND–113	17	ND	ND
Isorhamnetin‐3‐glucoside	ND–4	1	ND–0.14	ND
Syringetin‐3‐glucoside	1–27	7	ND	ND
Laricitrin‐glucoside	1–23	6
Myricetin‐3‐glucuronide	1–12	3.5
Myricetin‐3‐glucoside	1–57	15
Myricetin	2.70–28.78	11.59	ND	ND
Quercetin	3.49–37.36	16.18	ND–2.05	ND
Laricitrin	0.32–4.77	1.81	ND	ND
Kaempferol	ND–2.46	0.82	ND	ND
Isorhamnetin	ND–18.12	3.45	ND	ND
Total glycosides	1.42–55.69	16.16	ND–0.80	0.05
Total aglycones	7.42–76.51	33.85	ND–2.26	0.12

ND: not detected or quantified, blank: no data.

A metabolomic study of 91 grape varieties, conducted with hydrolysis of all the glycosides such that the flavonols were quantified solely as the aglycones, showed that the amount of myricetin averaged about the same as that of quercetin, at about 12 mg/kg, with lesser amounts of the other four aglycones (1–2 mg/kg). However, some varieties had much greater amounts of either myricetin or quercetin, with ratios ranging as high as 3–4 in either direction [3]. The same study observed that white varieties lack aglycones possessing B‐rings with three oxygenated substituents, suggesting a loss of the flavonoid 3ʹ,5ʹ‐hydroxylase enzyme activity [3].

15.3 Effects of growing conditions and winemaking

Initial work on Pinot Noir indicated that berry sunlight exposure strongly enhanced the levels of the flavonols [4]. This observation was later confirmed in Merlot, where a 10‐fold increase was observed between shaded and sun‐exposed clusters, while temperature had no effect [5]. Sunlight exposure upregulates genes encoding for flavonol synthase [6]. Since flavonols absorb UV light strongly at 360 nm, and they are located mostly in the outermost layer of cells in the berry, it appears that the plant produces these compounds as a natural sunscreen. Because flavonols are correlated with grape sun exposure, and sun exposure has been shown to correlate with many other quality parameters, flavonol concentrations have been proposed as a general quality marker for red grapes [7].

As mentioned earlier, flavonols must be extracted from skins during maceration, and are thus at higher concentrations in red wines. While the glycosides have good solubility, the aglycone quercetin is particularly insoluble in aqueous solutions. As wine ages, the glycosides are hydrolyzed (Chapter 25) and the resulting quercetin is then liable to precipitate and form a haze or deposit in the bottle. Rutin, the 3‐O‐rutinoside of quercetin is occasionally reported in wine, but a detailed study has shown that while rutin is found in grapes, it hydrolyzes in wine within 1–2 days, while other glycosides are stable during that time [2]. The aglycones can be removed by polyvinylpolypyrrolidone (PVPP) treatment, but this treatment is not very effective for glycoside removal [8]. However, it is possible to measure levels of quercetin in wine that are much higher than the point at which saturation occurs in model wine solutions. Some wines, for example, Sangiovese, can throw a precipitate of quercetin on aging, presumably from the hydrolysis of glycosides and resulting supersaturation. The flavonols, particularly quercetin, are well documented to provide a strong co‐pigmentation effect with anthocyanins (Chapter 16), which may explain their stability at supersaturated levels, although the relative importance of flavonols to co‐pigmentation under wine conditions is debated [9].

The flavonols are bitter, but at the levels found in wine it is not clear if these substances make a large contribution to flavor. One multivariate statistical study of different wines did not observe a correlation between bitterness and higher levels of flavonols [10] although it is possible that other compounds could have overwhelmed their effect. A different study that added back phenolic fractions found some bitterness associated with the fractions high in flavonols [11]. Others have found that flavonols possess “velvety astringency,” a concept discussed later (Chapter 33) [12].

While not often discussed in detail, grapes also contain small amounts of dihydroflavonols (flavanonols), first reported by Trousdale and Singleton [13], and by others more recently [2, 14].

References

1. 1. Castillo‐Munoz, N., Gomez‐Alonso, S., Garcia‐Romero, E., Hermosin‐Gutierrez, I. (2007) Flavonol profiles of Vitis vinifera red grapes and their single‐cultivar wines. Journal of Agricultural and Food Chemistry , 55 (3), 992–1002.
2. 2. Jeffery, D.W., Parker, M., Smith, P.A. (2008) Flavonol composition of Australian red and white wines determined by high‐performance liquid chromatography. Australian Journal of Grape and Wine Research , 14 (3), 153–161.
3. 3. Mattivi, F., Guzzon, R., Vrhovsek, U., et al. (2006) Metabolite profiling of grape: flavonols and anthocyanins. Journal of Agricultural and Food Chemistry , 54 (20), 7692–7702.
4. 4. Price, S.F., Breen, P.J., Valladao, M., Watson, B.T. (1995) Cluster sun exposure and quercetin in Pinot noir grapes and wine. American Journal of Enology and Viticulture , 46 (2), 187–194.
5. 5. Spayd, S.E., Tarara, J.M., Mee, D.L., Ferguson, J.C. (2002) Separation of sunlight and temperature effects on the composition of Vitis vinifera cv. Merlot berries. American Journal of Enology and Viticulture , 53 (3), 171–182.
6. 6. Downey, M.O., Harvey, J.S., Robinson, S.P. (2004) The effect of bunch shading on berry development and flavonoid accumulation in Shiraz grapes. Australian Journal of Grape and Wine Research , 10 (1), 55–73.
7. 7. Ritchey, J.G. and Waterhouse, A.L. (1999) A standard red wine: monomeric phenolic analysis of commercial Cabernet Sauvignon wines. American Journal of Enology and Viticulture , 50 (1), 91–100.
8. 8. Laborde, B., Moine‐Ledoux, V., Richard, T., et al. (2006) PVPP‐polyphenol complexes: a molecular approach. Journal of Agricultural and Food Chemistry , 54 (12), 4383–4389.
9. 9. Lambert, S.G., Asenstorfer, R.E., Williamson, N.M., et al. (2011) Copigmentation between malvidin‐3‐glucoside and some wine constituents and its importance to colour expression in red wine. Food Chemistry , 125 (1), 106–115.
10. 10. Saenz‐Navajas, M.P., Ferreira, V., Dizy, M., Fernandez‐Zurbano, P. (2010) Characterization of taste‐active fractions in red wine combining HPLC fractionation, sensory analysis and ultra performance liquid chromatography coupled with mass spectrometry detection. Analytica Chimica Acta , 673 (2), 151–159.
11. 11. Preys, S., Mazerolles, G., Courcoux, P., et al. (2006) Relationship between polyphenolic composition and some sensory properties in red wines using multiway analyses. Analytica Chimica Acta , 563 (1–2), 126–136.
12. 12. Hufnagel, J.C. and Hofmann, T. (2008) Orosensory‐directed identification of astringent mouthfeel and bitter‐tasting compounds in red wine. Journal of Agricultural and Food Chemistry , 56 (4), 1376–1386.
13. 13. Trousdale, E.K. and Singleton, V.L. (1983) Astilbin and engeletin in grapes and wine. Phytochemistry , 22 (2), 619–620.
14. 14. Vitrac, X., Castagnino, C., Waffo‐Teguo, P., et al. (2001) Polyphenols newly extracted in red wine from southwestern France by centrifugal partition chromatography. Journal of Agricultural and Food Chemistry , 49 (12), 5934–5938.