Abstract
Quercetin haze has been observed over the last few years in some aged Sangiovese wines. This problem could be due to an excess of the quercetin in the wine. Leaf removal increases the exposition of clusters to sunlight, which may enhance flavonol synthesis in the grapes. In this study, we evaluated the dynamics related to extractable flavonols in grapes grown in three usually defoliated Vitis vinifera (L.) cv. Sangiovese vineyards, whose wines showed quercetin precipitates. The particular structure of the vineyards in which the leaf removal experiments were carried out allowed the influence of vineyard, biotype and rootstock on grape flavonol contents at mid-maturation and technological maturity to be evaluated. The leaves were removed at pre-flowering (early) and at veraison (late). Leaf removal increased the content of extractable glycosidic flavonols in grapes at the two tested ripening stages. In addition, vineyard, biotype and rootstock affected the content of glycosidic flavonols and the interaction between the studied variables was significant. Even though leaf removal induced an increase in extractable quercetin glycosides which can increase the risk of quercetin haze in wine, an examination of the scientific literature on this topic showed that this risk does not depend on the absolute content of these compounds alone.
Highlights
Even though leaf removal induced an increase in extractable quercetin glycosides which can increase the risk of quercetin haze in wine, an examination of the scientific literature on this topic showed that this risk does not depend on the absolute content of these compounds alone
Since leaf removal increases the exposition of clusters to solar radiation, the aim of the present study was to determine the effect of this viticultural practice on flavonol synthesis in berries, which could be related to quercetin haze in wine
The contents of extractable flavonols, expressed as mg of quercetin equivalent per kg of grape berries (Tables 1 and 2), may seem low for Sangiovese grapes, which are usually considered to be rich in these compounds (Gambuti et al, 2020); the content of the flavonols expressed as individual glycosides fluctuating around 40 mg per kg and the fact that only a part of grape flavonols were determined with the method described in Materials and methods, confirm the richness in flavonols of Sangiovese grapes
Summary
Quercetin is a phenolic compound which belongs to the class of flavonols whose glycosides are located in the skins of white and red grapes (Cheynier and Rigaud, 1986; Makris et al, 2006, Mattivi et al, 2006; Castillo-Muñoz et al, 2007). In areas where the temperature reaches and exceeds 30 °C for long periods of time, the glycosilated flavonol content of grape usually decreases after reaching a maximum value (Keller and Hrazdina, 1998) It has not been determined whether this decrease is due to flavonol biosynthesis being blocked or to the fact that the degradation reactions exceeded the synthesis reactions. The solubility of quercetin in hydroalcoholic solution and in wine, along with the formation of quercetin deposits, has been extensively studied by Gambuti et al (2020) They found that this solubility depends on several variables, including the content of quercetin and its glycosides in wine, as well as wine composition in terms of other substances; they revealed that Sangiovese grapes and wines can be rich in quercetin and its glycosides. The particular structure of the vineyard in which the experiment was carried out allowed us to evaluate the influence of vineyard, biotype and rootstock on the OENO One 2021,4, 71-81 synthesis of quercetin derivatives at mid ripening and technological maturation
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