Luciano, Alessandra (2025) Understanding and Managing Quercetin Precipitation in Red Wines. [Tesi di dottorato]

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Abstract

One effect of climate change on grape composition is enhanced biosynthesis of quercetin and its glycosides in response to UV-B radiation. High concentrations of quercetin in wine can exceed its solubility threshold, resulting in undesirable precipitation that negatively affects clarity and consumer perception. This represents a quality and economic challenge for producers. This PhD project aims to improve understanding of the biochemical and physicochemical mechanisms that regulate quercetin stability and precipitation in red wines. The project will focus particularly on the factors that influence quercetin solubility and crystallisation. This PhD research has involved studies of model wine solutions and real wines, investigating multiple aspects of quercetin behaviour. Firstly, the impact of anthocyanins on quercetin solubility was evaluated by adding grape skin extracts, which are rich in anthocyanins, to model solutions containing quercetin. The results showed that increasing anthocyanin concentrations increased the solubility of quercetin and its glycosides, confirming the role of copigmentation interactions in stabilising quercetin in solution. Furthermore, Density Functional Theory calculations revealed that quercetin–flavylium complexes are more stable than alternative associations, such as catechin–flavylium complexes or flavylium self-association complexes. Additionally, these calculations showed that higher pH values reduce flavylium availability, thereby weakening copigmentation and favouring quercetin precipitation. The influence of environmental factors and nucleation seeds on quercetin crystallisation was evaluated by monitoring red wines spiked with quercetin and stored at 2°C and 20°C over a 10-day period. Contact with nucleation seeds was found to be the most significant factor driving precipitation, with quercetin concentration reducing by over 75% within three days at the lower temperature. This suggests that the controlled promotion of nucleation could be a practical strategy for accelerating stabilisation and mitigating precipitation risks in bottled wines. However, the addition of quercetin nucleation seeds is not applicable in wineries; these results suggested the necessity of evaluating the impact of other wine components on quercetin crystallisation. With this aim, the last experimental trial analysed 32 monovarietal Italian red wines from a single harvest that differed in terms of phenolic content .and base parameters. After one year of ageing, hydrolysis of quercetin glycosides occurred for all wines. While in 9 of them a deposit of quercetin was observed. Statistical Analysis of data related to wines showing or not the sediment, revealed that numerous factors influenced the solubility of quercetin, with tannin content being the most important factor increasing the solubility of quercetin rather than anthocyanin content. Regarding the risk of quercetin precipitation, PLS analysis indicates that the risk is enhanced in wines at higher pH and acetaldehyde levels. Collectively, these findings demonstrate that quercetin stability in red wines is governed by a complex interplay of factors, including anthocyanin-mediated copigmentation, pH, temperature, nucleation seeds, and the broader phenolic composition of the wine matrix. By elucidating these biochemical and physicochemical mechanisms, this work provides a comprehensive framework to predict and manage quercetin precipitation, ultimately supporting improved wine quality and colloidal stability under the challenging conditions imposed by climate change.

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