Quercetin in Wine—Formation, Fermentation, and Health

Quercetin is a flavonoid found in various fruits and vegetables, including grapes, where it contributes to pigmentation and potential health benefits. In wine, it affects taste and offers antioxidant properties. However, quercetin levels vary due to factors from grape cultivation to winemaking techniques.

Understanding how quercetin develops in grapes, changes during fermentation, and differs among wine types provides insight into wine quality and health effects.

Formation In Grapes

Quercetin biosynthesis in grapes is influenced by genetic, environmental, and physiological factors. As a flavonol, it belongs to the polyphenol class and is synthesized through the phenylpropanoid pathway. This process begins with phenylalanine converting into cinnamic acid via phenylalanine ammonia-lyase (PAL), followed by enzymatic reactions leading to flavonol production. Flavonol synthase (FLS) gene expression plays a key role in quercetin accumulation, with variations among grape cultivars. Red grape varieties, particularly those with thick skins, tend to have higher quercetin concentrations due to increased flavonol biosynthesis in response to environmental stimuli.

Sunlight exposure significantly affects quercetin levels in grape skins. Ultraviolet (UV) radiation, particularly UV-B, upregulates genes involved in flavonol biosynthesis, increasing quercetin accumulation. Research in the Journal of Agricultural and Food Chemistry found that grape clusters exposed to direct sunlight can have up to three times higher flavonol content than shaded clusters. This response serves as a protective mechanism, as flavonols absorb UV radiation and mitigate oxidative stress. Vineyard practices such as canopy thinning and leaf removal are used to optimize sunlight exposure, though excessive exposure can cause sunburn and stress-related issues.

Beyond light exposure, the developmental stage of the grape influences quercetin accumulation. Flavonol biosynthesis begins early in berry development, peaking during veraison—the transition from green to ripening. During this period, metabolic shifts increase anthocyanin production in red varieties and enhance flavonol synthesis. Some studies suggest that quercetin levels decline as grapes reach full ripeness due to flavonol degradation or incorporation into complex polyphenolic structures. This highlights the importance of harvest timing in determining flavonol composition.

Influences Of Soil And Climate

Soil composition and climate conditions significantly shape quercetin levels in grapes. Soil characteristics such as mineral content, pH, and water retention affect vine metabolism and secondary metabolite production. Studies show that well-drained soils with high organic matter promote balanced vine growth and optimal flavonol accumulation. In contrast, nutrient-poor or overly fertile soils can lead to stress-induced flavonol synthesis or dilution effects that lower quercetin concentrations.

Micronutrients like nitrogen, phosphorus, and potassium influence flavonoid biosynthesis. Research in Frontiers in Plant Science indicates that moderate nitrogen levels support flavonol synthesis by balancing vegetative growth and secondary metabolism. Excessive nitrogen suppresses flavonol accumulation by promoting canopy growth, reducing sunlight penetration. Potassium, essential for enzyme activation and osmotic regulation, enhances flavonoid concentrations when present at optimal levels. Soil pH also affects nutrient availability, with acidic soils often boosting flavonol synthesis by increasing metal ion solubility, which aids flavonoid biosynthesis.

Climatic factors such as temperature, precipitation, and solar radiation further impact quercetin accumulation. Temperature fluctuations influence enzymatic activity in flavonoid biosynthesis, with warm conditions generally increasing flavonol production. A study in Food Chemistry found that grapevines exposed to moderate heat stress (30–35°C) showed upregulated flavonol synthase (FLS) genes, leading to higher quercetin levels. However, excessive heat accelerates ripening, potentially reducing flavonol concentrations by shortening the biosynthesis window.

Water availability also affects quercetin content by influencing vine stress responses. Controlled water deficit strategies, such as regulated deficit irrigation (RDI), enhance flavonol accumulation by inducing mild drought stress. Research in Journal of Experimental Botany found that moderate water stress activates metabolic pathways that increase polyphenol production, including quercetin. However, prolonged drought or excessive rainfall can negatively impact vines, either by overstressing them or diluting flavonoid concentrations through excessive water uptake.

Changes During Fermentation

Fermentation alters quercetin’s concentration, solubility, and structure. Maceration plays a key role in determining initial quercetin levels in the must. Since quercetin is concentrated in grape skins, extended maceration increases extraction, especially in red winemaking. Alcohol enhances this process by disrupting cell membranes, facilitating flavonol release. Extraction is further influenced by temperature, pH, and enzyme activity, which affect quercetin’s solubility and stability.

Microbial metabolism also affects quercetin levels. Yeast strains, particularly Saccharomyces cerevisiae, interact with polyphenolic compounds, sometimes leading to quercetin loss as flavonols bind to mannoproteins and precipitate out of the wine. This effect is more pronounced in wines aged on lees, where prolonged yeast contact alters polyphenolic composition. Enzymatic activity during fermentation, including glycosidase action, may also hydrolyze quercetin glycosides into aglycone forms, affecting solubility and bioavailability.

Oxygen exposure during fermentation and aging influences quercetin stability. While controlled oxygenation can enhance color stability and tannin integration, excessive oxygen leads to flavonol degradation. This is particularly relevant in white winemaking, where oxidative conditions are more prevalent due to the absence of protective anthocyanins. Sulfur dioxide additions help prevent oxidative polymerization, though excessive sulfites can impact quercetin solubility and precipitation dynamics.

Analytical Detection

Measuring quercetin in wine requires precise analytical techniques due to its presence in free and glycosylated forms. Sample preparation often includes hydrolysis to break down quercetin glycosides into their aglycone form for consistent quantification. The complexity of the wine matrix, which contains organic acids, tannins, anthocyanins, and other flavonoids, necessitates methods with high specificity and sensitivity.

High-performance liquid chromatography (HPLC) is the standard for quercetin analysis, particularly when coupled with UV-visible or mass spectrometric detection. HPLC-UV efficiently separates and quantifies quercetin based on its absorbance at 370 nm, while HPLC-MS identifies molecular ion fragments. Advances in ultra-high-performance liquid chromatography (UHPLC) have improved resolution and analysis speed, allowing for more accurate quantification of trace quercetin levels. These techniques are widely used in research and quality control for assessing flavonol content in commercial wines.

Variation Among Wine Types

Quercetin concentration varies by grape variety, winemaking techniques, and aging conditions. Red wines typically contain higher quercetin levels than white wines due to extended skin contact during fermentation. Since quercetin is primarily found in grape skins, prolonged maceration in red winemaking extracts more flavonols, resulting in a richer polyphenolic profile. White wines, which undergo minimal skin contact, have significantly lower flavonol concentrations. Rosé wines, with shorter maceration periods than reds, exhibit intermediate quercetin levels.

Aging conditions also affect quercetin stability. Wines aged in oak barrels may experience slight flavonol reductions due to adsorption onto wood surfaces and oxidative changes. Conversely, bottle aging under controlled conditions can preserve or even enhance quercetin bioavailability, depending on interactions with tannins and other polyphenols. The variations among wine types highlight the complex relationship between grape composition and winemaking practices in determining quercetin levels.