Effects of Non-Grape Materials (MOG) on wine quercetin composition: insights from synthetic and Merlot grape juice fermentation

Abstract

Quercetin precipitation has become an increasingly common issue in red wine, often resulting in visually unpleasant sediments and diminished product quality. As mechanical harvesting becomes more widespread, the unintentional incorporation of material other than grapes (MOG), including leaves and stems, during harvest is becoming more prevalent. While MOG is known to influence the chemical and sensory properties of wine [1, 2], its potential role in affecting quercetin concentration and triggering precipitation remains poorly understood. Given that climate change is expected to further exacerbate quercetin instability, investigating the impact of MOG on wine composition has become increasingly relevant. This study investigated the effects of grape leaves and stems, added at 1%, 2%, and 3% (w/v), on fermentation kinetics, polyphenol content, color, and quercetin evolution during alcoholic fermentation in both synthetic and Merlot musts. Grape leaves and stems exerted distinct, dose-dependent effects on wine composition during fermentation. Grape leaves led to a steady increase in quercetin-3-glucuronide and quercetin-3-glucoside during the early to mid-fermentation stages. These levels remained stable in synthetic must but showed a slight decline in Merlot must toward the end of fermentation. Quercetin aglycone gradually accumulated from mid to late fermentation, especially in synthetic must. In contrast, grape stems caused only minor changes in all quercetin forms. Total polyphenol content also increased with both MOG types, primarily during early fermentation. Leaves promoted a stronger and more sustained release in synthetic must, whereas stems contributed to higher final polyphenol levels in Merlot must. Stems also significantly accelerated fermentation compared to leaves. While leaves were the main source of quercetin derivatives, stems notably increased catechin and proanthocyanidin concentrations, especially in synthetic must. In terms of color, leaves enhanced red pigmentation and darkened the wine, while stems reduced color intensity and increased lightness, shifting tonality toward yellow. These findings demonstrate that grape leaves and stems exert distinct, dose-dependent effects on wine composition. Grape leaves promote quercetin enrichment and may increase the risk of precipitation, whereas stems primarily affect fermentation kinetics and catechin concentration. Therefore, careful management of MOG levels is crucial, especially in mechanically harvested grapes and in regions vulnerable to quercetin instability and climate-related variability in wine quality.

References

[1] Capone DL, Barker A, Pearson W, Francis IL (2021) Influence of inclusion of grapevine leaves, rachis and peduncles during fermentation on the flavour and volatile composition of Vitis vinifera cv. Shiraz wine. Aust J Grape Wine Res 27:348–359.

[2] Guerrini, L., Masella, P., Angeloni, G., Calamai, L., Spinelli, S., Di Blasi, S., & Parenti, A. (2018) Harvest of Sangiovese grapes: the influence of material other than grape and unripe berries on wine quality. European Food Research and Technology 244:1487–1496.