Exploring the chemical properties of polyphenols in cold-hardy grape cultivars

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2024-05
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Cheng, Yiliang
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Watrelot, Aude A.
Boylston, Terri D.
Clark, Stephanie
Nonnecke, Gail R.
Rosentrater, Kurt A.
Talbert, Joey N.
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Food Science and Human Nutrition
Abstract
Cold-hardy interspecific grape cultivars, such as cv. ‘Marquette’ and ‘Frontenac’, are the crossbreeding of several Vitis species, including the native North American Vitis riparia and Vitis labrusca, and the traditional European Vitis vinifera. These grape cultivars are bred to withstand the harsh winter temperatures and are less susceptible to fungal diseases and pests than V. vinifera grape cultivars, which make them suitable for the Upper Midwest wine industry. However, interspecific grape cultivars present unique challenges such as high titratable acidity and distinctive phenolic composition, including a low concentration of condensed tannins and high concentration of anthocyanins. Such attributes can make the production of well-balanced red wine a challenge, necessitating adjustments to manage flavors, acidity and astringency, in the finished wine. The main goal of this Ph.D. program was to deepen the understanding of the chemical properties of phenolic compounds of these cold-hardy grape cultivars. The objectives were to 1) identify the effective viticultural and 2) winemaking practices to enhance the quality of the resulting red wines, and 3) investigate the critical factors that limit tannin retention in red wines. In the vineyard, the metabolism of polyphenols and organic acids in grapes can be affected by vineyard management techniques, such as leaf removal. For instance, increasing sunlight exposure to grape clusters through these canopy management techniques can reduce acidity and may enhance polyphenol development. While the effects of these techniques on V. vinifera grapevines have been extensively studied, research focusing specifically on interspecific cold-hardy cultivars remains limited. Over a two-year study period, a lower concentration of malic acid was observed in ‘Marquette’ grapes at harvest, as the result of post fruit set leaf removal treatment compared to the control. In addition, this treatment led to a higher concentration of total iron-reactive phenolics, color intensity, and anthocyanins by 26%, 21%, and 51%, respectively, at harvest compared to the control. Conversely, tannin concentrations in the juice dropped markedly from véraison to harvest, but without impact of the leaf removal treatment. The reduced extractability of tannins during berry development suggests a need for winemaking techniques that can enhance tannin extraction. In order to enhance tannin extraction from ‘Marquette’ grapes into red wines, four winemaking techniques were employed: Accentuated Cut Edges (ACE), macerating enzymes, saignée, and bentonite. These methods were chosen for their potential to break down grape skins mechanically and enzymatically, adjust the solid-to-juice ratio, and remove proteins that bind to tannins during fermentation and subsequent maceration, respectively. Individually, these techniques showed limited effectiveness in extracting condensed tannins. However, the combined application of ACE with macerating enzymes and of saignée with bentonite demonstrated a significant improvement in the overall phenolic and tannin concentrations. The synergetic effect of ACE and macerating enzymes indicated that more extensive breakdown of grape skin cell walls of cold-hardy grape cultivars, is required to enhance tannin extraction. Additionally, combining saignée and bentonite treatments resulted in an increase in tannin concentration at pressing due to the increased solid-to-liquid ratio and removal of proteins. The lack of effect of the individual treatment on tannin extraction might be due to stronger binding affinities or adsorption reactions between tannins and other macromolecules in the cells of cold-hardy grapes. The analysis of the soluble cell wall polysaccharide composition in treated must and finished ‘Marquette’ wines revealed that Accentuated Cut Edges (ACE) had no effect on polysaccharide degradation. In contrast, the application of macerating enzymes significantly modified the soluble polysaccharide profile, notably impacting the arabinose to galactose ratio. This change was largely due to the action of α-N-arabinofuranosidase present in the Rapidase® Clear Extreme enzyme, which facilitated the degradation of the natural sugar side chains, such as arabinogalactan proteins (AGP). Such a shift in composition was thought to affect tannin-polysaccharide interactions, potentially facilitating greater tannin extraction and influencing the wine's sensory properties. Understanding human perception of ‘Marquette’ wines subjected to various treatments is crucial to develop optimal practices to improve the overall wine quality that better meet consumer preferences. A descriptive sensory analysis conducted with trained panelists revealed that the Accentuated Cut Edges (ACE) treatment significantly enhanced honey and green-apple sensory notes of ‘Marquette’ wines after nine months of aging. The combination of ACE and enzyme treatments resulted in a higher color intensity compared to the control, perceived by panelists, despite the lack of differences observed by spectrophotometry. However, none of the applied treatments seemed to alter the perception of astringency, which could be attributed to the low concentration and small molecular size of the condensed tannins, as well as the release of rhamnogalacturonan (RG)-II within the ‘Marquette’ red wines. Astringency perception in red wines is the result of interactions between tannins and salivary proteins. Given that the distinguished properties of interspecific grape cultivars, such as ‘Marquette’ and ‘Frontenac’, tends to contain high level of anthocyanins in diglucoside form and pectins compared to V. vinifera cultivars like ‘Pinot noir’, one hypothesis explored was whether compounds from the flesh impact the retention of tannins and anthocyanins from grape skins or seeds under a wine-like condition. The study revealed that grape flesh significantly diminishes polyphenol concentrations, impacting both condensed tannins and anthocyanins (mono- and diglucosides). Interestingly, the non-V. vinifera cultivar, ‘Marquette’, demonstrated greater skin tannin retention compared to the V. vinifera cultivar, ‘Pinot noir’. This was likely due to the lower molecular mass of tannin of ‘Marquette’, and a potential competitive effect between anthocyanins and tannins for the binding sites in the flesh-derived macromolecules of ‘Marquette’ grapes. These studies revealed that the management of proteins and polysaccharides in grape tissues are key to achieving a higher condensed tannin concentrations in red wines made from cold-hardy grapes. The synergistic effects of applying multiple treatments have shown promising improvements in the quality of cold-hardy grape red wines, particularly in enhancing color stability and fruity aromas. Combining targeted vineyard management with innovative winemaking techniques is essential for enhancing the tannin concentration in red wines made from these grapes. The findings of these studies enhanced winemaking and chemical knowledge for cold-hardy grapes, providing insights for producers in challenging climates and promoting the production of high-quality wines reflective of their unique growing conditions.
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