Strategies for the improvement of tannin content in red, cold-hardy hybrid grapes and wines
Date
2023-12
Authors
Gapinski, Alexander D
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Watrelot, Aude A
Delchier, Nicolas
Slack, Suzanne M
Zabotina, Olga A
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Abstract
Phenolic compounds, especially tannins, are known to be important for red wine quality as they contribute astringency, protection from oxidation, and color stability. However, red wines made from cold-hardy hybrid grape cultivars have much lower tannin concentrations than red wines made from Vitis vinifera grape cultivars. Low tannin concentrations coupled with high acidity result in unbalanced red wines that have low consumer acceptance. The reasons for lower tannin concentrations in cold-hardy hybrid red wines could be due to lower content in grape skins and seeds throughout berry development and ripening, due to environmental factors such as nutrients in soil and sunlight exposure, or poor extractability from grape tissues and/or poor retention in wine during the winemaking process.
To better understand the content of phenolic compounds in cold-hardy hybrid grapes, the first study of this thesis evaluated the phenolics content of berry tissues and the basic chemical parameters of juices throughout development and ripening in 2021 and 2022. Three red cold-hardy hybrid grape cultivars (cv.) Crimson Pearl, Marquette, and Petite Pearl were compared to V. vinifera grape cv. Pinot noir. Crimson Pearl and Petite Pearl had lower acidity and Brix than Marquette at harvest. Crimson Pearl showed similar iron-reactive phenolics and tannin contents in juice, skins, and seeds as Petite Pearl and Marquette at harvest, which was lower compared to Pinot noir despite similar developmental trends of phenolic compounds. This suggested that the biosynthesis of tannins was reduced in cold-hardy grape cultivars compared to Pinot noir during berry development and ripening.
As a way to increase the content of tannin in cold-hardy grapes, the second study investigated the effect added nutrients as foliar sprays of phenylalanine (Phe, 100 mg/L) and prohexadione calcium (ProCa, 50 mg/L) throughout the 2022 growing season on Marquette grapes. Four treatments (control, Phe, ProCa, and ProCa+Phe) were assigned following a split-plot design and berries were sampled at five phenological time points. A linear regression model with random intercepts was used to analyze treatment effect compared to a water-treated control on berry weight, diameter, seeds per berry, basic chemical properties, tannin content, and iron-reactive phenolics content of juice, skins, and seeds. Phe showed a positive effect on juice tannin concentration at mid-ripening, but at harvest the model showed a negative effect on skin tannin and iron-reactive phenolics contents when Phe and both ProCa+Phe treatments were applied. Seed phenolic content was not affected by foliar-applied Phe or ProCa. Low application rate may be a reason for limited treatment effectiveness and further work on dose response may demonstrate foliar sprays of Phe and ProCa to be effective tools at managing phenolics content in Marquette grapes.
Phenolic concentration in red wines can also be managed in the winery with various winemaking practices and additives. The third study evaluated whole cluster (WC) fermentation as a winemaking practice to increase tannin content in Frontenac and Marquette red wines from Iowa and Minnesota. Stems are rich in phenolic compounds and could contribute an additional source along with grape skins and seeds. The chemistry and sensory perception of three conditions were compared: Control 0% WC (w/w), 25% WC (w/w), and 50% WC (w/w). The 50% WC wines contained significantly higher iron-reactive phenolics, tannin, and anthocyanin concentrations than control wines and color intensity remained unchanged after 4 months of aging. A tasting evaluation with the Marquette wines from both states revealed untrained panelists preferred 50% WC Marquette wines over control wines. This study showed promising results on the use of 50% whole clusters as a pre-fermentative technique to improve Marquette and Frontenac red wine quality.
Overall, the works presented in this thesis provide wine researchers and industry members with new information on phenolic content in cold-hardy hybrid grape cultivars and on the effectiveness of foliar-applied sprays in the vineyard and whole cluster winemaking on modulating the phenolic content in grape berry tissues and the resulting wines, respectively.
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Food Science and Human Nutrition
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