EFFECTS OF NITROGEN FERTILIZATION, GRAPE RIPENING, ALCOHOL ADJUSTMENTS, AND MACERATION TIME ON GRAPE AND WINE PHENOLIC COMPOSITION AND SENSORY PROFILES

Juliana  Pazos

This dissertation explores how vineyard and winemaking decisions alter grape and wine phenolic composition, focusing on tannins, as well as wine astringency perception. Chapters 2 and 3 analyze the effect of harvest time, pre-fermentation alcohol adjustments and extended maceration on wine phenolics and sensory attributes. Cabernet Sauvignon was harvested at three different stages (representing unripe, ripe and overripe fruit) and for each harvest, total soluble solids were adjusted to three concentrations (equivalent to ∼11 %, 14 %, and 16.5 % alcohol). In addition, each of the harvest-alcohol combinations were subjected to a 10-day maceration (CM) and an 8-month extended maceration (EM). EM increased wine tannin concentration by 290% in comparison to the CM. High alcohol increased tannin extraction, but only in combination with the EM process. Tannin size and composition analysis (by phloroglucinolysis) showed that EM increased seed tannin extraction, but high alcohol did not promote extraction from a particular tissue. Although tannin concentration in grapes changed with harvest time, no differences were observed in the wines. However, early harvest wine showed characteristics of greater seed tannin extraction. Polysaccharide composition and overall concentration in the wines were not clearly affected by maceration time, harvest or alcohol. Astringency perception strongly correlated with protein precipitable tannin concentration and inversely correlated to wine tannin size. Descriptive sensory analysis showed that maceration time and alcohol were the main drivers in overall differences in the wines considering wine aromas, taste and mouthfeel. Chapter 4 evaluates the impact of vineyard nitrogen (N) applications on grape and wine phenolic and protein content, and their relationship to sensory characteristics. Four soil N treatments of 0, 22.5, 45, and 90 kg N/ha (Control, Low, Medium, and High, respectively) and a foliar urea treatment of 17 kg N /ha (Urea) were applied on vines across four vintages. Overall, the vineyard treatments did not clearly alter grape anthocyanins or tannin size and content, but wine tannin concentration decreased with the High and Urea treatments. Those two treatments also showed shorter wine tannin sizes. In addition, the Urea, High and Medium treatments had more pathogenesis-related grape proteins than the Control and the percentage of bound tannins to proteins in wines increased in the High and Urea treatments in comparison to the Control. Therefore, it was concluded that greater vineyard N applications were promoting protein production that, during winemaking, would encourage tannin precipitation. This differences in tannin concentration were large enough to be perceived by panelists, who rated the High treatment lower in astringency than the Control in two out of the three analyzed vintages. In summary, this dissertation explores different mechanisms to alter wine tannin concentration and, as a result, modify astringency. Protein precipitable tannin concentration has shown to be a more relevant driver of wine astringency than tannin size.

EFFECTS OF NITROGEN FERTILIZATION, GRAPE RIPENING, ALCOHOL ADJUSTMENTS, AND MACERATION TIME ON GRAPE AND WINE PHENOLIC COMPOSITION AND SENSORY PROFILES

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