Linking the phenolic potential of grape berries with polyphenol distribution during wine making : a Sauvignon Blanc case study.

[摘要] ENGLISH ABSTRACT: The focus of this study was the phenolic potentials of grape and wine matrices of Sauvignon Blanc produced in either a high or low light microclimate. Phenolic compounds are important in grapes due to their protective function against environmental stress and fungal infection. In ripe grapes, polyphenols are unequally distributed as the skin is rich in flavonols, the seeds in flavan-3-ols, whereas the pulp contains low quantities of hydroxycinnamic acids. The polyphenol content of grapes can be altered through viticultural treatments such as manipulating the light microclimate of the bunches during development. Maintenance of a high light microclimate typically enhances polyphenol accumulation, thus increasing the phenolic potential of the grapes. The polyphenols that are present within the grapes are extracted to the derived juice and wine during winemaking where these compounds could influence colour as well as the sensory perception of wine. The extraction of phenolic compounds into the juice can be enhanced by modulating the winemaking steps, such as incorporating extended pre-fermentative skin contact. This modulation is mostly excluded in white wine-making due to potential negative implications of a high polyphenol content on white wine properties. In general, much less information on polyphenols is available for white grapes and wines.The aim of this study was to profile and quantify the phenolic potentials of Sauvignon Blanc grapes and to 'follow the compounds throughout the wine-making process, in all the grape and wine matrices, including the waste products. Pomace and juice sediments generated during winemaking and discarded as wastes can be considered as 'traps for phenolic compounds that accumulate in them. These waste matrices, when analysed alongside the juice and wine matrices could provide valuable information on the fate of polyphenols throughout the wine-making process.The approach in this study was to contrast Sauvignon Blanc grapes and wines that were exposed to two microclimates that differed in terms of the light exposure in the bunch zone. The study benefitted from a fully characterised model vineyard site where grapes from a high light (HL) and low light (LL) microclimate could be sourced. The first step was to profile and quantify the polyphenolic content and composition of the whole grapes, as well as their skins, pulps and seeds. The results confirmed previous findings that the berries from the HL environment accumulated significantly more polyphenols and that it was mostly the skins and to a lesser degree the pulp that responded to increased exposure by producing certain polyphenols. The flavonols found in the skins were strongly upregulated by the high light microclimate. The results confirmed that the two microclimates yielded berries with distinctly different phenolic potentials (a high phenolic potential, HP from the HL microclimate and a low phenolic potential (LP from the LL microclimate). In addition to the flavonols, certain hydroxycinnamic acids and flavan-3-ols could be quantified in this study.The next objective was to compare a standard white-wine making procedure (as the control) with modulated wine-making steps chosen to allow enhanced extraction of the polyphenols from the grape matrices of the HP and LP grapes. Enhanced extraction was modulated by implementing either pre-fermentative skin contact, fermentation in contact with the juice sediment, or a combination of these two treatments in the making of the wines. In addition to profiling and measuring the polyphenolic compounds in the juices, wines and waste matrices, the impact of the treatments on the sensorial quality of the wine was also evaluated from a mouthfeel perspective.Polyphenol transference occurred only partially from the grapes to the juice as no flavonols nor flavan-3-ols originating from the seeds were detected in the latter. During juice processing, total polyphenol levels also decreased considerably from the free run to the enzyme clarified juice as both hydroxycinnamic acids and flavan-3-ol concentrations decreased. This decrease was primarily linked to a decrease in catechin and caftaric acid which were the most abundant polyphenols in the juice matrix. Analysis of the corresponding sediment revealed that a considerable quantity of already extracted polyphenols settled out of suspension after enzyme clarification of the juice. Analysis of the pomace revealed that a fraction of polyphenols were also not extracted from the berry and remained in the pomace regardless of treatment, but the pomace was also considered as a phenolic trap due to flavonoid increases after pressing. The phenolic composition of the pomace also correlated with the fresh grape skin but contained lower quantities of polyphenols. The increase in total polyphenol content of the juices, which included hydroxycinnamic acids and flavan-3-ols, through pre-fermentative skin contact was in agreement with literature. The phenolic compounds that were increased in the juice were also higher in the corresponding sediments but did not clearly reflect in the pomace. The analysis method used to determine the cell wall composition of the pomace did not indicate significant alteration during the juice processing steps but the juice data revealed that sufficient deconstruction must have occurred to allow extraction of more phenolic compounds.In the standard and skin contact clarified juice made from high phenolic potential grapes, only the concentrations of catechin and caffeic were elevated in comparison to the low phenolic potential juice. High phenolic potential grapes in combination with skin contact yielded juices with higher total polyphenol content but mainly due to elevated levels of catechin.The phenolic profiles of the standard and skin contact wines correlated with the phenolic profiles of their preceding juices. Sediment contact during fermentation increased the total polyphenol content of all wines, specifically the flavan-3-ols, and more so in the high phenolic potential wine. The phenolic potential along with treatments induced perceptible differences on the sensorial characteristics of the wines. Standard high phenolic potential wines were less bitter than its low phenolic potential counterparts. Skin contact increased wine sweetness whereas sediment contact increased bitterness.This study provided new insight into the transference of polyphenols during white wine making as well as the potential implications of phenolic potential of grapes and additional wine making treatments on transference and the organoleptic properties of the wine.

[发布机构] Stellenbosch University