Browsing by Author "Greyling, Isabel"
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- ItemExtraction and bioconversion of aroma impact compounds from Sauvignon Blanc grapes to wine matrices during white wine production(Stellenbosch : Stellenbosch University, 2019-12) Greyling, Isabel; Vivier, Melane A.; Young, Philip R.; Zietsman, Anscha; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.ENGLISH ABSTRACT: The phenotypic response of Sauvignon Blanc grapes grown in high light (HL) conditions has been shown to acclimate to stress conditions via photoprotective responses that includes the upregulation of specific grape metabolites. These metabolites are typically concentrated in the berry skins and include aroma compounds/precursors, photosynthetic pigments and polyphenolic compounds which contribute to the distinctive aroma of the resulting wines. The fate of these aroma impact metabolites from the grape tissues during juice processing and fermentation up until the final wine has been studied and revealed a highly dynamic environment involving metabolite bioconversions and/or interaction. The aim of this study was to profile selected aroma-related metabolites in Sauvignon Blanc grape berries that were exposed to high light (HL) and low light (LL) microclimates and to follow the fate of the metabolites throughout the key processing stages of winemaking. Two winemaking procedures that could potentially affect the extraction of aroma-related metabolites from grape skins and juice sediment (particulate grape debris) were evaluated. The impact of the various factors (vineyard microclimate, skin contact and fermentation with grape sediment) on the sensory profile of the wine was also determined. Berries grown in the HL environment contained higher concentrations of grape derived aroma-linked metabolites (including IBMP, limonene, linalool, 6-MHO and hexyl formate) confirming previously published results. These berries were therefore characterised as having higher aromatic potential (HAP) compared to the berries from the LL microclimate with lower aromatic potential (LAP). During juice processing there was a dramatic decrease in the aromatic potential in both HAP and LAP juices and the chemical analysis of the sediment could account for many of the “lost” compounds. Thus, we established that the sediment represents a reservoir of untapped aromatic potential. The distinction between HAP and LAP berries persisted throughout juice processing up to the final wines when prepared by standard winemaking (Std) procedures. The main difference between HL-Std and LL-Std wines where in the concentration of grape derived aroma compounds with the formercontaining significantly higher concentrations of IBMP, hotrienol, linalool and β-damascenone. Confirming previous studies, the LL-Std wines had a vegetative sensorial character and the HL-Std wines were fruity. Modulations to the standard winemaking procedure to investigate enhanced extraction of aroma metabolites included a skin contact treatment before pressing and fermentation in contact of the sediment formed during juice clarification. Skin contact (Sc) mitigated the loss in aroma potential seen during juice processing and increased hexyl formate, hotrienol and IBMP concentration in both LAP and HAP juice. The corresponding skin contact wines contained higher concentrations of linalool, IBMP and 3-MH. Interestingly LL-Sc wines had a similar chemical and especially sensory profile to HL-Std and HL-Sc wines. This demonstrated that the aromatic potential of LAP berries (with additional extraction) is adequate to alter the aromatic potential of wine. Wines fermented in contact with the sediment were enriched by the metabolites that were detected in the sediment and this enrichment effect was stronger than what was achieved with skin contact. These wines had significantly higher concentrations of hexyl formate, IBMP and 3MH, but had lower ester levels and malodours were perceived during sensorial analysis. This study provided insights into the transfer and fate of the aromatic potential of Sauvignon Blanc from the grapes, throughout juice processing onto the final wine. It highlights the impacts of viticultural manipulations and winemaking steps that aims to improve aroma compound extraction on the aromatic potential and sensory profile of the wine.