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Factors influencing the colour and phenolic composition of Shiraz wine during winemaking

Garrido Banuelos, Gonzalo (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Phenolic compounds are considered important quality contributors of red wines. However, lot of unknowns exist in terms of their role in wine due to their complexity and continuous evolution which starts in fresh grapes, during alcoholic fermentation and wine ageing. In this study, we have evaluated different parameters which could affect the colour and phenolic composition of Shiraz wines. All the work has been carried out within the same vineyard during three harvest seasons. This study has interconnected topics in order to reach a broader understanding on how certain factors influencing the phenolic composition in fresh grapes can affect phenolic extractability and subsequent wine ageing. In the first part of the study, the colour and phenolic composition of wines was changed by harvesting the grapes at different ripening levels. Differences observed in the phenolic composition in young red wines were also followed during prolonged wine ageing. The initial phenolic composition of young wines influenced the wines’ colour and phenolic evolution during bottle storage. The sensory profiles of the wines were also influenced by the grape ripening and ageing. This work also evaluated the impact of vintage, grape ripeness and alcoholic fermentation on the transformation of the grape berry cell wall. All these factors have been shown to affect the final colour and phenolic composition of young red wines. Grape ripeness was relevant for the study, but the vintage effect was shown to have the major impact on the phenolic content and the cell wall composition in fresh grapes. Nonetheless, compositional changes in the grape berry cell wall, partly influenced by the de-pectination during grape ripening and fermentation, also affected the release of grape phenolics into the wines. In addition, the extraction of grape proanthocyanidins with a longer polymer length was also partly influenced by the cell wall breakdown during fermentation, enhanced in riper berries. The last part of the study has shown the influence of different anthocyanin/tannin ratios and oxidation on the evolution of colour, phenolics and the precipitate formed over time in a wine-like system (WL). The use of advanced chromatographic and untargeted techniques has allowed us to measure the impact of different seed additions on the phenolic composition and development of the WL. A larger amount of tannins, extracted from higher additions of grape seeds, influenced the initial phenolic concentration and also the polymerisation reactions over time, which were enhanced in the presence of oxygen. Differences in the precipitate formed over time were also influenced by the initial amount of seeds used. This work highlights the importance of a better understanding of the grape cell wall composition and its evolution. This can lead to better control of the extraction of grape phenolic compounds and their ratios during the winemaking process, which can also influence the ageing of the wines.

AFRIKAANSE OPSOMMING: Fenoliese verbindings word beskou as belangrike bydraers tot die gehalte van rooiwyne. Daar is egter nog baie onduidelikheid oor hul rol in wyn as gevolg van hul kompleksiteit en voortdurende evolusie wat begin in vars druiwe, tydens alkoholiese fermentasie en tydens veroudering. In hierdie studie het ons verskillende faktore geëvalueer wat die kleur en fenoliese samestelling van Shiraz-wyne kan beïnvloed. Al die werk is gedoen op dieselfde wingerd oor ʼn periode van drie oesseisoene. Hierdie studie omvat onderlinge verwante onderwerpe om 'n breër begrip te verkry van hoe sekere faktore wat die fenoliese samestelling in vars druiwe beïnvloed, fenoliese ekstraksie kan beïnvloed en die daaropvolgende wynveroudering. In die eerste deel van die studie is die kleur en fenoliese samestelling van wyne verander deur die druiwe op verskillende rypheidsvlakke te oes. Verskille wat waargeneem is in die fenoliese samestelling van jong rooiwyne is ook waargeneem tydens langdurige wynveroudering. Die aanvanklike fenoliese samestelling van jong wyne het die kleur- en fenoliese evolusie van die wyne tydens bottel veroudering beïnvloed. Die sensoriese profiele van die wyne is ook deur druif rypwording en veroudering beïnvloed. Hierdie werk het ook die impak van oesjaar, druifrypheid en alkoholiese fermentasie op die transformasie van die druifkorrel-selwandsamestelling geëvalueer. Al hierdie faktore het die finale kleur en fenoliese samestelling van jong rooiwyne beïnvloed. Druif rypheid was relevant vir die studie, maar daar is aangetoon dat die seisoenale effek die grootste impak op die fenoliese inhoud en die selwandsamestelling van vars druiwe het. Tog het komposisionele veranderinge in die druifkorrel selwand, deels beïnvloed deur depektienase tydens druif rypwording en fermentasie, ook die vrystelling van druif fenole in die wyne beïnvloed. Daarbenewens was die ekstraksie van druif proantosianidiene met 'n langer polimeerlengte ook deels beïnvloed deur die afbraak van die selwand tydens fermentasie, hierdie verskynsel is verhoog in ryper druiwe. Die laaste deel van die studie het die invloed van verskillende antosianien/tannienverhoudings en oksidasie op die evolusie van die kleur, fenoliese samestelling en die presipitaat wat oor tyd in 'n wynagtige medium gevorm het, getoon. Die gebruik van gevorderde chromatografie en ongeteikende tegnieke het ons toegelaat om die impak van verskillende druiwepit toevoegings op die fenoliese samestelling en ontwikkeling van die wynagtige medium te meet. Die groter hoeveelheid tanniene, verkry uit die hoër toevoegings van druiwepitte, het die aanvanklike fenoliese konsentrasie sowel as die polimerisasie-reaksies oor tyd beïnvloed. Hierdie reaksies was ook verhoog in die teenwoordigheid van suurstof. Die verskille in die presipitaat wat oor tyd gevorm het, is beïnvloed deur die aanvanklike hoeveelhede sade wat gebruik is. Hierdie werk beklemtoon die belangrikheid van 'n beter begrip van die selwand se samestelling en evolusie, met die doel om die ekstraksie van druif fenoliese verbindings en hul verhoudings beter te beheer tydens die wynmaak proses, wat ook wynveroudering ook kan beïnvloed.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103560
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