ENGLISH ABSTRACT: Sauvignon blanc wines have become increasingly popular in South Africa as it is a cultivar that can be
easily manipulated in the vineyard and cellar to produce a range of wine styles. These wines are usually
given aroma descriptors such as green pepper, grassy and asparagus; while other more tropical aromas
include passion fruit and guava. These aromas are thought to be mainly caused by methoxypyrazines
and volatile thiols. These compounds are known to be character impacting compounds of Sauvignon
blanc and are present in the grapes in the aromatic form (methoxypyrazines) or as non‐aromatic
precursors (thiols) that can be released by the yeast during fermentation. Other aroma compounds such
as esters, higher alcohols, fatty acids and monoterpenes are compounds that could potentially influence
the aroma bouquet of a wine significantly. These aroma compounds exist either as precursors in the
grapes (monoterpenes) or arise due to yeast metabolism during fermentation (esters, higher alcohols,
fatty acids) and often display fruity, floral and pleasant aromas.
In the cellar, winemaking practices can be manipulated to a certain extent to achieve the desired wine
style. Winemaking tools such as temperature, skin contact, pressing conditions, oxygen (O2), sulphur
dioxide (SO2) and yeast strain are only a few factors influencing the outcome of a wine. In general, South
African winemakers maintain a very reductive environment during Sauvignon blanc wine production by
using inert gasses, thereby causing the production costs to increase. There is sufficient evidence to
support the reductive handling of white wine, however there seems to be a lack of information as to
why the must should be treated reductively before fermentation. The over all goal of this study was thus
to investigate the effect of different O2 and SO2 additions to Sauvignon blanc must before settling,
specifically focussing on the typical aroma compounds often found in these wines.
Chapter 2 gives an overview of the oxidation reactions occurring in must (enzymatic oxidation) and wine
(chemical oxidation). This chapter also reports the origin of the specific Sauvignon blanc aroma
compounds and their reaction to different must and wine treatments with a focus on oxidation. Chapter
3 reports research results focussing on the effect of the different must treatments on the character
impacting compounds of Sauvignon blanc wines, specifically the methoxypyrazines and the volatile
thiols. The effect of the treatments on the polyphenols and glutathione content in the must and wine
was also investigated. Oxidation in the absence of SO2 led to a decrease in glutathione and certain
phenolic compounds in the must. In general, volatile thiols were protected against oxidation by SO2,
even when O2 was present in the must. Methoxypyrazines concentrations were not significantly
influenced by the treatments. Chapter 4 elucidates the effect of the treatments on other yeast and
grape derived aroma compounds often found in Sauvignon blanc wines, such as the esters, higher
alcohols, fatty acids and monoterpenes. In general, the effect of SO2 seemed to have the greatest
influence on the produced aroma compounds.
The results reported in this thesis could possibly change the way South African Sauvignon blanc musts
are handled in future during the winemaking process. It is clear that O2 and SO2 management in the
cellar is of critical importance for the winemaker to produce wines of high quality. Future work is
important to fully understand the mechanisms and evolution of important aroma compounds of
Sauvignon blanc wines during the winemaking process.
AFRIKAANSE OPSOMMING: Sauvignon blanc wyn aroma word gewoonlik beskryf met terme soos groen rissie, grasagtig en aspersie
terwyl ander tropiese aromas soos grenadella en koejawel ook dikwels voorkom. Die manipulasie van
Sauvignon blanc in die wingerd en in die kelder tydens wynmaak, gee die wynprodusent die vryheid om
‘n wye reeks wyn style te produseer. Dit maak Sauvignon blanc baie populêr in die Suid‐Afrikaanse
wynindustrie. Die bogenoemde aromas word waargeneem in die wyn as gevolg van die
teenwoordigheid van sekere aroma komponente genaamd metoksipirasiene en vlugtige tiole. Hierdie
komponente lewer ‘n unieke bydrae tot die aroma samestelling van Sauvignon blanc wyne en kom voor
in die druiwe in die aromatiese vorm (metoksipirasiene) of as nie‐aromatiese voorlopers (tiole) wat
tydens alkoholiese fermentasie deur die gis vrygestel kan word. Komponente soos esters, hoër alkohole,
vetsure en monoterpene kan ook ‘n potensiële bydra lewer tot die algehele aroma van Sauvignon blanc
wyne en kom voor in die druiwe (monoterpene) of ontstaan as gevolg van gis metabolisme gedurende
alkoholiese fermentasie (esters, hoër alkohole, vetsure). Hierdie geur komponente word dikwels beskryf
as vrugtig, blomagtig en oor die algemeen aangenaam.
Tydens wynmaak kan die wyn tot ‘n mate gemanipuleer word om ‘n spesifieke wynstyl te bekom.
Hulpmiddels soos temperatuur, dopkontak, pers omstandighede, suurstof (O2), swawel dioksied (SO2) en
gisras is slegs ‘n paar faktore wat die algemene uitkoms van ‘n wyn kan beïnvloed. Oor die algemeen
word Sauvignon blanc in Suid‐Afrika baie reduktief behandel tydens wynbereiding. Dit vereis sekere
hulpmiddels, soos die gebruik van inerte gas, wat die produksiekoste dikwels verhoog. Navorsing
ondersteun die reduktiewe behandeling van wit wyn, maar dit wil voorkom asof daar ‘n tekort aan
navorsing is wat die reduktiewe behandeling van die sap voor fermentasie regverdig. Die algemene doel
van die studie is dus om die effek van verskillende O2 en SO2 byvoegings tot Sauvignon blanc sap (voor
afsak) te ondersoek met die fokus op die tipiese aroma komponente wat in die wyn voorkom.
Hoofstuk 2 lewer ‘n algemene oorsig van die tipes oksidasie reaksies wat voorkom in sap (ensiematiese
oksidasie) en wyn (chemiese oksidasie). Spesifieke Sauvignon blanc aroma komponente word ook
ondersoek in terme van die oorsprong van die komponente asook die reaksie wat plaasvind met
verskillende mos en wyn behandelings, met ‘n fokus op oksidasie. In hoofstuk 3 word die effek van die
verskillende mos behandelings op tipiese Sauvignon blanc aroma komponente, spesifiek
metoksipirasiene en vlugtige tiole, ondersoek. Die effek van die behandelings op die polifenole en
glutatioon inhoud in die mos en wyn word ook gerapporteer. Oksidasie van die sap in die afwesigheid
van SO2, het ‘n afname in glutatioon en sekere polifenol konsentrasies veroorsaak. Dit wil voorkom asof
die produksie van vlugtige tiole oor die algemeen beskerm word teen oksidasie indien daar genoegsame
SO2 teenwoordig is. Hierdie effek word ondervind selfs as die sap met suursof versadig word. Die effek
van die behandelings op die konsentrasies van metoksipirasiene was nie beduidend nie. Hoofstuk 4
rapporteer die effek van die behandelings op ander aroma komponente soos esters, hoër alkohole,
vetsure en monoterpene. Oor die algemeen wil dit voorkom asof die effek van SO2 beduidend was en
waarskynlik die grootste invloed op die produksie van hierdie aroma komponente het.
Na aanleiding van die resultate bevind in hierdie tesis, is daar ‘n moontlikheid dat die manier waarop
Sauvignon blanc wyne geproduseer word in Suid‐Afrika, moontlik kan verander in die toekoms. Vir die
wynmaker om hoë kwaliteit Sauvignon blanc wyne te produseer, is O2 en SO2 bestuur in die kelder van
kardinale belang. Verdere navorsing moet steeds gedoen word om die meganisme en evolusie van
belangrike aroma komponente in Sauvignon blanc wyne tydens die wynmaakproses, ten volle te
verstaan.
