Thesis (MScAgric)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Grape must is a complex medium, and during wine production numerous biochemical pathways
and metabolic reactions are taking place simultaneously to produce a specific taste and aroma.
Microorganisms, specifically yeast, play a key role in the formation of metabolites formed during
alcoholic fermentation. Sauvignon blanc, a well studied grape cultivar, is known to have a
versatile range of aroma profiles ranging from “green” to “tropical”. It has been broadly stated
that a “green” Sauvignon blanc can be created in the vineyard and a “tropical” Sauvignon blanc
can be created by selecting a specific yeast strain, and that the balance between “green” and
“tropical” characters is essential for the final aroma profile. Except for grape-derived varietal
aromatic compounds such as methoxypyrazines (green), volatile thiols (tropical) and
monoterpenes (floral), yeast derived volatile compounds including esters, higher alcohols, fatty
acids and carbonyl compounds will also contribute to the final wine aroma.
The main aim of this study was to assess how viticultural treatment-derived differences in
grape must, can impact on aroma production when this grape must is fermented with different
commercial wine yeast strains. The viticulture treatment focused on light intensity modulated
through canopy treatment. Volatile aroma differences were compared for canopy and yeast
treatments, specifically focusing on the fermentation derived bouquet (esters, higher alcohols,
volatile fatty acids, carbonyl compounds and monoterpenes).
Results showed significant differences between initial must compositions, including
titratable acidity, malic acid and yeast assimilable nitrogen. The volatile aroma compounds were also significantly impacted although no noticeable effect on the overall fermentation kinetics was observed.
Depending on the yeast strain differences in volatile compounds varied. A clear vintage
effect is noticeable between volatile compounds affected by the treatments. Data generated in
2012 shows clear differences between ethyl- and acetate esters and could clearly be grouped
according to yeast strain through multivariate analysis. Sensory evaluation results could clearly be distinguished according to canopy treatment
and to a lesser degree according to yeast strain used. This indicates that although yeast has a
more prominent impact on the fermentative bouquet that develops during alcoholic fermentation
the overriding aroma is primarily derived from grape-derived metabolites which can be
manipulated by canopy treatments. None the less the difference in fermentation bouquet does
add to the complexity of the wine especially in the case of fermentation derived “tropical”
aromas including guava and passion fruit. In some cases where shaded grapes had higher
ester concentrations, the resultant wine also had higher aroma quality.
This study has contributed to a better understanding of the complex relationships between
canopy manipulation and yeast selection on aroma formation. The analysis of volatile aroma alone however is not enough to understand the final perception of wine taste and further indepth
studies of the viticultural and oenological factors is needed.
In particular, this project has focused on a single vineyard over only two vintages. The
general validity of the conclusions derived from this study therefore will require additional data
sets. / AFRIKAANSE OPSOMMING: Druiwemos is ‘n komplekse medium en tydens wynbereiding is daar verskeie biochemiese weë
en metaboliese reaksies wat gelyktydig plaasvind om ‘n spesifieke smaak en aroma te
produseer. Mikro-organismes, veral gis, speel ‘n sleutelrol in die vorming van metaboliete
tydens alkoholiese gisting. Sauvignon blanc, ‘n goed bestudeerde druifkultivar, besit ‘n
veelsydige reeks aromaprofiele wat wissel van “groen” tot “tropies”. Oor die algemeen word dit
voorgehou dat ‘n “groen” Sauvignon blanc in die wingerd geskep word, terwyl ‘n “tropiese”
Sauvignon blanc geskep kan word deur ‘n spesifieke gisras te selekteer, en die balans tussen
“groen” en “tropiese” karakters is noodsaaklik vir die finale aromaprofiel. Behalwe vir
druifafgeleide kultivarafhanklike aromatiese verbindings soos metoksipirasiene (groen), vlugtige
tiole (tropies) en monoterpene (blomagtig), sal gisafgeleide vlugtige komponente, waaronder
esters, hoër alkohole, vetsure en karbonielverbindings, ook tot die finale wynaroma bydra.
Die hoofdoelwit van hierdie studie was om te bepaal hoe verskille in druiwemos wat
afkomstig is van wynkundige behandeling ‘n impak op aromaproduksie kan hê wanneer hierdie
druiwemos met verskillende kommersiële wyngisrasse gegis word. Die wynkundige behandeling
het gefokus op ligintensiteit wat deur lowerbehandeling gereguleer is. Vlugtige aromaverskille is
op grond van lower- en gisbehandelings vergelyk, met ‘n spesifieke fokus op die
gistingsafgeleide boeket (esters, hoër alkohole, vlugtige vetsure, karbonielverbindings en
monoterpene).
Die resultate het beduidende verskille getoon tussen aanvanklike mossamestellings,
waaronder titreerbare suurheid, appelsuur en gis-assimileerbare stikstof. Daar was ook ‘n
noemenswaardige impak op die vlugtige aromaverbindings, hoewel geen merkbare effek op die
algehele gistingskinetika waargeneem kon word nie.
Die verskille in vlugtige verbindings het gewissel op grond van die gisras. ‘n Duidelike
oesjaareffek was merkbaar tussen vlugtige verbindings wat deur die behandelings geaffekteer
is. Data wat in 2012 gegenereer is, toon duidelike verskille tussen etiel- en asetaatesters en kon
duidelik m.b.v. meervariantanalise volgens gisras gegroepeer word. Die resultate van die sensoriese evaluering kon duidelik volgens lowerbehandeling
onderskei word, en tot ‘n mindere mate volgens die gisras wat gebruik is. Dít dui daarop dat
hoewel gis ‘n meer prominente impak het op die gistingsboeket wat tydens alkoholiese gisting
ontwikkel, is die oorheersende aroma hoofsaaklik afgelei van druifafgeleide metaboliete wat
deur lowerbehandelings gemanipuleer kan word. Nietemin dra die verskil in gistingsboeket by
tot die kompleksiteit van die wyn, veral in die geval van gistingsafgeleide “tropiese” aromas,
insluitend koejawel en grenadella. In sommige gevalle waar beskadude druiwe hoër
esterkonsentrasies gehad het, het die gevolglike wyn ook ‘n hoër aromakwaliteit gehad.
Hierdie studie dra by tot ‘n beter begrip van die effek van die komplekse verhoudings
tussen lowermanipulasie en gisseleksie op aromavorming. ‘n Analise van vlugtige aroma alleen is egter nie voldoende om die finale persepsie van wynsmaak te begryp nie en bykomende
diepgaande studies van die wingerdkundige en wynkundige faktore word benodig.
Hierdie projek het in die besonder gefokus op ‘n enkele wingerd oor slegs twee oesjare. Die
algemene geldigheid van die afleidings wat van hierdie studie gemaak word, sal dus
bykomende datastelle vereis. / The National Research Foundation and Postgraduate Merit Bursary for financial
support
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/80181 |
| Date | 03 1900 |
| Creators | Von Mollendorff, Anke |
| Contributors | Bauer, Florian F., Du Toit, Maret, Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 110 p. : ill. |
| Rights | Stellenbosch University |
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