Thesis (PhD)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: Key research areas in modern enology are related to the production of wine of
consistent quality, as well as to the improvement of existing wine quality through the
enhancement of the sensory properties of wine. The formation of glycerol during
alcoholic fermentation is highly relevant to both these issues. Since the early years of
the zo" century, glycerol has been positively associated with the quality of wine and it
is thought to impart important mouth-feel sensations such as "viscosity",
"smoothness" and "body". In general, it is considered that glycerol concentrations
higher than those normally found in wine, can contribute towards the improvement of
wine quality. It has also been suggested that increased concentrations of glycerol
can enhance the aroma of wine. On the basis of these perceptions, several strategies
have been developed to favour the production of glycerol during the fermentation
process and over a period of years, a large volume of data has been collected that
relates to various aspects regarding glycerol production during alcoholic
fermentation.
To date, however, several aspects regarding the relationship between glycerol
and wine quality remain unclear. The reasons for this situation can mainly be
ascribed to the lack of reliable analytical data to serve as a basis for investigating the
relationship between glycerol and wine quality, as well as the preponderance of
empirical and anecdotal evidence. Despite numerous opinions regarding optimal
glycerol concentrations in wine, glycerol is indeed seldom assayed on a routine basis
and targets with respect to specific wine grape cultivars and glycerol concentrations
have largely remained unspecified. To date, very little information regarding glycerol
concentrations in South African wines has been published. The analytical techniques
that are most frequently used for the quantification of glycerol in grape juice,
fermenting must and wine are not easily automated and this aspect placed severe
limitations on the generation of large volumes of analytical data on glycerol
concentrations in these matrices.
This project was undertaken with the aim to holistically address some of the
unresolved issues relating to the relationship between glycerol and wine quality. This
also implied the development and optimisation of analytical techniques suitable for
the rapid and accurate determination of glycerol in fermentation media, as well as in
finished wine. In the first stage of this project a quantitative database was established
that contained the analytical data on the glycerol concentrations of a statistically
significant number of wines of adjudged quality, as well as additional information for
each wine regarding the geographic origin, vintage, routine chemical analyses and
the yeast strain(s) used for the production of the wine. The relevance of glycerol in
wine for the modern South African winemaker was evaluated through the
establishment of a quantitative database that contained the opinions of an expert
panel of 15 South African winemakers, enologists and wine chemists on topics
relating to glycerol in wine.
In the second stage of the project the data captured in the databases were
used to investigate aspects regarding the relationship between glycerol and wine quality. From the data captured in the qualitative database, it was clear that the topic
regarding glycerol in wine was important to the South African winemakers and it was
also evident that there was a need for the development and optimisation of methods
suitable for the routine analysis of the glycerol concentrations in grape juice,
fermenting must and wine. The opinions of the panel members also highlighted the
issue that the mouth-feel property of wine was considered to be an area where the
quality of some wines could be further improved.
The quantitative database contained the information on the glycerol
concentrations of 450 commercial South African table wines of adjudged quality. The
premium cultivars Chardonnay, Chenin blanc, Sauvignon blanc, Cabernet
Sauvignon, Merlot, Shiraz and Pinotage were used for the purpose of investigating
the relationship between glycerol concentration and wine quality. The wines
represented a wide variety of wine styles, including dry white, off-dry white, dry red
and late harvest wines. The average glycerol concentration was significantly
associated with the wine style. In white wines the average glycerol concentration was
much lower than in the red wines (6.82 gIL versus 10.49 gIL, respectively). No
significant relationship between the final glycerol concentration and the geographic
origin, vintage and the yeast strain used for the fermentation was found. Wine quality
could not be significantly associated with glycerol concentration in the red wines. In
the white wines, the relationship between glycerol concentration and wine quality was
significant, but due to the very small differences in the average glycerol
concentrations of the wines of different quality ratings, the statistical significance is
probably of little practical value.
The effect of glycerol on the volatility of a selection of esters and higher alcohols
was also investigated. Solid-phase microextraction, followed by gas chromatography,
was used to analyse the composition of the headspace at equilibrium between the
liquid phase and the gas phase of a model wine, and a dry white wine that contained
a basal concentration of 5.4 gIL glycerol. Results showed that incremental increases
in the glycerol concentrations over a range of 1 -10 gIL in the model wine and in a
Chenin blanc wine, were not accompanied by a proportional increase or decrease in
the abundance of the aroma components in the headspace. The volatile components
tested were isoamyl acetate, ethyl butyrate, ethyl valerate, ethyl lactate, ethyl
hexanoate, hexyl acetate, isoamyl alcohol and isobutanol. For all the volatile
components tested, the difference between the headspace composition of samples
containing the lowest glycerol concentration, and those containing the highest
glycerol concentration, was not significant. However, sufficient experimental evidence
was obtained to indicate that increasing glycerol concentrations had an effect on the
volatility of aroma components, and that the effect is of a complex and non-linear
nature.
In the third phase of the work Fourier transform infrared spectroscopy (FT-IR)
was used to establish and optimise methods for the accurate and rapid quantification
of glycerol in wine. For this purpose calibrations were developed for the quantification
of glycerol in dry wine and late harvest wines. The accuracy of prediction was
evaluated by means of the standard error of prediction that was 0.38 gIL for the dry
wines and 0.65 gIL for the sweet wines. Large variations are introduced in the FT-IR spectra of wine by factors such as process technology, cultivar and geographic
origin, and this variation can have an effect on the accuracy of the analytical data
generated when employing FT-IR spectroscopy. Using glycerol prediction in wine as
a model system, principal component analysis of the FT-IR spectra was done in order
to establish quality control measures for the detection of poorly predicted, or outlier
samples. A classification model, based on principal component analysis, was
established that enabled the interpretation and classification of the outlier samples in
the data set in 100% of the cases tested. This work forms the basis for expanding the
quality control measures for the detection of wines of which the FT-IR spectra are
highly unnatural, as well as for establishing quality control measures to ensure that
accurate analytical data are generated when FT-IR is used.
FT-IR spectroscopy was also used to develop a rapid screen for the evaluation of
the fermentation profiles of wine yeasts. For this purpose, a selection of wine yeasts,
which included commercial wine yeasts frequently used in winemaking in South
Africa, as well as a selection of hybrid Saccharomyces cerevisiae yeasts, that were
obtained through a selective breeding strategy aimed at increasing glycerol
concentrations were used. Calibrations necessary for the accurate quantification of
glycerol, volatile acidity, ethanol, reducing sugar and glucose, in Chenin blanc must
and a synthetic must were developed and optimised. This work forms the basis upon
which the scope of the analysis, both in terms of the number of components that can
be measured, as well as the medium in which the yeasts are being evaluated, can be
enlarged. This would be valuable for future applications in both the research as well
as the industrial environment. The method that was developed serves to illustrate
how this application can play a supportive role in yeast development programmes,
through the speeding up of the initial stages of yeast strain evaluation. / AFRIKAANSE OPSOMMING: Navorsing in moderne wynkunde is sterk gefokus op die produksie van wyn waarvan
die kwaliteit van In volhoubare goeie peil is, sowel as die verdere verbetering van
bestaande wynkwaliteit, deur In verhoging van die sensoriese eienskappe van wyn.
Die vorming van gliserol tydens alkoholiese fermentasie het betrekking op beide
hierdie aspekte. In Opvatting wat wyd gehuldig word en wat reeds sedert die vroeë
jare van die 20e eeu geld, impliseer dat gliserol In positiewe bydrae lewer tot
wynkwaliteit. Oor die algemeen word dit beskou dat gliserolvlakke hoër as wat
normaalweg in wyn aangetref word, kan bydra om die kwaliteit van die wyn nog
verder te vehoog. Daar is ook spekulasie dat verhoogde gliserolvlakke in wyn die
intensiteit van die aroma van wyn kan verhoog. Hierdie opvattings het tot gevolg
gehad dat veskeie strategieë ontwikkel is om die gliserol vlakke wat tydens die
fermentasieproses gevorm word, te verhoog.
Baie min inligting oor die gliserolinhoud van Suid-Afrikaanse wyn is tot onlangs
gepubliseer. Ten spyte van die opvattings oor gliserol en wynkwaliteit wat reeds oor
In lang tydperk gehuldig word, bly sekere aspekte van die verwantskap tussen
gliserol en wynkwaliteit nog steeds onduidelik. Redes vir hierdie situasie kan
hoofsaaklik toegeskryf word aan die totale afwesigheid van betroubare en
substansiële eksperimentele data wat as basis kan dien vir die evaluering van die
algemene opvattings aangaande die verwantskap tussen gliserol en wynkwaliteit. Die
inligting wat wel beskikbaar is, is verder ook oorwegend van In empiriese aard.
Huidig word gliserol selde op In roetine basis in die analitiese laboratorium bepaal,
hoofsaaklik omdat die bestaande metodes tydrowend is en nie maklik geoutomatiseer
kan word nie.
In hierdie studie is In kwantitatiewe databasis opgestel waarin die inligting ten
opsigte van die gliserolvlakke van 450 kommersiële Suid-Afrikaanse tafelwyne
waarvan die kwaliteit beoordeel is, vervat is. Die kultivars Chardonnay, Chenin blanc,
Sauvignon blanc, Cabernet Sauvignon, Merlot, Shiraz en Pinotage is gebruik vir
hierdie doel en die seleksie van wyne was verteenwoordigend van In wye
verskeidenheid wynstyle, insluitende droë wit-, halfdroë wit-, droë rooi- en laatoeswyne.
Die gemiddelde gliserolvlakke in die witwyne was heelwat laer as die
gemidelde gliserolvlakke in die droë rooiwyne (6.82 gIL teenoor 10.49 gIL,
onderskeidelik). Geen beduidende verwantskap kon aangetoon word tussen die
gliserolkonsentrasie in die wyn en die geografiese oorsprong, oesjaar, en die gisras
wat gebruik is in die produksie van die wyn nie. Die kwaliteit van rooiwyn kon nie
beduidend met die gliserolkonsentrasie geassosieer word nie. In die geval van wit
wyn was die verwantskap statisties beduidend, maar die verskille was klein en
moontlik nie van veel praktiese waarde nie.
Die aktualiteit van gliserol vir die moderne Suid-Afrikaanse wynmaker is
geëvalueer op grond van die opinies van In paneel van 15 Suid-Afrikaanse kundiges, rakende aspekte wat verband hou met die algemene opvattings oor gliserol en wyn.
Die paneel het bestaan uit wynmakers, wynkundiges en chemici. Die opinie van die
panel is deur middel van "n vraelys bekom en is vervat in "n kwalitatiewe databasis.
Die resultate van hierdie menigspeiling het getoon dat gliserol wel vir die Suid-
Afrikaanse wynmakers belangrik is en dit het ook die behoefte uitgewys vir die
ontwikkeling en optimisering van metodes wat geskik is vir die roetine analyses van
gliserol in wyn.
Die invloed van gliserol op die vlugtigheid van "n seleksie van hoëralkohole en
esters is ondersoek in "n model wyn sowel as "n wit wyn, waarvan die basiese
gliserolkonsentrasie 5.4 gIL was. Soliede-fase mikroekstraksie van die gasfase van
wyn is opgevolg met "n gaschromatografiese analise. Resultate het getoon dat "n
stapsgewyse toename in die gliserol konsentrasie, oor "n konsentrasie reeks van 0 -
10 gIL in die model wyn, en 0 - 15 gIL in die wit wyn, nie gepaard gegaan het met "n
reglynige toename of afname in die konsentrasie van die aromakomponente nie. Vir
al die komponente wat ondersoek is, was die samestelling van die gasfase in
monsters wat die laagste gliserol konsentrasie gehad het, teenoor dié wat die
hoogste gliserol konsentrasie gehad het, nie beduidend nie. Nietemin het die
resultate getoon dat gliserol wel "n effek het op die vlugtigheid van die aroma
komponente wat in hierdie ondersoek gebruik is, maar dat die aard van die effek
kompleks en nie-liniê is.
Fourier-transformasie-infrarooispektroskopie (FT-IR) is gebruik om die metodes
vir die analise van gliserol in wyn sodanig te optimiser, dat vinnige en akkurate
bepalings op "n roetine basis in wyn gedoen kan word. Kalibrasies is ontwikkel vir die
kwantifisering van gliserol in droëwyn en laatoeswyn. Die standaard voorspellingsfout
van die kalibrasies wat ontwikkel is, was 0.38 gIL in droëwyn en 0.65 gIL in die geval
van laatoeswyn. Inherente variasie in die FT-IR-spektra word deur eksterne faktore
soos die proses tegnologie wat gebruik is om die wyn te berei, die kultivar en
geografiese oorsprong van die wyn veroorsaak en hierdie variasie kan "n effek hê op
die voorspelingsakkuraatheid van FT-IR spektroskopie. Om die effek van hierdie
variasie op die akkuraarheid van die analitiese data te evalueer, is die voorspelling
van gliserol as modelsisteem gebruik. Hoofkomponentanalise van die FT-IR spektra
is gedoen om dié wyne met onnatuurlike FT-IR-spektra te identifiseer, ten einde
beheer oor die akkuraatheid van die analitiese data uit te oefen. "n Verdere model
wat ook gebaseer is op hoofkomponentanalise van die FT-IR-wynspektra is ontwikkel
vir die interpretasie en klassifikasie van wynmonsters met onnatuurlike FT-IRspektra.
Met die model wat ontwikkel is kon onnatuurlike wynmonsters met "n
akkuraatheid van 100% gëidentifiseer word. Hierdie werk vorm die basis waarop
kwaliteitsbeheer verder uitgebrei kan word sodat wyne met onnatuurlike spektra
gëidentifiseer kan word, sowel as om te verseker dat akkurate data gegenereer word
as FT-IR as analitiese metode gebruik word vir die kwantifisering van gliserol in wyn.
FT-IR spektroskopie is ook gebruik om "n vinnige skandeerproses te ontwikkel
waarmee die fermentasieprofiele van wyngiste geëvalueer kan word. Die seleksie giste wat vir hierdie doel gebruik is, sluit kommersiële Suid-Afrikaanse wyngiste in,
sowel as hibriede Saccharomyces cerevisia-giste wat normaalweg hoër vlakke van
gliserol tydens die fermentasieproses produseer. Kalibrasies is ontwikkel vir die
akkurate kwantifisering van gliserol, vlugtige suur, alkohol, reduserende suiker en
glukose, in Chenin blanc-mos sowel as in 'n sintetiese medium. Hierdie werk vorm
die basis waarop verdere uitbreidings gedoen kan word, sodat meer metaboliete
gemeet kan word en 'n groter verskeidenheid van fermentasiemedia gebruik kan
word. Hierdie ontwikkeling is waardevol vir toekomstige toepassings in die
navorsings- sowel as die industriële omgewing. Die metode wat ontwikkel is illustreer
ook hoe hierdie toepassing 'n ondersteunende rol kan speel in wyngisontwikkelingsprogramme
deur die aanvanklike evalueringsproses van die giste te
versnel.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/53745 |
| Date | 03 1900 |
| Creators | Nieuwoudt, Helene Hesta |
| Contributors | Prior, B. A., Bauer, F. F., Pretorius, I. S., Stellenbosch University. Faculty of AgriScience. Dept. of Viticulture and Oenology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 145 pages : illustrations |
| Rights | Stellenbosch University |
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