Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Yeast strains contend with numerous stresses during winemaking. An inability to perceive and
initiate the physiological changes needed to adapt to stress, has been linked to slow or incomplete
(residual sugar > 4 g/L) fermentations. Wine yeast strains differ in genotype; this is manifested as
differences in their stress tolerance, and fermentation performance.
The first goal of this study was to evaluate how the initial sugar (200 or 240 g/L) and nitrogen
(50, 100, 250, or 400 mg/L) content, and the fermentation temperature (15°C or 20°C) affected the
fermentation performance of 17 commercial wine yeast strains. Fermentation performance was
evaluated based on the fermentation kinetics (lag phase, maximum fermentation rate and total
weight loss by CO2 evolution), residual sugar content and yeast dry weight. The results demonstrate
that the fermentation performances of commercial yeast cultures are significantly and differently
affected by initial nitrogen and sugar levels, as well as the fermentation temperature. Additionally,
excess nitrogen had a negative impact on the fermentation kinetics and sugar consumption.
Nitrogen deficiency is a common cause of slow and incomplete fermentations, as it affects yeast
growth and thus fermentation rates. Nitrogen supplements are routinely added at the onset of
fermentation, reducing the risk of problematic fermentations. Therefore characterising the
fermentative ability of a strain over a range of oenologically relevant conditions, could aid
winemakers in selecting a yeast strain capable of fermenting a grape must (of known sugar and
nitrogen levels) to completion at the desired fermentation temperature.
Investigations on fermentation related stress generally focus on its influence on
fermentation rate and sugar consumption. However, from a winemaking perspective, the strain’s
ability to produce the desired volatile aroma compounds is equally important. Yet, literature
provides little insight into the influence stress has on the volatile aroma profile; this is surprising as
wine aroma is closely linked to wine quality and consumer liking.
The final goal of this study was to evaluate changes to the volatile aroma profiles produced
by five commercial yeast strains, in response to hyperosmotic and temperature stress. The
concentrations of the aroma compounds were quantified using a gas chromatograph coupled to a
flame ionization detector. The results show that hyperosmotic and temperature stress caused
significant changes in the levels of a number of aroma compounds. Furthermore, the changes
observed differed among the evaluated strains, as well as for the fermentation stress treatments
studied.
Future aims should be directed towards the potential application of yeast strain selection as
a means to avoid problematic fermentations in grape must; in addition to the further
characterisation of the relationship between stress and the resultant volatile aroma profile in wine. / AFRIKAANSE OPSOMMING: Gisrasse moet verskeie stresfaktore afweer tydens die wynmaak proses. Die onvermoë van ‘n
wyngis om stres waar te neem en die nodige fisiologiese veranderinge te inisieer om aan te pas by
die strestoestande word met slepende of onvolledige fermentasies (met ‘n residuele suiker van
meer as 4 g/L) geassosieer. Wyngisrasse verkil in genotipe; wat as groot verskille in die graad van
strestoleransie, en dus ook fermentasie sukses geopenbaar word.
Die eerste doelwit van hierdie studie was om te evalueer hoe die suiker (200 of 240 g/L) en
stikstof (50, 100, 250, of 400 mg/L), asook die fermentasie temperatuur (15°C of 20°C) die
fermentasie prestasie van 17 kommersiële wyngiskulture beïnvloed. Die sukses van fermentasie is
geëvalueer op grond van fermentasie kinetika (sloerfase, maksimum fermentasiespoed en totale
gewigsverlies as CO2 verlies), die residuele suiker inhoud en die gis droë massa.
Die resultate demonstreer dat die fermentasie sukses van kommersiële giskulture
beduidend en verskillend beïnvloed word deur die aanvangsstikstof en – suikerkonsentrasies,
asook die fermentasie temperatuur. Daarbenewens, wanneer stikstof in oormaat teenwoordig is
kan dit ‘n negatiewe impak op fermentasietempo en suiker metabolisme hê. Beperkende vlakke van
stikstof ‘n algemene oorsaak van slepende of onvolledige fermentasies, aangesien stikstof die groei
en gevolglik ook die fermentasiespoed van gis beïnvloed. Stikstofaanvullings word dikwels tot
druiwemos toegevoeg aan die begin van gisting, wat die risiko van probleemfermentasies verlaag.
Dus kan die karakterisering van die fermentasievermoë van ‘n gisras vir ‘n reeks wynkundig
relevante kondisies die wynmaker help om ‘n gisras te selekteer wat in staat is om ‘n druiwemos
(waarvan die suiker en stikstofvlakke bekend is) droog te gis by die gewenste temperatuur.
Meeste studies wat fermentasieverwante stress ondersoek, fokus op die die invloed
daarvan op fermentasietempo en suikerverbruik. Van ‘n wynmaakperspektief is die gis se vermoë
om die gewensde vlugtige aroma komponente te produseer egter ewe belangrik as die vermoë om
fermentasie te voltooi. Tog verskaf die literatuur min insig tot die invloed van stres op die vlugtige
aromaprofiel; wat verbasend is aangesien die aromaprofiel ‘n belangrike faktor is van die
waargenome wynkwaliteit en daarom ook verbruikersvoorkeur.
Die finale doelwit van hierdie projek was om die veranderinge tot die vlugtige aromaprofiel
geproduseer deur vyf kommersiële gisrasse in reaksie op hiperosmotiese stres en temperatuur
stres te evalueer. Die konsentrasies van die aromakomponente is gekwantifiseer deur gas
chromatografie gekoppel aan vlam‐ioniserende deteksie. Die resultate wys dat hiperosmotiese‐ en
temperatuur stres beduidende veranderinge meebring in die vlakke van ‘n aantal
aromakomponente. Verder is die waargenome veranderinge ook verskillend vir die geëvalueerde
gisrasse, asook vir die verskille stresbehandelings wat ondersoek is. Toekomstige studies behoort gerig te wees op die toepassing van gis seleksie om potensiële
probleemfermentasies in druiwemos te voorkom; asook die verdere karakterisering van die
verhouding tussen omgewingstresfaktore en die gevolglike vlugtige aromaprofiel in wyn.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20336 |
| Date | 03 1900 |
| Creators | Fairbairn, Samantha |
| Contributors | Smith, Anita, Bauer, Florian, Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 83 p. : ill. |
| Rights | Stellenbosch University |
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