Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Nitrogen composition of grape must is highly variable and impacts on the health of the
fermenting yeast population as well as the formation of aroma and flavour compounds in wine.
Insufficient yeast assimilable nitrogen (YAN), mostly consisting of amino acids and ammonium,
can lead to stuck or sluggish fermentations as well as the formation of undesirable compounds
such as H2S. Furthermore, it is well established that the total concentration of YAN and the
specific amino acid composition have a significant impact on the final aroma and flavour of
wines. However, the impact of individual amino acids and of specific amino acid compositions
on fermentation kinetics and on the production of aroma and flavour impact compounds under
winemaking conditions is not well understood.
The first goal of this study was to evaluate the effect of single amino acids on growth
kinetics and major volatile production of two industrial wine yeast strains under conditions
resembling wine fermentations. To facilitate these fermentation conditions while also allowing
for easy reproducibility and manipulation of the initial components, a synthetic grape media was
utilized. Biomass formation, exponential growth rate, lag phase, and fermentation duration were
utilized to evaluate the efficiency of single amino acids.
The data show that previously observed trends in laboratory strains mostly apply to these
conditions and strains. In general, the efficiency of amino acids to be used as nitrogen sources
and the production of major volatiles due to their presence followed the same patterns for both
industrial yeast strains. However, the production of the secondary metabolites butanol,
propanol, acetic acid, and ethyl acetate were found to be produced in different final
concentrations dependent upon the yeast strain.
The branched-chained and aromatic amino acids (BCAAs) treatments were observed to
have the most dramatic effects on major volatile production. Investigating the relationships
between the initial concentration of the BCAAs and the final concentrations of major volatile
compounds, it was found that the production of fusel alcohols and fusel acids due to the
degradation of BCAAs by S. cerevisiae could be predicted from the initial concentration of
BCAAs. While under simple nitrogen conditions the production of several other secondary metabolites such as butanol, propionic acid, valeric acid, decanoic acid and 2-phenylethyl
acetate were found to be correlated to the initial concentration of BCAAs in the media.
Future studies should focus on the validation of these trends in aroma production in real
grape musts under various fermentation temperatures for a number of industrial wine yeast
strains. / AFRIKAANSE OPSOMMING: Die stikstof samestelling van druiwemos is hoogs veranderlik en impakteer op die
kerngesondheid van die fermenterende gis populasie asook die produksie van aroma- en
geurverbindings in wyn. Onvoldoende assimileerbare stikstof (ook genoem “yeast assimilable
nitrogen” (YAN)), wat meestal bestaan uit aminosure en ammonium, kan aanleiding gee tot
steek- of slepende fermentasies, asook die vorming van ongewensde verbindings soos H2S. Dit
is alombekend dat die totale konsentrasie van YAN en dan ook die spesifieke aminosuur
samestelling ‘n noemenswaardige impak op die finale aroma en smaak van wyn het. Die
invloed van individuele am inosure en spesifieke aminosuur samestellings op die fermentasie
kinetika, asook die produksie van verbindings met ‘n impak op die wyn aroma en smaak, word
egter nie deeglik verstaan nie.
Die eerste doelwit van die studie was om twee industriële gisrasse te gebruik om die effek
van enkel aminosure op die groei-kinetika en produksie van die vername vlugtige verbindings
onder wynmaak toestande te bepaal. Kunsmatige, gedefinieerde druiwermosmedium is gebruik
om wynmaak toestande te simuleer en ook herhaalbaarheid en manipulering van die
aanvanklike samestelling van die medium te verseker.
Die studie het vorige tendense wat opgemerk is in die evaluasie van laboratorium rasse
onder soortgelyke toestande bevestig. Die doeltreffendheid waarmee aminosure oor die
algemeen gebruik word as stikstofbron, asook die produksie van die vernaamste vlugtige
verbindings wat gekoppel is aan hulle teenwoordigheid, het ‘n vergelykbare patroon vir beide
rasse gevolg. Die sekondêre metaboliete butanol, propanol, asetaat en etiel-asetaat is egter wel
in verskillende eindkonsentrasies geproduseer deur die verskillende gisrasse. Die vertakte-ketting en aromatiese aminosuur (“branched-chained and aromatic amino
acids” (BCAAs)) behandelings het die mees dramatiese effek op die produksie van die
vernaamste vlugtige komponente gehad. Ondersoek na die verwantskap tussen die
aanvanklike konsentraies van die BCAAs en die finale konsentrasies van dié verbindings het
aangedui dat die produksie van hoër alkohole en sure, as gevolg van die afbraak van BCAAs
deur S. cerevisiae, met behulp van die aanvanklike konsentrasie van die BCAAs voorspel kon
word. Terselfdertyd is gevind dat onder eenvoudige stikstoftoestande, verskeie ander sekondêre metaboliete soos butanol, propionaat, valeriaat, dekanoësuur en 2-fenieletielasetaat,
gekorreleer kan word met die aanvanklike BCAAs in die media.
Verdere studies moet poog om hierdie tendense ten opsigte van aromaproduksie te bevestig en
wel deur gebruik te maak van ware druiwemos, verskeie fermentasietemperature, asook ’n
verskeidenheid van wyngisrasse.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/85797 |
| Date | 12 1900 |
| Creators | McKinnon, Alexander |
| Contributors | Bauer, Florian, Smit, Anita, Stellenbosch University. Faculty of AgriSciences. Dept. of Institute for Wine Biotechnology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Format | ix, 74 p. : ill. |
| Rights | Stellenbosch University |
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