Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: During wine fermentation, numerous grape must constituents serve as nutrients to wine yeast
(Saccharomyces cerevisiae), which enable their growth and successful completion of alcoholic
fermentation. Many of these nutritional factors, in particular nitrogen, also act as precursors for yeastderived
flavour compounds such as higher alcohols, esters and volatile fatty acids. Yeast nitrogen
metabolism thus plays a determining role in wine aroma and quality. Not only is the nitrogen source,
concentration and supplementation timing important, but various environmental factors and the genetic
constitution of the yeast strain used for fermentation will also contribute to fermentation outcomes.
The main goal of this work was to explore the complex interactions between a number of contributing
factors; namely nitrogen source, timing of addition, yeast strain and fermentation matrix. Broadly, this
study assessed the impact of seven different nitrogen combinations, added either to the initial grape must
or after the onset of fermentation, on fermentation performance and aroma compound production by nine
commercial wine yeast strains. Fermentations were done in synthetic grape must, and validated for a
subset of parameters in real grape must. The nitrogen treatments were designed according to the generally
established order of preference of S. cerevisiae for individual amino acids as source of nitrogen under
fermentative conditions, and the potential of certain amino acids to participate in metabolic pathways that
produce specific aroma compounds.
The results reveal that different nitrogen combinations can lead to unexpected aroma outcomes,
depending strongly on the genetic background of individual yeast strains and the timing of nitrogen
addition. Certain nitrogen treatments consistently resulted in significant increases or decreases in specific
aroma compound concentrations in comparison to the treatment fermented on ammonium as only
nitrogen source, for multiple yeast strains. These compounds were classified as nitrogen treatment
dependent. Other aroma compounds were produced similarly for all nitrogen treatments and were
designated as nitrogen treatment independent. The presence of specific amino acid groups (for example
the branched-chain and aromatic amino acids) could be correlated to significantly altered production
patterns of related (such as higher alcohols) or unrelated (diethyl succinate) aroma compounds relative to
the other nitrogen treatments. Taken together, a number of interesting and novel hypotheses regarding the
metabolic pathways involved could be derived from the data.
Ultimately, this initial assessment of interactive effects during fermentation will contribute to practical
guidelines for winemakers to allow matching grape must constituents (such as nutrients) with the intrinsic
aroma production capabilities of specific yeast strains in order to modulate wine aroma, style and quality. / AFRIKAANSE OPSOMMING: Tydens wynfermentasie dien talle druiwemosbestanddele as voedingstowwe vir wyngis (Saccharomyces
cerevisiae) wat hul groei bevorder en hul in staat stel om alkoholiese fermentasie suksesvol te voltooi.
Baie van hierdie voedingstowwe, veral stikstof, dien ook as voorlopers vir geurkomponente afkomstig
van gismetabolisme, soos hoër alkohole, esters en vlugtige vetsure. Die stikstofmetabolisme van gis speel
dus ‘n bepalende rol in wynaroma en -kwaliteit. Nie net is die stikstofbron, konsentrasie en
tydsberekening van stikstof toevoeging belangrik nie, maar verskeie omgewingsfaktore, asook die
genetiese samestelling van die gisras aangewend vir fermentasie, sal bydra tot die fermentasie uitkomste.
Die hoofdoel van hierdie werk was om die komplekse interaksies tussen ‘n aantal bydraende faktore te
ondersoek; naamlik die stikstofbron, tyd van stikstof toevoeging, gisras en fermentasiematriks. Breedweg
het hierdie studie die impak van sewe verskillende stikstofkombinasies, toegedien tot die druiwemos voor
of na die aanvang van fermentasie, op die suksesvolle verloop van fermentasie en die produksie van
aromakomponente deur nege kommersiële wyngisrasse bepaal. Fermentasies is in sintetiese druiwemos
uitgevoer, en ‘n deelversameling van die fermentasies in regte druiwesap te herhaal. Die
stikstofbehandelings is ontwerp in ooreenstemming met die algemeen vasgestelde voorkeurvolgorde van
S. cerevisiae vir individuele aminosure as stikstofbron onder fermentatiewe kondisies, en die potensiaal
van sekere aminosure om mee te doen in metaboliese paaie wat spesifieke aromaverbindings produseer.
Die resultate toon dat verskillende stikstofkombinasies tot onverwagte aroma-uitkomste kan lei wat sterk
afhanklik is van die genetiese agtergrond van individuele gisrasse en die tyd van stikstof byvoeging.
Sekere stikstofbehandelings het konsekwent, vir veelvuldige gisrasse, tot beduidende toenames of
afnames in die konsentrasies van spesifieke aromakomponente gelei in vergelyking met die behandeling
wat ammonium as enigste stikstofbron bevat het. Hierdie verbindings is as stikstofbehandeling afhanklik
geklassifiseer. Ander aromaverbindings is soortgelyk vir alle stikstofbehandelings geproduseer en is
aangewys as stikstofbehandeling onafhanklik. Die teenwoordigheid van spesifieke aminosuurgroepe
(byvoorbeeld die vertakte-ketting en aromatiese aminosure) kon gekorreleer word met beduidende
veranderings in produksiepatrone van verwante (soos hoër alkohole) of onverwante (dietielsuksinaat)
aromakomponente relatief tot die ander stikstofbehandelings. Alles inaggenome kon ‘n aantal interessante
en nuwe hipoteses rakende die betrokke metabolise padweë van die data afgelei word.
Uiteindelik sal hierdie aanvaklike bepaling van interaktiewe effekte tydens fermentasie bydra tot
praktiese riglyne vir wynmakers, wat hulle in staat sal stel om druiwesapbestanddele (soos nutriënte) te
strook met die intrinsieke aromaproduksie kapasiteite van spesifieke gisrasse, en sodoende wynaroma,
styl en kwaliteit te moduleer. / Oenobrands and THRIP for funding this project
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/80076 |
| Date | 03 1900 |
| Creators | Smit, Anita Yolandi |
| Contributors | Bauer, Florian F., 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 | English |
| Type | Thesis |
| Format | 113 p. : ill. |
| Rights | Stellenbosch University |
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