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Organic acid metabolism in Saccharomyces cerevisiae : genetic and metabolic regulation

Chidi, Boredi Silas (2016-03)

Thesis (PhD)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Organic acids are major contributors to the organoleptic properties of wine. Each acid indeed contributes to the overall acidity of the product, which is an essential feature of wine quality. In addition, and an aspect that has been neglected in many evaluations in the past, each acid also imparts its own sensory characteristic to the wine. Changes in organic acid profiles therefore define relevant sensory features of wine beyond the general perception of acidity. The main objective of this study was to investigate how different yeast strains and a number of environmental factors (such as aeration, initial pH, temperature and sugar content) influence the organic acid levels in fermenting musts at three critical physiological stages (exponential, early stationary and late stationary phase). Five commercial wine yeast strains (VIN13, EC1118, BM45, 285 and DV10) were selected and these strains were subjected to two widely differing fermentation conditions. The data showed significant variation in organic acid concentrations in the final product depending on the yeast strain, and a more multifactorial experimental design was adopted to investigate the impact of environmental parameters. The impact on both grape-derived (tartaric, citric and malic acid) and fermentation-derived (succinic, acetic and pyruvic acid) acids was evaluated. Condition-dependent shifts in the production of specific organic acids were observed. The multifactorial experimental design evaluated environmental parameters that can be at least partially controlled or managed in the cellar. The influence of individual and /or combinatorial factors such as temperature, pH and sugar content of the must were also shown to affect acid profiles of the synthetic wines. A further goal of this project was to identify genes that are involved in organic acid metabolism. Transcriptome data of the five yeast strains was analyzed in order to identify genes which showed differential expression between strains and/or time points paralleled by differences in organic acids for the same comparisons. A correlation model was constructed for genes identified in this manner and model predictions were compared/aligned to observed changes in acid levels in response to deletion of the target genes. This approach provided some predictive capacity for modelling the impact of target genes on acid levels. Although some predictions based on gene expression to acid correlations were not validated experimentally, the analysis as a whole provided new insights into organic acid evolution mechanisms of different strains at different stages of fermentation. Overall, the use of a multifactorial experimental design in the current study confirmed existing knowledge and sheds new light on factors which, either on their own or in combination with other factors, impact on individual organic acids in wine. As a practical outcome, the data can serve for the development of guidelines for winemakers with regard to strain selection and management of fermentation parameters in order to better control wine acidity and wine organic acid profiles.

AFRIKAANSE OPSOMMING: Organiese sure is vername bydraers tot die organoleptiese kenmerke van wyn. Trouens dra elke suur by tot die algehele suurheid van die produk, wat ’n noodsaaklike kenmerk van wynkwaliteit is. Daarbenewens – en dit is ’n aspek wat in baie analises in die verlede afgeskeep is – verleen elke suur ook sy eie sensoriese kenmerk aan die wyn. Veranderinge in organiese suurprofiele definieer dus die relevante sensoriese kenmerke van wyn verby die algemene waarneming van suurheid. Die vernaamste doelwit van hierdie studie was om te ondersoek hoe verskillende gisrasse en ’n aantal omgewingsfaktore (soos belugting, aanvanklike pH, temperatuur en suikergehalte) die vlakke van organiese suur op drie kritiese stadiums in gistende mos beïnvloed (eksponensieel, vroeë stasionêre en laat stasionêre fase). Vyf kommersiële wyngisrasse (VIN13, EC1118, BM45, 285 en DV10) is geselekteer en aan twee baie verskillende gistingstoestande blootgestel. Die data toon noemenswaardige verskille in die konsentrasies van organiese suur in die finale produk, afhangend van die gisras, en ’n meer multifaktoriale eksperimentele ontwerp is gekies om die impak van omgewingsparameters te ondersoek. Die impak op beide druifafgeleide (wynsteen-, sitroen- en melksuur) en gistingsafgeleide (suksien-, asyn en piruvaatsuur) sure is geëvalueer. Toestand-afhanklike skuiwe in die produksie van spesifieke organiese sure is waargeneem. Die multifaktoriale eksperimentele ontwerp het omgewingsparameters geëvalueer wat ten minste gedeeltelik in die kelder beheer of bestuur kan word. Daar is aangedui dat die invloed van individuele en/of gesamentlike faktore soos die temperatuur, pH en suikergehalte van die mos ’n invloed het op die suurprofiele van die sintetiese wyne. Nóg ’n doelwit van hierdie projek was om die gene te identifiseer wat in metabolisme van organiese suur betrokke is. Transkriptoomdata van die vyf gisrasse is geanaliseer om die gene te identifiseer wat differensiële uitdrukking tussen rasse en/of tydpunte getoon het, parallel aan verskille in organiese sure vir dieselfde vergelykings. ’n Korrelasiemodel is gekonstrueer vir die gene wat op hierdie wyse geïdentifiseer is en modelvoorspellings is vergelyk/belyn met die waargenome veranderinge in suurvlakke in reaksie op die delesie van die teikengene. Hierdie benadering het ’n mate van voorspellende kapasiteit verskaf vir die modellering van die impak van teikengene op suurvlakke. Hoewel sommige voorspellings op die basis van geenuitdrukking op suurkorrelasies nie eksperimenteel bevestig is nie, het die analise in sy geheel insigte verskaf in die meganisme van die evolusie van organiese sure van verskillende rasse tydens verskillende fases van gisting. Oor die algemeen het die gebruik van ’n multifaktoriale eksperimentele ontwerp in die huidige studie die bestaande kennis bevestig en nuwe lig gewerp op faktore wat alleen, of in kombinasie met ander faktore, ’n impak het op die individuele organiese sure in wyn. As ’n praktiese uitkoms kan die data dien vir die ontwikkeling van riglyne vir wynmakers met betrekking tot rasseleksie en die bestuur van gistingsparameters om sodoende beter beheer te verkry oor wynsuurheid asook die organiese suurprofiel van wyn.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98474
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