en en fi en en en en en en en en en en en en en en pl en en en af en en en en en en en en en en fr en en en en en en
Kinetic modelling of wine fermentations : why does yeast prefer glucose to fructose
JavaScript is disabled for your browser. Some features of this site may not work without it.
ENGLISH ABSTRACT: In the present-day competitive global market, wine industries are constantly  aiming to improve the wine-making process,including the role of yeast. The  most commonly used wine yeast is Saccharomyces cerevisiae, which is able to  produce high quality wines, but problem fermentations do sometimes arise.  The occurrence of stuck and sluggish fermentations pose a serious problem  leading to loss of productivity and quality. Although the precise mechanism  leading to stuck fermentations is unknown, they are often correlated with  high fructose to glucose ratios in the wine-must. S. cerevisiae is a glucophylic  yeast, indicating its preference for consuming glucose over fructose. Both these  hexose sugars are present in unfermented wine must, mostly in equal concentrations.  As fermentation progresses, glucose is consumed at a faster rate than  fructose, leading to an increase in the fructose to glucose ratio. Yeast are left  with the undesirable fructose at the later stages of fermentation, when the  environmental stresses on the yeast can lead to stuck or sluggish fermentation.  This residual fructose can lead to undesirable sweetness, as fructose is  about twice as sweet as glucose. Even with the extensive research into yeast  metabolism, there is as yet no definitive explanation as to why yeasts ferment  glucose faster than fructose.  This study aimed to investigate the mechanism responsible for the faster consumption  of glucose over fructose of a commercially used wine yeast strain  S. cerevisiae VIN 13. The first two steps of sugar metabolism, uptake and  phosphorylation, were investigated as the possible sites of discrepancy in fermentation  rates. Enzyme rates and affinities for both glucose and fructose as substrates for the relevant enzymes were experimentally determined. These  kinetic parameter values were used to improve an existing model of yeast glycolytic  pathway to model wine fermentations. The feasibility of constructing  and validating a kinetic model of wine fermentations were investigated, by  comparing model predicted fluxes with experimentally determined fluxes.  Another aspect of this study was an investigation into the effect of hexose  sugar type on fermentation profiles. Wine fermentations were done with only  one hexose sugar as carbon source to determine if it has an effect on the flux  through metabolism.  This work succeeded in the construction of a kinetic model that distinguished  between glucose and fructose as carbon source. The glucose was consumed  faster than fructose, with control lying in the hexose transport step. It was  also established that fermentation prfiles of fermentations with only one sugar  was the same for both one sugar type fermentations. Fermentation with either  glucose or fructose as the sole carbohydrate source had the same specfic  production and consumption rates as normal fermentations with both sugars.  Construction of detailed kinetic models can aid in the metabolic and cellular  engineering of novel yeast strains. By identifying the importance of hexose  transport, and thus the glucophilic character of the yeast, in flux control, yeast  transporters can be targeted for strain improvement. This may in turn lead to  more effective fermentation practices for controlling problem fermentations, or  to the development of novel strains that utilizes fructose in the same manner as  glucose, and in so doing lower the risk of stuck or sluggish wine fermentation.
AFRIKAANSE OPSOMMING: In die hedendaagse kompeterende wynmark is wynmakers aanhoudend besig  om die wynmaak proses te verbeter en dit sluit die verbetering van wyngis  in. Die mees algemeenste gebruikte wyngis is Saccharomyces cerevisiae, omdat  dit wyn van gehalte produseer, maar probleem fermentasies kom wel voor.  Die verskynsel van vasval of stadige fermentasies kan lei tot die verlies van  produksie en kwaliteit. Die oorsaak van probleem fermentasies is gewoontlik  veelvoudig, maar die verhouding van glukose tot fruktose in die wyn-mos kan  ongunstig raak om fermentasies te onderhou. S. cerevisiae is 'n glukofiliese  gis, wat sy voorkeur om glukose bo fruktose te gebruik beskryf. Albei hierdie  heksose suikers is teenwoordig in ongefermenteerde wyn-mos, meestal in gelyke  hoeveelhede. Soos fermentasies vorder word glukose vinniger verbruik as  fruktose wat lei tot 'n toename in die fruktose tot glukose verhouding. Die gis  moet dus die fruktose in die later stadium van fermentasie gebruik wanneer  die omgewings druk op die gis kan lei tot probleem fermentasies. Die oorblywende  fruktose kan lei tot ongewenste soetheid aangesien fruktose twee keer  soeter is as glukose. Selfs met die ekstensiewe navorsing met betrekking tot  gis metabolisme is daar nog nie 'n verduideliking hoekom gis glukose vinniger  as fruktose gebruik nie.  Hierdie studie het beoog om die meganisme wat lei tot die vinniger verbruik  van glukose oor fruktose te ondersoek vir 'n kommersieël gebruikte gis S. cerevisiae VIN 13. Die eerste twee stappe van suiker metabolisme, suiker opname  en fosforilasie, was ondersoek as die moontlike punt van die verskil in fermentasie  tempo. Ensiem snelhede en affiniteite vir beide glukose en fruktose as substrate vir die ensieme van belang was eksperimenteel bepaal. Hierdie  waardes is gebruik om 'n bestaande model van gis glikolise aan te pas vir wyn  fermentasies. Die uitvoerbaarheid van saamstel en valideer van 'n kinetiese  model van wyn fermentasies was ondersoek, deur model voorspelde fluksie  waardes met eksperimentele fluksie waardes te vergelyk.  'n Ander aspek van die studie was die ondersoek van die effek van heksose  suiker tipe op fermentasie profiel. Wyn fermentasies is gedoen met slegs een  heksose suiker as koolstof bron om te bepaal of dit 'n invloed het op die fluksie  deur metabolisme.  Hierdie werk het daarin geslaag om 'n kinetiese model saamtestel wat onderskei  tussen glukose en fruktose as koolstof bron. Die glukose is vinniger verbruik as  fruktose, met beheer gesetel in die heksose opname stap. Dit was ook vasgestel  dat fermentasie profiele van fermentasies met slegs een suiker nie verskil het  vir fermentasies met slegs fruktose of glukose. Fermentasies met slegs een  suiker het dieselfde spesifieke produksie en konsumpsie tempo gehad as die  normale fermentasie met albei suikers. Die konstruksie van 'n gedetailleerde  kinetiese model kan gebruik word in die metaboliese en sellulêre ontwikkeling  van nuwe gisstamme. Deur die ontdekking van die belangrikheid van heksose  opname in fluksie beheer, wat lei tot die glukofiliese karakter van gis, kan gis  opname geteiken word vir gis ontwikkeling. Dit mag om die beurt lei tot meer  effektiewe fermentasie praktyk in die beheer van probleem fermentasies, of die  ontwikkeling van nuwe stamme wat fruktose in dieselfde manier as glukose  benut, en sodoende die risiko van vasval of stadige wyn fermentasies verlaag.
Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/80316
This item appears in the following collections:
Masters Degrees (Biochemistry) [104]
All of SUNScholarCommunities & CollectionsBy TitleBy AuthorBy AdvisorBy DateBy SubjectBy TypeThis CollectionBy TitleBy AuthorBy AdvisorBy DateBy SubjectBy Type
Copyright © Stellenbosch University | ISSN-L 2310-7855