Kinetic modelling of wine fermentations : why does yeast prefer glucose to fructose

[摘要] ENGLISH ABSTRACT: In the present-day competitive global market, wine industries are constantlyaiming to improve the wine-making process,including the role of yeast. Themost commonly used wine yeast is Saccharomyces cerevisiae, which is able toproduce high quality wines, but problem fermentations do sometimes arise.The occurrence of stuck and sluggish fermentations pose a serious problemleading to loss of productivity and quality. Although the precise mechanismleading to stuck fermentations is unknown, they are often correlated withhigh fructose to glucose ratios in the wine-must. S. cerevisiae is a glucophylicyeast, indicating its preference for consuming glucose over fructose. Both thesehexose sugars are present in unfermented wine must, mostly in equal concentrations.As fermentation progresses, glucose is consumed at a faster rate thanfructose, leading to an increase in the fructose to glucose ratio. Yeast are leftwith the undesirable fructose at the later stages of fermentation, when theenvironmental stresses on the yeast can lead to stuck or sluggish fermentation.This residual fructose can lead to undesirable sweetness, as fructose isabout twice as sweet as glucose. Even with the extensive research into yeastmetabolism, there is as yet no definitive explanation as to why yeasts fermentglucose faster than fructose.This study aimed to investigate the mechanism responsible for the faster consumptionof glucose over fructose of a commercially used wine yeast strainS. cerevisiae VIN 13. Thefirst two steps of sugar metabolism, uptake andphosphorylation, were investigated as the possible sites of discrepancy in fermentationrates. Enzyme rates and affinities for both glucose and fructose as substrates for the relevant enzymes were experimentally determined. Thesekinetic parameter values were used to improve an existing model of yeast glycolyticpathway to model wine fermentations. The feasibility of constructingand validating a kinetic model of wine fermentations were investigated, bycomparing model predictedfluxes with experimentally determinedfluxes.Another aspect of this study was an investigation into the effect of hexosesugar type on fermentation profiles. Wine fermentations were done with onlyone hexose sugar as carbon source to determine if it has an effect on thefluxthrough metabolism.This work succeeded in the construction of a kinetic model that distinguishedbetween glucose and fructose as carbon source. The glucose was consumedfaster than fructose, with control lying in the hexose transport step. It wasalso established that fermentation prfiles of fermentations with only one sugarwas the same for both one sugar type fermentations. Fermentation with eitherglucose or fructose as the sole carbohydrate source had the same specficproduction and consumption rates as normal fermentations with both sugars.Construction of detailed kinetic models can aid in the metabolic and cellularengineering of novel yeast strains. By identifying the importance of hexosetransport, and thus the glucophilic character of the yeast, influx control, yeasttransporters can be targeted for strain improvement. This may in turn lead tomore effective fermentation practices for controlling problem fermentations, orto the development of novel strains that utilizes fructose in the same manner asglucose, and in so doing lower the risk of stuck or sluggish wine fermentation.

[发布机构] Stellenbosch University