Stress, fermentation performance and aroma production by yeast

[摘要] ENGLISH ABSTRACT:Yeast strains contend with numerous stresses during winemaking. An inability to perceive andinitiate the physiological changes needed to adapt to stress, has been linked to slow or incomplete(residual sugar > 4 g/L) fermentations. Wine yeast strains differ in genotype; this is manifested asdifferences in their stress tolerance, and fermentation performance.The first goal of this study was to evaluate how the initial sugar (200 or 240 g/L) and nitrogen(50, 100, 250, or 400 mg/L) content, and the fermentation temperature (15°C or 20°C) affected thefermentation performance of 17 commercial wine yeast strains. Fermentation performance wasevaluated based on the fermentation kinetics (lag phase, maximum fermentation rate and totalweight loss by CO2 evolution), residual sugar content and yeast dry weight. The results demonstratethat the fermentation performances of commercial yeast cultures are significantly and differentlyaffected by initial nitrogen and sugar levels, as well as the fermentation temperature. Additionally,excess nitrogen had a negative impact on the fermentation kinetics and sugar consumption.Nitrogen deficiency is a common cause of slow and incomplete fermentations, as it affects yeastgrowth and thus fermentation rates. Nitrogen supplements are routinely added at the onset offermentation, reducing the risk of problematic fermentations. Therefore characterising thefermentative ability of a strain over a range of oenologically relevant conditions, could aidwinemakers in selecting a yeast strain capable of fermenting a grape must (of known sugar andnitrogen levels) to completion at the desired fermentation temperature.Investigations on fermentation related stress generally focus on its influence onfermentation rate and sugar consumption. However, from a winemaking perspective, the strain'sability to produce the desired volatile aroma compounds is equally important. Yet, literatureprovides little insight into the influence stress has on the volatile aroma profile; this is surprising aswine aroma is closely linked to wine quality and consumer liking.The final goal of this study was to evaluate changes to the volatile aroma profiles producedby five commercial yeast strains, in response to hyperosmotic and temperature stress. Theconcentrations of the aroma compounds were quantified using a gas chromatograph coupled to aflame ionization detector. The results show that hyperosmotic and temperature stress causedsignificant changes in the levels of a number of aroma compounds. Furthermore, the changesobserved differed among the evaluated strains, as well as for the fermentation stress treatmentsstudied.Future aims should be directed towards the potential application of yeast strain selection asa means to avoid problematic fermentations in grape must; in addition to the furthercharacterisation of the relationship between stress and the resultant volatile aroma profile in wine.

[发布机构] Stellenbosch University