Stress, fermentation performance and aroma production by yeast

ENGLISH ABSTRACT: Yeast strains contend with numerous stresses during winemaking. An inability to perceive and initiate 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 as differences 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 the fermentation performance of 17 commercial wine yeast strains. Fermentation performance was evaluated based on the fermentation kinetics (lag phase, maximum fermentation rate and total weight loss by CO2 evolution), residual sugar content and yeast dry weight. The results demonstrate that the fermentation performances of commercial yeast cultures are significantly and differently affected 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 yeast growth and thus fermentation rates. Nitrogen supplements are routinely added at the onset of fermentation, reducing the risk of problematic fermentations. Therefore characterising the fermentative ability of a strain over a range of oenologically relevant conditions, could aid winemakers in selecting a yeast strain capable of fermenting a grape must (of known sugar and nitrogen levels) to completion at the desired fermentation temperature. Investigations on fermentation related stress generally focus on its influence on fermentation rate and sugar consumption. However, from a winemaking perspective, the strain’s ability to produce the desired volatile aroma compounds is equally important. Yet, literature provides little insight into the influence stress has on the volatile aroma profile; this is surprising as wine 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 produced by five commercial yeast strains, in response to hyperosmotic and temperature stress. The concentrations of the aroma compounds were quantified using a gas chromatograph coupled to a flame ionization detector. The results show that hyperosmotic and temperature stress caused significant changes in the levels of a number of aroma compounds. Furthermore, the changes observed differed among the evaluated strains, as well as for the fermentation stress treatments studied. Future aims should be directed towards the potential application of yeast strain selection as a means to avoid problematic fermentations in grape must; in addition to the further characterisation of the relationship between stress and the resultant volatile aroma profile in wine.