Abstract
Sluggish or stuck fermentations cause significant financial losses for winemakers each year. In order to investigate the reasons for problems during spontaneous fermentation of Riesling must in a well-known German vineyard of the lower Moselle, yeast strains involved in must fermentation were identified during winemaking in the two years 2011 and 2012. Identification of the yeast isolates was performed by applying analyses of the ITS-1-5.8-ITS2 region and restriction fragment analyses of different gene sequences. It revealed that Saccharomyces (S.) bayanus and not Saccharomyces cerevisiae was the main fermenting yeast. Either S. bayanus finished the fermentation or led to stuck fermentation. After about four weeks of stuck, fermentation continued spontaneously S. bayanus was replaced by the triple hybrid S. cerevisiae × S. kudriavzevii × S. bayanus. The triple hybrid strain HL 78 was able to utilize fructose more efficiently than S. bayanus strain HL 77. The fructophilic character of the triple hybrid strain correlated with an enhanced uptake of radiolabeled fructose compared to glucose. In contrast to the usual starter culture S. cerevisiae, both isolates, S. bayanus strain HL 77 and the triple hybrid strain HL 78, could grow in the absence of ammonium when amino acids were present. However, the triple hybrid was able to consume glucose and especially fructose at lower amino acid concentrations. Thus, the triple hybrid strain HL 78 was a suitable strain to overcome stuck fermentation without changing the fermentation conditions and the aroma profile desired by the selected winery. It has already been successfully used to restart stuck fermentation. The procedure described here could be a useful approach for wine makers facing problems during spontaneous fermentation. Since the application of genetically modified yeast strains is not allowed for starter culture, based on these studies we suggest the generation of hybrid strains with desired phenotypical features from mother yeasts strains/species of a certain winery and their application in case that during a spontaneous fermentations a sluggish or stuck fermentation is observed.
Highlights
Despite the advantages of starter cultures, some winegrowers prefer a spontaneous fermentation to produce wines of high quality with a distinctive and characteristic sensory profile
Since the application of genetically modified yeast strains is not allowed for starter culture, based on these studies we suggest the generation of hybrid strains with desired phenotypical features from mother yeasts strains/species of a certain winery and their application in case that during a spontaneous fermentations a sluggish or stuck fermentation is observed
The yeast strain most frequently found in the winery investigated during fermentation was Saccharomyces bayanus during both years of sampling, in 2011 and 2012, S. cerevisiae is known as the main wine yeast in general and S. bayanus is found rarely (Dittrich & Großmann, 2005)
Summary
Despite the advantages of starter cultures, some winegrowers prefer a spontaneous fermentation to produce wines of high quality with a distinctive and characteristic sensory profile. The risk of stuck fermentation increases with the application of spontaneous fermentation, as these microbial processes rely on the complex indigenous microbiota of the grapes and the winery. González et al (2006) described the hybrid S. cerevisiae × S. kudriavzevii which could be advantageous for specific fermentation conditions due to the combined characteristics of both parents. Several authors have isolated this type of hybrid in Europe (Bradbury et al, 2006; González et al, 2008; Lopes et al, 2010), the strains of S. kudriavzevii known so far have been isolated from decaying leaves in Japan (Naumov et al, 2000) and from oak bark samples in Portugal (Sampaio & Gonçalves, 2008), but not yet from grapes or must
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