Non-Saccharomyces yeasts diversity on grape berries and molecular characterization of Starmerella bacillaris yeasts strains having high invertase activity

The diversity of yeast species on grape berries changes depending on various factors. Determination of indigenous yeast intensity and diversity on grape berries is important for increasing the sensory characteristics of wine as well as fermentation efficiency. The natural yeast biota of grape berries affects wine flavour and quality by producing some secondary metabolites and hydrolytic enzymes. Despite the application of different nonconventional yeasts in food and fermentation industries, many significant researches are conducted in finding and improving the new strains having industrially important enzyme activities. Invertase enzyme has a vital role in the food industry in which it increases the sweetness of food without crystallizing them. Here, we studied yeast diversity in grape berries from selected localities and also their invertase activity. We collected grape berries from Alphonse, K?nal? Yap?ncak, Çavu?, Efes Karas?, Cinsaut, Atasar?s? and Isabella grape varieties cultivated in Bozcaada island and Gelibolu peninsula. Twenty-one yeast species belonging to seven genera were identified. The yeast strains having high invertase activities were identified with 5.8S-ITS rDNA sequencing technique. The diversity of yeast biota on berries collected from Gelibolu was greater than that of Bozcaada. Metschnikowia pulcherrima, Cryptococcus laurentii and Rhodotorula glutinis yeast species were dominant yeast species on grape berries. A total of 294 sucrose grown yeast strains showed growth on sucrose, and 19 of them exhibit the highest invertase activity that is not glucose repressible. These 19 yeast strains were identified as Starmerella bacillaris using 5.8S ITS rDNA region and the phylogenetic analysis was inferred with the Maximum Parsimony method.

Disentangling the genetic bases of Saccharomyces cerevisiae nitrogen consumption and adaptation to low nitrogen environments in wine fermentation

The budding yeast Saccharomyces cerevisiae has been considered for more than 20 years as a premier model organ- ism for biological sciences, also being the main microorganism used in wide industrial applications, like alcoholic fermentation in the winemaking process. Grape juice is a challenging environment for S. cerevisiae , with nitrogen deficiencies impairing fermentation rate and yeast biomass production, causing stuck or sluggish fermentations, thus generating sizeable economic losses for wine industry. In the present review, we summarize some recent efforts in the search of causative genes that account for yeast adaptation to low nitrogen environments, specially focused in wine fermentation conditions. We start presenting a brief perspective of yeast nitrogen utilization under wine fermentative conditions, highlighting yeast preference for some nitrogen sources above others. Then, we give an outlook of S. cerevisiae genetic diversity studies, paying special attention to efforts in genome sequencing for population structure determination and presenting QTL mapping as a powerful tool for phenotype-genotype correlations. Finally, we do a recapitulation of S. cerevisiae natural diversity related to low nitrogen adaptation, specially showing how different studies have left in evidence the central role of the TORC1 signalling pathway in nitrogen utilization and positioned wild S. cerevisiae strains as a reservoir of beneficial alleles with potential industrial applications (e.g. improvement of industrial yeasts for wine production). More studies focused in disentangling the genetic bases of S. cerevisiae adaptation in wine fermentation will be key to determine the domestication effects over low nitrogen adaptation, as well as to definitely proof that wild S. cerevisiae strains have potential genetic determinants for better adaptation to low nitrogen conditions.

The ICY1 gene from Saccharomyces cerevisiae affects nitrogen consumption during alcoholic fermentation

Background Saccharomyces cerevisiae is the main microorganism responsible for alcoholic fermentation. In this process, the consumption of nitrogen is of great importance since it is found in limiting quantities and its deficiency produces sluggish and/or stuck fermentations generating large economic losses in the wine-making industry. In a previous work we compared the transcriptional profiles between genetically related strains with differences in nitrogen consumption, detecting genes with differential expression that could be associated to the differences in the levels of nitrogen consumed. One of the genes identified was ICY1. With the aim of confirming this observation, in the present work we evaluated the consumption of ammonium during the fermentation of strains that have deleted or overexpressed this gene. Results Our results confirm the effect of ICY1 on nitrogen uptake by evaluating its expression in wine yeasts during the first stages of fermentation under low (MS60) and normal (MS300) assimilable nitrogen. Our results show that the mRNA levels of ICY1 diminish when the amount of assimilable nitrogen is low. Furthermore, we constructed strains derived from the industrial strain EC1118 as a null mutant in this gene as well as one that overexpressed it. Conclusions Our results suggest that the expression of ICY1 is regulated by the amount of nitrogen available in the must and it is involved in the consumption of ammonium, given the increase in the consumption of this nitrogen source observed in the null mutant strain.

Screening Wild Yeast Strains for Alcohol Fermentation from Various Fruits

Wild yeasts on the surface of various fruits including grapes were surveyed to obtain yeast strains suitable for fermenting a novel wine with higher alcohol content and supplemented with rice starch. We considered selected characteristics, such as tolerance to alcohol and osmotic pressure, capability of utilizing maltose, and starch hydrolysis. Among 637 putative yeast isolates, 115 strains exhibiting better growth in yeast-peptone-dextrose broth containing 30% dextrose, 7% alcohol, or 2% maltose were selected, as well as five alpha-amylase producers. Nucleotide sequence analysis of the 26S rDNA gene classified the strains into 13 species belonging to five genera; Pichia anomala was the most prevalent (41.7%), followed by Wickerhamomyces anomalus (19.2%), P. guilliermondii (15%), Candida spp. (5.8%), Kodamaea ohmeri (2.5%), and Metschnikowia spp. (2.5%). All of the alpha-amylase producers were Aureobasidium pullulans. Only one isolate (NK28) was identified as Saccharomyces cerevisiae. NK28 had all of the desired properties for the purpose of this study, except alpha-amylase production, and fermented alcohol better than commercial wine yeasts.