Mating of natural Saccharomyces cerevisiae strains for improved glucose fermentation and lignocellulosic inhibitor tolerance.

Natural Saccharomyces cerevisiae isolates from vineyards in the Western Cape, South Africa were evaluated for ethanol production in industrial conditions associated with the production of second-generation biofuels. The strains displayed high phenotypic diversity including the ability to grow at 45 °C and in the presence of 20% (v/v) ethanol, strain YI13. Strains HR4 and YI30 were inhibitor-tolerant under aerobic and oxygen-limited conditions, respectively. Spore-to-spore hybridization generated progeny that displayed heterosis, including increased ethanol productivity and improved growth in the presence of a synthetic inhibitor cocktail. Hybrid strains HR4/YI30#6 and V3/YI30#6 were able to grow at a high salt concentration (2 mol/L NaCl) with V3/YI30#6 also able to grow at a high temperature (45 °C). Strains HR4/YI30#1 and #3 were inhibitor-tolerant, with strain HR4/YI30#3 having similar productivity (0.36 ± 0.0036 g/L per h) as the superior parental strain, YI30 (0.35 ± 0.0058 g/L per h). This study indicates that natural S. cerevisiae strains display phenotypic variation and heterosis can be achieved through spore-to-spore hybridization. Several of the phenotypes (temperature-, osmo-, and inhibitor tolerance) displayed by both the natural strains and the generated progeny were at the maximum conditions reported for S. cerevisiae strains.

Evaluating fermentation characteristics of Kazachstania spp. and their potential influence on wine quality.

The current study is the first one to demonstrate the wine fermentation potential of members of several species of the genus Kazachstania including strains derived from grape must. The fermentation characteristics were evaluated in synthetic grape juice medium and in Sauvignon blanc. Our data show that none of the species evaluated could ferment to dryness in monoculture fermentations. However, at least 75% of the sugar was consumed before the fermentations got stuck. In mixed-culture fermentations with Saccharomyces cerevisiae diverse aroma profiles were evident especially in Sauvignon blanc fermentations. Four distinct potential aroma associations were identified: (i) Kazachstania solicola-vinegar and solvent-like, (ii) Kazachstania hellenica-spirituous, cheesy, (iii) Kazachstania aerobia CBS-fruity, floral (iv) K. aerobia IWBT, Kazachstania unispora and Kazachstania servazii-rancid, harsh. Furthermore, strain variation was apparent as the two K. aerobia strains displayed distinct karyotypes and aroma potential. Our data show that although members of the genus Kazachstania are typically encountered at low frequency in grape must, some of the species have positive aroma attributes that should be explored further.