Inferring ethanol tolerance of Saccharomyces and non-Saccharomyces yeasts by progressive inactivation

Rational Selection of Mixed Yeasts Starters for Agave Must Fermentation

Tequila and mezcal are both traditional Mexican liquors that are produced from cooked Agave spp. must fermentation and usually rely on spontaneous or pure Saccharomyces cerevisiae strain inoculation. In order to contribute to the rational selection of yeast starters for tequila and mezcal productions, we tested a collection of 25 yeasts originally isolated from mezcal musts, spanning 10 different yeast species. These strains were first characterized in a semi synthetic medium (labeled as M2, having 90 g/L fructose and 10 g/L glucose of initial hexoses) at 48 h of culture, observing a differential pattern in the consumption of sugars and productivity. Selected Saccharomyces strains left around 10 g/L of fructose and showed higher fermentation performance. However, some non-Saccharomyces strains, specifically from Torulospora (Td), Kluyveromyces (Km), and Zygosaccharomyces (Zb) genera, consumed almost all the sugar (i.e., Km1Y9 with <5 g/L) and had a high productivity of ethanol. In general, all Saccharomyces strains presented a high production of ethyl-butyrate, ethyl-decanoate, and ethyl-hexanoate with peaks of 10, 38, and 3 μg/L, respectively. In addition, some Kluyveromyces and Torulospora strains showed a high production of phenyl ethyl acetate (i.e., Km1D5 with up to 1400 μg/L); isoamyl acetate (i.e., Km1D5 and Td1AN2 with more than 300 μg/L), and hexyl acetate (i.e., Td1AN2 with 0.3 μg/L). Representative strains of the most productive genera (Saccharomyces, Torulospora, and Kluyveromyces) were selected to evaluate their fermentative performance and survival in a mixed culture on a medium based on Agave tequilana must, and their population kinetics was characterized using specific fluorescent in situ hybridization (FISH) probes in a qualitative and semi-quantitative analysis during fermentation. We observed that the mixture ratios of 0.1:1:1 or 1:1:1 (Saccharomyces:Kluyveromyces:Torulospora), maintained good fermentation productivities, with alcohol yields above 0.45 g/g, and allowed a high survival rate of the non-Saccharomyces strains during the fermentation process. Finally, mixed inoculum fermentations on A. tequilana must medium, including different Saccharomyces strains and the finally selected Torulospora and Kluyveromyces strains, showed the best production parameters in terms of ethanol, carbon dioxide, glycerol, and acetic acid values, as well as improved volatile metabolite profiles as compared to the pure cultures. All these data were used to propose a methodology of selection of strains to be used as a pure or mixed starter for tequila and mezcal fermentations, with high primary metabolite productivity and desired aromatic profile.

Community structure and metabolic potentials of the traditional rice beer starter 'emao'

The emao, a traditional beer starter used in the North-East regions of India produces a high quality of beer from rice substrates; however, its microbial community structure and functional metabolic modules remain unknown. To address this gap, we have used shot-gun whole-metagenome sequencing technology; accordingly, we have detected several enzymes that are known to catalyze saccharification, lignocellulose degradation, and biofuel production indicating the presence of metabolic functionome in the emao. The abundance of eukaryotic microorganisms, specifically the members of Mucoromycota and Ascomycota, dominated over the prokaryotes in the emao compared to previous metagenomic studies on such traditional starters where the relative abundance of prokaryotes occurred higher than the eukaryotes. The family Rhizopodaceae (64.5%) and its genus Rhizopus (64%) were the most dominant ones, followed by Phaffomycetaceae (11.14%) and its genus Wickerhamomyces (10.03%). The family Leuconostocaceae (6.09%) represented by two genera (Leuconostoc and Weissella) was dominant over the other bacteria, and it was the third-highest in overall relative abundance in the emao. The comprehensive microbial species diversity, community structure, and metabolic modules found in the emao are of practical value in the formulation of mixed-microbial cultures for biofuel production from plant-based feedstocks.