The isolation of pentose-assimilating yeasts and their xylose fermentation potential

Abstract

For the implementation of cellulosic ethanol technology, the maximum use of lignocellulosic materials is important to increase efficiency and to reduce costs. In this context, appropriate use of the pentose released by hemicellulose hydrolysis could improve de economic viability of this process. Since the Saccharomyces cerevisiae is unable to ferment the pentose, the search for pentose-fermenting microorganisms could be an alternative. In this work, the isolation of yeast strains from decaying vegetal materials, flowers, fruits and insects and their application for assimilation and alcoholic fermentation of xylose were carried out. From a total of 30 isolated strains, 12 were able to assimilate 30 g L−1 of xylose in 120 h. The strain Candida tropicalis S4 produced 6 g L−1 of ethanol from 56 g L−1 of xylose, while the strain C. tropicalis E2 produced 22 g L−1 of xylitol. The strains Candida oleophila G10.1 and Metschnikowia koreensis G18 consumed significant amount of xylose in aerobic cultivation releasing non-identified metabolites. The different materials in environment were source for pentose-assimilating yeast with variable metabolic profile.

Valorisation of agroindustrial residues acid hydrolyzates as carbon sources for ethanol production by native yeast strains with different fermentative capabilities//Valorización de hidrolizados ácidos de residuos agroindustriales como fuente de carbono para la producción de etanol por levaduras nativas con capacidades fermentativas diferentes

“…Similarly, yeasts were able to grow in the basal culture medium containing xylose as a carbon source, showing a slower lag or adaptation phase (24 h) in comparison with the medium which contained glucose, at 8 h (Figure 2). Candida intermedia CBE002 showed a higher biomass yield (0.35 g/g of xylose), and reached a 9.71 g/L biomass at 72 h of incubation, twice the growth observed in the other selected strains (Table 3), similar to the results reported by Martins et al (2018). The authors isolated thirty yeasts which were capable of consuming xylose as a carbon source; finding that Pichia guilliermondii G1.2 and G4.2 have the maximum biomass yield (0.3 g biomass/g xylose).…”

“…The study of yeasts that are capable of using pentoses, mainly xylose, becomes relevant (Bellasio et al, 2015). Yeast members of the Candida genus are widely reported as a xylose fermenter (Schirmer-Michel et al, 2008;Martins et al, 2018;Pérez-Cadena et al, 2018;Shariq and Sohail, 2018), as well as yeasts of the Pichia, Kluyveromyces (Mussatto et al, 2012) and Wickerhamomyces (Bazoti et al, 2017) genera.…”

“…In order to carry out an efficient conversion of lignocellulosic biomass into bioethanol, the use of pentoses released from hemicellulose hydrolysis is of utmost importance. In nature, there are about 100 types of microorganisms that can metabolize this substrate (Ge et al, 2017;Martins et al, 2018). Yeasts from the Candida, Pichia, and Pachysolen genera are able to ferment xylose into ethanol, whether aerobically or anaerobically.…”

“…Another challenge is to find yeasts that are tolerant to the various disintegration products generated from thermal and chemical lignocellulosic biomass pre-treatments, which are limiting factors in the fermentation process (Selim et al, 2018). Martins et al (2018) conducted a search for yeasts capable of fermenting synthetic xylose into ethanol, finding the C. tropicalis S4 strain which metabolizes 56 g/L of synthetic xylose to produce 6 g/L of ethanol. In addition, yeasts of the Candida and Wickerhamomyces genera are able to ferment sugars obtained from hydrolysis of pretreated sugarcane bagasse by steam explosion into ethanol; however, the strains show a preference for glucose, by consuming it almost entirely, but not for other sugars present in the hydrolysate (Bazoti et al, 2017).…”

Engineered Saccharomyces cerevisiae for lignocellulosic valorization: a review and perspectives on bioethanol production

“…Still, some of them cannot grow on xylose, which can be explained by deficient regulation of xylose pathway expression or enzymes [ 70 ]. Several xylose-assimilating yeasts have been isolated from different sources, but only a small percentage is capable of producing ethanol from this pentose [ 71 – 73 ]. These naturally xylose-fermenting yeast, such as Scheffersomyces stipitis (formerly known as Pichia stipitis), Candida tropicalis or Spathaspora passalidarum can convert xylose into ethanol, however low tolerance to ethanol and lignocellulosic-derived inhibitors are major drawbacks, as well as strict culture conditions requirements (e.g., pH and dissolved oxygen levels) to maintain the xylose fermentation performance [ 74 , 75 ].…”