Cellulolytic enzyme expression and simultaneous conversion of lignocellulosic sugars into ethanol and xylitol by a new Candida tropicalis strain

Abstract

BackgroundLignocellulosic ethanol production involves major steps such as thermochemical pretreatment of biomass, enzymatic hydrolysis of pre-treated biomass and the fermentation of released sugars into ethanol. At least two different organisms are conventionally utilized for producing cellulolytic enzymes and for ethanol production through fermentation, whereas in the present study a single yeast isolate with the capacity to simultaneously produce cellulases and xylanases and ferment the released sugars into ethanol and xylitol has been described.ResultsA yeast strain isolated from soil samples and identified as Candida tropicalis MTCC 25057 expressed cellulases and xylanases over a wide range of temperatures (32 and 42 °C) and in the presence of different cellulosic substrates [carboxymethylcellulose and wheat straw (WS)]. The studies indicated that the cultivation of yeast at 42 °C in pre-treated hydrolysate containing 0.5 % WS resulted in proportional expression of cellulases (exoglucanases and endoglucanases) at concentrations of 114.1 and 97.8 U g−1 ds, respectively. A high xylanase activity (689.3 U g−1 ds) was also exhibited by the yeast under similar growth conditions. Maximum expression of cellulolytic enzymes by the yeast occurred within 24 h of incubation. Of the sugars released from biomass after pretreatment, 49 g L−1 xylose was aerobically converted into 15.8 g L−1 of xylitol. In addition, 25.4 g L−1 glucose released after the enzymatic hydrolysis of biomass was fermented by the same yeast to obtain an ethanol titer of 7.3 g L−1.ConclusionsDuring the present study, a new strain of C. tropicalis was isolated and found to have potential for consolidated bioprocessing (CBP) applications. The strain could grow in a wide range of process conditions (temperature, pH) and in the presence of lignocellulosic inhibitors such as furfural, HMF and acetic acid. The new yeast produced cellulolytic enzymes over a wide temperature range and in the presence of various cellulosic substrates. The cellulolytic enzymes produced by the yeast were effectively used for the hydrolysis of pretreated biomass. The released sugars, xylose and glucose were, respectively, converted into xylitol and ethanol. The potential shown by the new inhibitor tolerant cellulolytic C. tropicalis to produce ethanol or xylitol is of great industrial significance.

Biorefinery cascade processing for creating added value on tomato industrial by-products from Tunisia

“…Recent innovations about lignocellulosic biomass hydrolysis include acid or base treatments, which are not yet satisfactory because of the ecological waste issues [22, 23]. Hydrolysis with hemicellulolytic and cellulolytic enzymes had also been commonly used for lignocellulose degradation [2, 24]. However, enzymatic hydrolysis is time consuming and is not enough to break down the hemicellulose and cellulose complex structures to simple sugars [2, 25, 26].…”

Process Optimization of Steam Explosion Parameters on Multiple Lignocellulosic Biomass Using Taguchi Method—A Critical Appraisal

“…The highest accumulation of furfural and HMF was found to be 110 mg g −1 DM in CS corresponding to ∼9.6 g L −1 which would be detrimental to bioconversion (van der Pol et al, 2014; Figure 3C). For WS, M and W the maximum individual inhibitor accumulation were below 4 g L −1 for HMF and furfural, which is important as up to 4 g L −1 of furfural or HMF has been shown to not inhibit microbial growth (Geddes et al, 2011;Mattam et al, 2016;Wang et al, 2016). Previous publications already mentioned have a range of inhibitors concentrations from 7.5 to 45.8 g kg −1 (Castro et al, 2014) 3.62 to 5.65 g L −1 (Negro et al, 2014) and 3.77 g Kg −1 (Zhang et al, 2014) for SE-PT biomass.…”

Second-Generation Bioethanol Production through a Simultaneous Saccharification-Fermentation Process Using Kluyveromyces Marxianus Thermotolerant Yeast

“…Hasunuma and Kondo [30] presented a review of the development of yeast cells for second-generation bioethanol production through CBP. Within their study, they conclude that the combination of cell surface [31][32][33][34] review the possibilities of recombinant yeasts generation for second-generation bioethanol production through CBP.…”