Optimized Bioconversion of Xylose Derived from Pre-Treated Crop Residues into Xylitol by Using Candida boidinii

Crop residues can serve as low-cost feedstocks for microbial production of xylitol, which offers many advantages over the commonly used chemical process. However, enhancing the efficiency of xylitol fermentation is still a barrier to industrial implementation. In this study, the effects of oxygen transfer rate (OTR) (1.1, 2.1, 3.1 mmol O2/(L × h)) and initial xylose concentration (30, 55, 80 g/L) on xylitol production of Candida boidinii NCAIM Y.01308 on xylose medium were investigated and optimised by response surface methodology, and xylitol fermentations were performed on xylose-rich hydrolysates of wheat bran and rice straw. High values of maximum xylitol yields (58–63%) were achieved at low initial xylose concentration (20–30 g/L) and OTR values (1.1–1.5 mmol O2/(L × h)). The highest value for maximum xylitol productivity (0.96 g/(L × h)) was predicted at 71 g/L initial xylose and 2.7 mmol O2/(L × h) OTR. Maximum xylitol yield and productivity obtained on wheat bran hydrolysate were 60% and 0.58 g/(L × h), respectively. On detoxified and supplemented hydrolysate of rice straw, maximum xylitol yield and productivity of 30% and 0.19 g/(L × h) were achieved. This study revealed the terms affecting the xylitol production by C. boidinii and provided validated models to predict the achievable xylitol yields and productivities under different conditions. Efficient pre-treatments for xylose-rich hydrolysates from rice straw and wheat bran were selected. Fermentation using wheat bran hydrolysate and C. boidinii under optimized condition is proved as a promising method for biotechnological xylitol production.

Improving xylitol yield by deletion of endogenous xylitol-assimilating genes: a study of industrial Saccharomyces cerevisiae in fermentation of glucose and xylose

ABSTRACT Engineered Saccharomyces cerevisiae can reduce xylose to xylitol. However, in S.cerevisiae, there are several endogenous enzymes including xylitol dehydrogenase encoded by XYL2, sorbitol dehydrogenases encoded by SOR1/SOR2 and xylulokinase encoded by XKS1 may lead to the assimilation of xylitol. In this study, to increase xylitol accumulation, these genes were separately deleted through CRISPR/Cas9 system. Their effects on xylitol yield of an industrial S. cerevisiae CK17 overexpressing Candida tropicalis XYL1 (encoding xylose reductase) were investigated. Deletion of SOR1/SOR2 or XKS1 increased the xylitol yield in both batch and fed-batch fermentation with different concentrations of glucose and xylose. The analysis of the transcription level of key genes in the mutants during fed-batch fermentation suggests that SOR1/SOR2 are more crucially responsible for xylitol oxidation than XYL2 under the genetic background of S.cerevisiae CK17. The deletion of XKS1 gene could also weaken SOR1/SOR2 expression, thereby increasing the xylitol accumulation. The XKS1-deleted strain CK17ΔXKS1 produced 46.17 g/L of xylitol and reached a xylitol yield of 0.92 g/g during simultaneous saccharification and fermentation (SSF) of pretreated corn stover slurry. Therefore, the deletion of XKS1 gene provides a promising strategy to meet the industrial demands for xylitol production from lignocellulosic biomass.

Xylitol Production from Exhausted Olive Pomace by Candida boidinii

In this work, the production of xylitol from a hemicellulosic hydrolysate of exhausted olive pomace (EOP), a residue originated in the olive oil production process by Candida boidinii, was assessed. The hydrolysate was obtained by dilute acid pretreatment of EOP at 170 °C and 2% H2SO4 (w/v). A previous detoxification step of the hydrolysate was necessary, and its treatment with activated charcoal and ion-exchange resin was evaluated. Prior to fermentation of the hydrolysate, fermentation tests in synthetic media were performed to determine the maximum xylitol yield and productivity that could be obtained if inhibitory compounds were not present in the medium. In addition, the glucose existing in the media exerted a negative influence on xylitol production. A maximum xylitol yield of 0.52 g/g could be achieved in absence of inhibitor compounds. Fermentation of the hemicellulosic hydrolysate from EOP after detoxification with ion-exchange resin resulted in a xylitol yield of 0.43 g/g.

Evaluation of Candida guilliermondii cell growth and one-pot fermentation for xylitol bioproduction

Xylitol is a polyalcohol with a high sweetener power that presents healthy benefits when compared to sucrose, such as prevention of caries and metabolism only partially dependent on insulin. Its bioproduction, besides being within the context of Biorefiney and Green Chemistry, presents advantages when compared to the traditional chemical process. This study analyzed Candida guilliermondii IM/UFRJ 50088 cells growth for three different operational strategies, two of them conducted on shake flasks and the other one held on instrumented bioreactor. The latter provided the best results, achieving a maximum cell productivity (Qx max) of 0.61 g/L.h while reaching cell concentrations higher than 20 g/L. For the one-pot fermentation assay, despite low xylitol yield (YP/S≈0,1g/g) obtained, which shows that further research is necessary, the propagation steps were successful.Â

Microbial Bioconversion Process of Glucose for the Production of Xylitol

Xylitol is the first rare sugar that has global market because of its excellent properties. Considering its superiority to chemosynthesis, biosynthesis of xylitol became hot issue in recent studies. The production of xylitol from glucose experienced a development from three-step process to two-step process, or even only one-step process. The microbial and enzymatic process involving key enzymes, molecular cloning and expression and transgenic bacteria construction is introduced in this paper. This study may provide novel thought to explore new resource for better control of biological reaction conditions and obtainment of higher xylitol yield.

Improvement of biotechnological xylitol production by glucose during cultive of Candida guilliermondii in sugarcane bagasse hydrolysate

The effect of glucose on xylose-to-xylitol bioconversion by Candida guilliermondii was examined by adding it to sugarcane bagasse hydrolysate medium to obtain different glucose:xylose ratios (1:25, 1:12, 1:5 and 1:2.5). Under experimental conditions, increasing glucose:xylose ratio improved the assimilation of the xylose present in the hydrolysate by yeast, resulting in biomass increase, and in the formation of xylitol and glycerol/ethanol by-products. Maximum values of xylitol yield (0.59 g g-1) and volumetric productivity (0.53 g l-1.h-1) were obtained with glucose:xylose ratio of 1:5, resulting in the higher conversion efficiency (64.3%).

Evaluation of nutrient supplementation to charcoal-treated and untreated rice straw hydrolysate for xylitol production by Candida guilliermondii

Xylitol was produced by Candida guilliermondii from charcoal-treated and untreated rice straw hemicellulosic hydrolysate with or without nutrients (ammonium sulphate, calcium chloride, rice bran extract). Both, xylitol yield and volumetric productivity decreased significantly when the nutrients were added to treated and untreated hydrolysates. In the treated hydrolysate, the efficiency of xylose conversion to xylitol was 79% when the nutrients were omitted. The results demonstrated that rice straw hemicellulosic hydrolysate treated with activated charcoal was a cheap source of xylose and other nutrients for xylitol production by C. guilliermondii. The non-necessity of adding nutrients to the hydrolysate media would be very advantageous since the process becomes less costly.

Reduced Oxidative Pentose Phosphate Pathway Flux in Recombinant Xylose-Utilizing Saccharomyces cerevisiae Strains Improves the Ethanol Yield from Xylose

ABSTRACT In recombinant, xylose-fermenting Saccharomyces cerevisiae, about 30% of the consumed xylose is converted to xylitol. Xylitol production results from a cofactor imbalance, since xylose reductase uses both NADPH and NADH, while xylitol dehydrogenase uses only NAD+. In this study we increased the ethanol yield and decreased the xylitol yield by lowering the flux through the NADPH-producing pentose phosphate pathway. The pentose phosphate pathway was blocked either by disruption of the GND1 gene, one of the isogenes of 6-phosphogluconate dehydrogenase, or by disruption of the ZWF1 gene, which encodes glucose 6-phosphate dehydrogenase. Decreasing the phosphoglucose isomerase activity by 90% also lowered the pentose phosphate pathway flux. These modifications all resulted in lower xylitol yield and higher ethanol yield than in the control strains. TMB3255, carrying a disruption of ZWF1, gave the highest ethanol yield (0.41 g g−1) and the lowest xylitol yield (0.05 g g−1) reported for a xylose-fermenting recombinant S. cerevisiae strain, but also an 84% lower xylose consumption rate. The low xylose fermentation rate is probably due to limited NADPH-mediated xylose reduction. Metabolic flux modeling of TMB3255 confirmed that the NADPH-producing pentose phosphate pathway was blocked and that xylose reduction was mediated only by NADH, leading to a lower rate of xylose consumption. These results indicate that xylitol production is strongly connected to the flux through the oxidative part of the pentose phosphate pathway.