Strategies for eliminating l-arabinitol in the bioconversion of xylitol

Research progress in the biosynthesis of xylitol: feedstock evolution from xylose to glucose

Xylitol, as an important food additive and fine chemical, has a wide range of applications, including food, medicine, chemical, and feed. This review paper focuses on the research progress of xylitol biosynthesis, from overcoming the limitations of traditional chemical hydrogenation and xylose bioconversion, to the full biosynthesis of xylitol production using green and non-polluting glucose as substrate. In the review, the molecular strategies of wild strains to increase xylitol yield, as well as the optimization strategies and metabolic reconfiguration during xylitol biosynthesis are discussed. Subsequently, on the basis of existing studies, the paper further discusses the current status of research and future perspectives of xylitol production using glucose as a single substrate. The evolution of raw materials from xylose-based five-carbon sugars to glucose is not only cost-saving, but also safe and environmentally friendly, which brings new opportunities for the green industrial chain of xylitol.

Arabinose as an overlooked sugar for microbial bioproduction of chemical building blocks

The circular economy is anticipated to bring a disruptive transformation in manufacturing technologies. Robust and industrial scalable microbial strains that can simultaneously assimilate and valorize multiple carbon substrates are highly desirable, as waste bioresources contain substantial amounts of renewable and fermentable carbon, which is diverse. Lignocellulosic biomass (LCB) is identified as an inexhaustible and alternative resource to reduce global dependence on oil. Glucose, xylose, and arabinose are the major monomeric sugars in LCB. However, primary research has focused on the use of glucose. On the other hand, the valorization of pentose sugars, xylose, and arabinose, has been mainly overlooked, despite possible assimilation by vast microbial communities. The present review highlights the research efforts that have explicitly proven the suitability of arabinose as the starting feedstock for producing various chemical building blocks via biological routes. It begins by analyzing the availability of various arabinose-rich biorenewable sources that can serve as potential feedstocks for biorefineries. The subsequent section outlines the current understanding of arabinose metabolism, biochemical routes prevalent in prokaryotic and eukaryotic systems, and possible products that can be derived from this sugar. Further, currently, exemplar products from arabinose, including arabitol, 2,3-butanediol, 1,2,3-butanetriol, ethanol, lactic acid, and xylitol are discussed, which have been produced by native and non-native microbial strains using metabolic engineering and genome editing tools. The final section deals with the challenges and obstacles associated with arabinose-based production, followed by concluding remarks and prospects.

Optimization of xylitol production from xylose by a novel arabitol limited co-producing Barnettozyma populi NRRL Y-12728

Xylitol is a widely marketed sweetener with good functionality and health-promoting properties. It can be synthetized by many yeast species in a one-step reduction of xylose. Arabinose is a common contaminant found in xylose and there is ongoing interest in finding biocatalysts that selectively produce xyltiol. From a screen of 99 yeasts, Barnettozyma populi Y-12728 was found to selectively produce xylitol from both mixed sugars and corn stover hemicellulosic hydrolysate. Here, fermentation conditions for xylitol production from xylose by B. populi were optimized. The medium for xylitol production was optimized through response surface methodology. The yeast produced 31.2 ± 0.4 g xylitol from xylose (50 g L^-1) in 62 h using the optimized medium. The optimal pH for xylitol production was 6.0. Glucose (10 g L^-1), acetic acid (6.0 g L^-1), HMF (4 mM) and ethanol (2.0 g L^-1) inhibited the xylitol production. The glucose inhibition was entirely mitigated by using a 2-stage aeration strategy, indicating that the yeast was inhibited by ethanol produced from glucose under low aeration. This culture strategy will greatly benefit xylitol production from hemicellulosic hydrolysates, which often contain glucose. This is the first report on optimization of xylitol production by a Barnettozyma species.