Production of xylitol by aConiochaeta ligniariastrain tolerant of inhibitors and defective in growth on xylose

Expanding sugar alcohol industry: Microbial production of sugar alcohols and associated chemocatalytic derivatives

Sugar alcohols are polyols that are widely employed in the production of chemicals, pharmaceuticals, and food products. Chemical synthesis of polyols, however, is complex and necessitates the use of hazardous compounds. Therefore, the use of microbes to produce polyols has been proposed as an alternative to traditional synthesis strategies. Many biotechnological approaches have been described to enhancing sugar alcohols production and microbe-mediated sugar alcohol production has the potential to benefit from the availability of inexpensive substrate inputs. Among of them, microbe-mediated erythritol production has been implemented in an industrial scale, but microbial growth and substrate conversion rates are often limited by harsh environmental conditions. In this review, we focused on xylitol, mannitol, sorbitol, and erythritol, the four representative sugar alcohols. The main metabolic engineering strategies, such as regulation of key genes and cofactor balancing, for improving the production of these sugar alcohols were reviewed. The feasible strategies to enhance the stress tolerance of chassis cells, especially thermotolerance, were also summarized. Different low-cost substrates like glycerol, molasses, cellulose hydrolysate, and CO₂ employed for producing these sugar alcohols were presented. Given the value of polyols as precursor platform chemicals that can be leveraged to produce a diverse array of chemical products, we not only discuss the challenges encountered in the above parts, but also envisioned the development of their derivatives for broadening the application of sugar alcohols.

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.

Biosynthetic strategies to produce xylitol: an economical venture

Xylitol is a natural five-carbon sugar alcohol with potential for use in food and pharmaceutical industries owing to its insulin-independent metabolic regulation, tooth rehardening, anti-carcinogenic, and anti-inflammatory, as well as osteoporosis and ear infections preventing activities. Chemical and biosynthetic routes using D-xylose, glucose, or biomass hydrolysate as raw materials can produce xylitol. Among these methods, microbial production of xylitol has received significant attention due to its wide substrate availability, easy to operate, and eco-friendly nature, in contrast with high-energy consuming and environmental-polluting chemical method. Though great advances have been made in recent years for the biosynthesis of xylitol from xylose, glucose, and biomass hydrolysate, and the yield and productivity of xylitol are substantially improved by metabolic engineering and optimizing key metabolic pathway parameters, it is still far away from industrial-scale biosynthesis of xylitol. In contrary, the chemical synthesis of xylitol from xylose remains the dominant route. Economic and highly efficient xylitol biosynthetic strategies from an abundantly available raw material (i.e., glucose) by engineered microorganisms are on the hard way to forwarding. However, synthetic biology appears as a novel and promising approach to develop a super yeast strain for industrial production of xylitol from glucose. After a brief overview of chemical-based xylitol production, we critically analyzed and comprehensively summarized the major metabolic strategies used for the enhanced biosynthesis of xylitol in this review. Towards the end, the study is wrapped up with current challenges, concluding remarks, and future prospects for designing an industrial yeast strain for xylitol biosynthesis from glucose.