Polyol production by culture of methanol-utilizing yeast

Sustainable biorefinery approach by utilizing xylose fraction of lignocellulosic biomass

Lignocellulosic biomass (LCB) has been a lucrative feedstock for developing biochemical products due to its rich organic content, low carbon footprint and abundant accessibility. The recalcitrant nature of this feedstock is a foremost bottleneck. It needs suitable pretreatment techniques to achieve a high yield of sugar fractions such as glucose and xylose with low inhibitory components. Cellulosic sugars are commonly used for the bio-manufacturing process, and the xylose sugar, which is predominant in the hemicellulosic fraction, is rejected as most cell factories lack the five‑carbon metabolic pathways. In the present review, more emphasis was placed on the efficient pretreatment techniques developed for disintegrating LCB and enhancing xylose sugars. Further, the transformation of the xylose to value-added products through chemo-catalytic routes was highlighted. In addition, the review also recapitulates the sustainable production of biochemicals by native xylose assimilating microbes and engineering the metabolic pathway to ameliorate biomanufacturing using xylose as the sole carbon source. Overall, this review will give an edge on the bioprocessing of microbial metabolism for the efficient utilization of xylose in the LCB.

Biovalorisation of crude glycerol and xylose into xylitol by oleaginous yeast Yarrowia lipolytica

Background

Xylitol is a commercially important chemical with multiple applications in the food and pharmaceutical industries. According to the US Department of Energy, xylitol is one of the top twelve platform chemicals that can be produced from biomass. The chemical method for xylitol synthesis is however, expensive and energy intensive. In contrast, the biological route using microbial cell factories offers a potential cost-effective alternative process. The bioprocess occurs under ambient conditions and makes use of biocatalysts and biomass which can be sourced from renewable carbon originating from a variety of cheap waste feedstocks.

Result

In this study, biotransformation of xylose to xylitol was investigated using Yarrowia lipolytica, an oleaginous yeast which was firstly grown on a glycerol/glucose for screening of co-substrate, followed by media optimisation in shake flask, scale up in bioreactor and downstream processing of xylitol. A two-step medium optimization was employed using central composite design and artificial neural network coupled with genetic algorithm. The yeast amassed a concentration of 53.2 g/L xylitol using pure glycerol (PG) and xylose with a bioconversion yield of 0.97 g/g. Similar results were obtained when PG was substituted with crude glycerol (CG) from the biodiesel industry (titer: 50.5 g/L; yield: 0.92 g/g). Even when xylose from sugarcane bagasse hydrolysate was used as opposed to pure xylose, a xylitol yield of 0.54 g/g was achieved. Xylitol was successfully crystallized from PG/xylose and CG/xylose fermentation broths with a recovery of 39.5 and 35.3%, respectively.

Conclusion

To the best of the author’s knowledge, this study demonstrates for the first time the potential of using Y. lipolytica as a microbial cell factory for xylitol synthesis from inexpensive feedstocks. The results obtained are competitive with other xylitol producing organisms.

Microbial conversion of xylose into useful bioproducts

Microorganisms can produce a number of different bioproducts from the sugars in plant biomass. One challenge is devising processes that utilize all of the sugars in lignocellulosic hydrolysates. D-xylose is the second most abundant sugar in these hydrolysates. The microbial conversion of D-xylose to ethanol has been studied extensively; only recently, however, has conversion to bioproducts other than ethanol been explored. Moreover, in the case of yeast, D-xylose may provide a better feedstock for the production of bioproducts other than ethanol, because the relevant pathways are not subject to glucose-dependent repression. In this review, we discuss how different microorganisms are being used to produce novel bioproducts from D-xylose. We also discuss how D-xylose could be potentially used instead of glucose for the production of value-added bioproducts.

Studies on xylitol production by metabolic pathway engineered Debaryomyces hansenii

Debaryomyces hansenii is one of the most promising natural xylitol producers. As the conversion of xylitol to xylulose mediated by NAD(+) cofactor dependent xylitol dehydrogenase (XDH) reduces its xylitol yield, xylitol dehydrogenase gene (DhXDH)-disrupted mutant of D. hansenii having potential for xylose assimilating pathway stopping at xylitol, was used to study the effects of co-substrates, xylose and oxygen availability on xylitol production. Compared to low cell growth and xylitol production in cultivation medium containing xylose as the only substrate, XDH disrupted mutants grown on glycerol as co-substrate accumulated 2.5-fold increased xylitol concentration over those cells grown on glucose as co-substrate. The oxygen availability, in terms of volumetric oxygen transfer coefficient, kLa (23.86-87.96 h(-1)), affected both xylitol productivity and yield, though the effect is more pronounced on the former. The addition of extra xylose at different phases of xylitol fermentation did not enhance xylitol productivity under experimental conditions.

Xylitol production from D-xylose and horticultural waste hemicellulosic hydrolysate by a new isolate of Candida athensensis SB18

This paper describes the production of xylitol from d-xylose and horticultural waste hemicellulosic hydrolysate by a new strain of Candida athensensis SB18. Strain SB18 completely consumed 250 and 300 g L(-1) D-xylose and successful converted it to xylitol in the respective yield of 0.83 and 0.87 g g(-1), resulting in 207.8 and 256.5 g L(-1) of xylitol, respectively. The respective volumetric productivity were 1.15 and 0.97 g L(-1) h(-1). Approximately 100.1 g L(-1) of xylitol was obtained from the bioconversion of detoxified horticultural waste hemicellulosic hydrolysate using strain SB18. The yield and productivity were 0.81 g g(-1) xylose and 0.98 g L(-1) h(-1), respectively. Strain C. athensensis SB18 was able to completely utilize glucose, mannose, xylose and partially arabinose. This work demonstrates that stain C. athensensis SB18 is a promising strain for high-titer and high-yield xylitol production and it has great potential in bioconversion of hemicellulosic hydrolysate.

Microbial and bioconversion production of D-xylitol and its detection and application

D-Xylitol is found in low content as a natural constituent of many fruits and vegetables. It is a five-carbon sugar polyol and has been used as a food additive and sweetening agent to replace sucrose, especially for non-insulin dependent diabetics. It has multiple beneficial health effects, such as the prevention of dental caries, and acute otitis media. In industry, it has been produced by chemical reduction of D-xylose mainly from photosynthetic biomass hydrolysates. As an alternative method of chemical reduction, biosynthesis of D-xylitol has been focused on the metabolically engineered Saccharomyces cerevisiae and Candida strains. In order to detect D-xylitol in the production processes, several detection methods have been established, such as gas chromatography (GC)-based methods, high performance liquid chromatography (HPLC)-based methods, LC-MS methods, and capillary electrophoresis methods (CE). The advantages and disadvantages of these methods are compared in this review.

Xylitol conversion by fermentation using five yeast strains and polyelectrolyte-assisted ultrafiltration

Batch fermentations for xylitol production were conducted using Candida boidinii (BCRC 21432), C. guilliermondii (BCRC 21549), C. tropicalis (BCRC 20520), C. utilis (BCRC 20334), and P. anomala (BCRC 21359) together with a mixture of sugars simulating lignocellulosic hydrolysates as the carbon source. C. tropicalis had the highest bioconversion yield (Y(P/S)) of 0.79 g g(-1) (g xylitol x g xylose(-1)) over 48 h. Additional fermentations with C. tropicalis achieved Y(P/S) values of 0.6 and 0.39 g g(-1) after 96 and 72 h using urea and soybean meal as the nitrogen sources, respectively. Ethanol and arabitol were also produced in all fermentation. Xylitol in the fermentation broth was recovered by cross-flow ultrafiltration. With prior application of 2 mg polydiallyl dimethylammonium chloride l(-1) on the membrane surface, protein in the permeate was reduced from 7.1 to 1.5 mg l(-1 )after 2 h.

Effect of medium osmolarity on the bioproduction of glycerol and ethanol by Hansenula anomala growing on glucose and ammonium

The osmotolerant yeast Hansenula anomala survives in media at low water activity resulting from increasing NaCl concentrations in the culture medium by producing compatible solutes. High salinity resulted in the use of a large part of the assimilated carbon substrate (glucose) for cell maintenance (28%), required for intracellular synthesis compounds and for osmotic cell regulation. The maintenance coefficient for non-growth-associated glucose consumption was found to be 0.38 mmol glucose g biomass(-1) h(-1). For decreasing water activity, there is a competition between the pathways leading to glycerol and ethanol production, until an experimental ethanol/total glycerol ratio reached a value 3.4 for 2 mol l(-1) NaCl (close to the theoretical value of 4)-illustrating the osmo-dependent channelling of carbon towards polyols production. This competition leads to a cessation of ethanol production during the stationary state before that of glycerol. Since osmotic adjustment occurred mainly during growth, glycerol production during stationary state can be clearly related to another mechanism other than osmotic: it was excreted by a fermentative mechanism to ensure energy for cell maintenance.