Improvement on d-xylose to Xylitol Biotransformation by Candida guilliermondii Using Cells Permeabilized with Triton X-100 and Selected Process Conditions

Fungal biorefinery for sustainable resource recovery from waste

Depletion of natural resources and negative impact of fossil fuels on environment are becoming a global concern. The concept of biorefinery is one of the alternative platforms for the production of biofuels and chemicals. Valorisation of biological resources through complete utilization of waste, reusing secondary products and generating energy to power the process are the key principles of biorefinery. Agricultural residues and biogenic municipal solid wastes are getting importance as a potential feedstock for the generation of bioproducts. This communication reviews and highlights the scope of yeast and fungi as a potent candidate for the synthesis of gamut of bioproducts in an integrated approach addressing sustainability and circular bioeconomy. It also provides a close view on importance of microbes in biorefinery, feedstock pretreatment strategies for renewable sugar production, cultivation systems and yeast and fungi based products. Integrated closed loop approach towards multiple product generation with zero waste discharge is also discussed.

Improved upstream processing for detoxification and recovery of xylitol produced from corncob

This work deals with the development of an improved process for xylitol production from corn cob hydrolysate by biotechnological routes emphasizing the detoxification of corncob acid hydrolysate. The acid hydrolysate obtained by acid hydrolysis of corn cob was concentrated and detoxified by activated charcoal, membrane process and ion exchange resin process. The resultant partially purified corncob hydrolysate was used in fermentation. The fermentation of acid hydrolysate containing 56.5 g/L xylose was carried out in a 14 L fermenter at pH 4.5 for 48 h with 150 rpm stirring rate at 30 °C. A xylitol yield of 62% was achieved from the partially purified acid hydrolysate medium during fermentation using Candida tropicalis MTCC 6192. The purity of xylitol was increased to 92-94% upon downstream processing of carbonation, subsequently ion exchange process and activated charcoal.

Production of xylitol by expressing xylitol dehydrogenase and alcohol dehydrogenase from Gluconobacter thailandicus and co-biotransformation of whole cells

In the present study, recombinant strains were constructed for xylitol production by cloning and expressing the novel xylitol dehydrogenase (xdh) and alcohol dehydrogenase (adh) genes in E. coli BL21 (DE3) from Gluconobacter thailandicus CGMCC1.3748. The optimum pH, temperature, specific activity and kinetic parameters were further investigated for purified XDH. The co-culture of G. thailandicus (30 g/L), BL21-xdh (20 g/L) and BL21-adh (20 g/L) produced 34.34 g/L of xylitol after 48 h in the presence of 40 g/L d-arabitol and 2% ethanol. The concentration of xylitol produced in this co-biotransformation was found to be 2.7-folds higher than the xylitol yield of G. thailandicus alone, while the yield was increased by 4.8% when compared to that of G. thailandicus mixed with BL21-xdh under the similar experimental conditions.

Xylitol production by Debaryomyces hansenii and Candida guilliermondii from rapeseed straw hemicellulosic hydrolysate

This study evaluated the possibility of using rapeseed straw hemicellulosic hydrolysate as a fermentation medium for xylitol production. Two yeast strains, namely Debaryomyces hansenii and Candida guilliermondii, were used for this bioconversion process and their performance to convert xylose into xylitol was compared. Additionally, different strategies were evaluated for the hydrolysate detoxification before its use as a fermentation medium. Assays in semi-defined media were also performed to verify the influence of hexose sugars on xylose metabolism by the yeasts. C. guilliermondii exhibited higher tolerance to toxic compounds than D. hansenii. Not only the toxic compounds present in the hydrolysate affected the yeast's performance, but glucose also had a negative impact on their performance. It was not necessary to completely eliminate the toxic compounds to obtain an efficient conversion of xylose into xylitol, mainly by C. guilliermondii (Y_P/S=0.55g/g and 0.45g/g for C. guilliermondii and D. hansenii, respectively).