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Xylitol, as an alternative low calorie sweetener is well accepted in formulations of various confectioneries and healthcare products. Worldwide it is industrially produced by catalytic hydrogenation of pure d-xylose solution under high temperature and pressure. Biotechnological xylitol production is a potentially attractive replacement for chemical process, as it occurs under much milder process conditions and can be based on sugar mixtures derived from low-cost industrial and agri-waste. However, microbial fermentation route of xylitol production is not so far practiced industrially. This review highlights the challenges and prospects of biotechnological xylitol production considering possible genetic modifications of fermenting microorganisms and various aspects of industrial bioprocessing and product downstreaming.

Ethanol is considered the most potential next generation automotive fuel because it is carbon-neutral and could be produced from renewable resources like lignocellulosic biomass. There are some technological barriers such as pretreatment, saccharification of cellulose and hemicellulose matrix, and simultaneous fermentation of hexose and pentose sugars which needs to be addressed for efficient conversion of lignocellulosic biomass to bioethanol. This paper reviews the various process options and kinetic models adopted towards resolving the technological challenges to develop a low-cost commercial process.

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Xylanase is an enzyme in high demand for various industrial applications, such as those in the biofuel and pulp and paper fields. In this study, xylanase-producing microbes were isolated from the gut of the wood-feeding termite at 50°C. The isolated microbe produced thermostable xylanase that was active over a broad range of temperatures (40-90°C) and pH (3.5-9.5), with optimum activity (4,170 ± 23.5 U mg⁻¹) at 60°C and pH 4.5. The enzyme was purified using a strong cation exchanger and gel filtration chromatography, revealing that the protein has a molecular mass of 205 kDa and calculated pI of 5.38. The half-life of xylanase was 6 h at 60°C and 2 h at 90°C. The isolated thermostable xylanase differed from other xylanases reported to date in terms of size, structure, and mode of action. The novelty of this enzyme lies in its high specific activity and stability at broad ranges of temperature and pH. These properties suggest that this enzyme could be utilized in bioethanol production as well as in the paper and pulp industry.

Several efforts have been made during the last three decades to develop successful lignocellulose-based technologies for the production of fuels and chemicals. However, such technologies still seemed to be emerging, because of the high technical risks involved and huge capital investments. This paper describes a holistic approach toward utilization of sugar cane bagasse as lignocellulosic feedstock into fuel (ethanol), chemical (furfural), and energy (electricity), using a biorefinery approach instead of co-generation. The proposed scheme could be integrated with existing sugar or paper mills, where the availability of biomass feedstock is in abundance. Fermentable sugar components (xylose and glucose) from sugar cane bagasse have been extracted employing acid hydrolysis and enzymatic saccharification. Recovery and reuse of saccharifying enzyme was a major process advantage. The pentose fraction was efficiently utilized for yeast biomass generation and furfural production. High-temperature fermentation of a hexose stream by thermophilic yeast Kluyveromyces sp. IIPE453 (MTCC 5314) with cell recycle produced ethanol with an overall yield of 88% ± 0.05% and a productivity of 0.76 ± 0.02 g/L h–1. A complete material balance on two consecutive process cycles, each starting with 1 kg of feedstock, resulted in an overall yield of 366 mL of ethanol, 149 g of furfural, and 0.30 kW of electricity.

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A yeast strain Kluyveromyces sp. IIPE453 (MTCC 5314), isolated from soil samples collected from dumping sites of crushed sugarcane bagasse in Sugar Mill, showed growth and fermentation efficiency at high temperatures ranging from 45 degrees C to 50 degrees C. The yeast strain was able to use a wide range of substrates, such as glucose, xylose, mannose, galactose, arabinose, sucrose, and cellobiose, either for growth or fermentation to ethanol. The strain also showed xylitol production from xylose. In batch fermentation, the strain showed maximum ethanol concentration of 82 +/- 0.5 g l(-1) (10.4% v/v) on initial glucose concentration of 200 g l(-1), and ethanol concentration of 1.75 +/- 0.05 g l(-1) as well as xylitol concentration of 11.5 +/- 0.4 g l(-1) on initial xylose concentration of 20 g l(-1) at 50 degrees C. The strain was capable of simultaneously using glucose and xylose in a mixture of glucose concentration of 75 g l(-1) and xylose concentration of 25 g l(-1), achieving maximum ethanol concentration of 38 +/- 0.5 g l(-1) and xylitol concentration of 14.5 +/- 0.2 g l(-1) in batch fermentation. High stability of the strain was observed in a continuous fermentation by feeding the mixture of glucose concentration of 75 g l(-1) and xylose concentration of 25 g l(-1) by recycling the cells, achieving maximum ethanol concentration of 30.8 +/- 6.2 g l(-1) and xylitol concentration of 7.35 +/- 3.3 g l(-1) with ethanol productivity of 3.1 +/- 0.6 g l(-1) h(-1) and xylitol productivity of 0.75 +/- 0.35 g l(-1) h(-1), respectively.