Current trends in biotechnological production of xylitol and future prospects

This review describes recent research developments on biological conversion of hemicellulosic biomass towards production of xylitol by taking advantage of power of biotechnology. Xylitol is a five-carbon sugar alcohol with established commercial uses in different healthcare sectors and especially as an alternative sweetener for diabetic persons. Xylitol can be synthesized either by chemical hydrogenation of xylose or by fermentation. The precursor xylose is produced from biomass by chemical or enzymatic hydrolysis and can be converted to xylitol primarily by yeast strains which offer the possibilities of economic production by reducing required energy when compared to chemical production. Biomass hydrolysis under an acidic environment is the most commonly used practice and is influenced by various process parameters. Several microbial growth inhibitors are produced during chemical hydrolysis that reduce xylitol production from xylose, a detoxification step is therefore essential. Enzymatic hydrolysis has advantages over chemical conversion although more research is necessary to reduce inhibition due to structural variation from different substrates or plant species. Enzymatic xylitol production is mostly an integral process of microbial species belonging to the Candida genus. Extensive research has been performed to screen for xylitol producing microbial strains as well as to understand microbial metabolism, the xylitol metabolic pathway, cofactor requirements, development of robust recombinant strains, optimization of bioconversion parameters and xylitol production strategies using free and immobilized cells. The imperative role of hydrolysis of xylose containing biomass and subsequent process parameters has major impact on economis of bioconversion. The review identifies ways forward for improved enzymatic xylitol production to compete with current chemical processes.