Hemicellulose Sugar Fermentation: Hydrolysate Challenges, Microorganisms, and Value-Added Products

Abstract

The efficient use of xylose represents one of the most important pillars for the economic development of converting lignocellulosic biomass into value-added by-products with potential use in various activities, such as pharmaceutical, food, or biofuel industries. The conversion of lignocellulosic biomass into bioproducts of industrial value needs to be subjected to pretreatment, enzymatic hydrolysis, fermentation, and distillation or purification step. Xylose is one of the main sugars obtained by hydrolysis of biomass hemicellulose fraction. However, xylose metabolism is restricted to a few microorganisms. This makes it difficult to establish an industrial scale of production to obtain value-added products. In addition, the use of lignocellulosic pretreatment can result in sugar degradation products: furfural (sugar pentose degradation) and 5-hydroxymethyl furfural (5-HMF) (sugar hexoses degradation); formic and levulinic acids from the degradation of furfural and 5-HMF and acetic acid from the acetyl group structures of hemicellulose; and phenolics compounds released from lignin. All of these compounds are related to losses in the fermentation process. Therefore, it is important to define the best strategies for choosing a pretreatment in order to obtain high sugar release and low conversion of contaminants. Processes that aim at detoxifying the fermentative medium must be carried out. In addition, it is necessary to establish ideal cultivation conditions, such as the concentration of dissolved oxygen and pH regulation of the médium, which controls that lead to better xylose conversion rates. Studies have been conducted with potential pentose fermenting microorganisms, such as Scheffersomyces stipitis, Spathaspora hagerdaliae, Candida shehate (ethanol production); Meyerozyma guilliermondii; Spathaspora passalidarum (xylitol production); Klebsiella oxytoca; Klebsiella pneumonia; Bacillus licheniformis (2,3-butanediol (BDO) production). Therefore, research should be focused on genetic changes in yeast strains, for the production of high-value compounds as ways to establishing gains of the biofuels production, chemicals, and food. This chapter discusses the main implications involving the conversion of hemicellulose sugar into value-added products and the main microorganisms used in these processes.KeywordsLignocellulosic biomassXylose fermentationValue-added products