Bioremediation of Lignocellulosic Waste Coupled to Production of Bioethanol

ROSALES SORO, MARÍA DEL M.; AHMED, PABLO M.; DE FIGUEROA, LUCÍA I.C.; PAJOT, HIPÓLITO F.

Strategies for Bioremediation of Organic and Inorganic Pollutants

CRC Press Taylor & Francis Group

Año: 2018; p. 28 - 46

The first-generation ethanol employed nowadays is produced from comestible stocks such as corn and sugarcane, raising several concerns about world hunger and making the search for new raw materials a priority concern (Tenenbaum 2008). Second generation ethanol is produced from lignocellulosic biomass generated as an agricultural and industrial waste. According to the data of the United States Department of Energy (http://www.energy.gov/), ethanol from lignocellulose could decrease the release of greenhouse gasses by 85%, as compared to the use of gasoline. Such waste does not have alternative uses. However, producing ethanol from lignocellulosic biomass is quite a complicated process involving multiple processes and profitable technologies which have not been developed yet (Kumar et al. 2008). In the saccharification process, monomeric sugars are released from celluloses and hemicelluloses leading mainly to the accumulation of glucose and xylose. The fermentation of such monomeric sugars in pilot factories as IoGen, Abengoa, is performed by Saccharomyces cerevisiae. The technology for the production of ethyl alcohol from glucose or sucrose with baker?s yeast has been around for centuries. However, S. cerevisiae ferments only hexoses leaving xylose unconsumed. Therefore, the search of yeast strains capable of fermenting all sugars released from lignocellulose has received special attention recently (Li et al. 2015).Metabolic engineering permits improving fermentation characteristics of yeasts(Jeffries and Jin 2004). Significant progress has been made in the heterologous gene expression of bacterial and fungal xylose reductases (XR) and xylose isomerases (XI) in S. cerevisiae cells to increase ethanol yield from xylose (Moyses et al. 2016). One of the most promising approaches is to obtain a modified and robust industrial strain of S. cerevisiae by genetic engineering and subsequently taking adaptive evolution with selection pressure for optimal xylose utilization (dos Santos et al. 2016).This chapter discusses the advantages of using lignocellulosic biomass to produce ethanol explaining the need for biomass pretreatment to enhance the hydrolysis of cellulose by the rigid association of cellulose with lignin. Particular emphasis will be placed on the cellulose hydrolysis to produce fermentescible sugars, the convertion of these sugars into ethanol, and the improvement of engineered S. cerevisiae strains for xylose fermentation.