Extraction and characterization of hemicelluloses from Pinus radiata and its feasibility for bioethanol production

Galactoglucomannans are the major hemicellulosic fraction present in softwoods, and is mainly composed by the hexoses: galactose, glucose and mannose. The  efficient and selective isolation of these hemicelluloses is regarded as one of the  most critical obstacles to overcome for their utilization. The objective of the current work was to extract and characterize acidic and neutral aqueous solutions of   hemicelluloses from Pinus radiata wood chips and investigate their feasibility for bioethanol production. Hemicelluloses in P. radiata represented 26 g/100 g wood (o.d.w.) and hexoses are responsible for approximately 64% of this amount.   According to the different extraction conditions, approximately 50% of the  hemicellulosic fraction was solubilized and recovered after precipitation with ethanol.  The recovered hemicellulosic fractions were in the form of oligomers with weight- average molecular weigth (Mw) varying from 4x103 to 4x105 g/mol. Hemicellulosic oligomers were hydrolyzed with dilute sulfuric acid and the hydrolysates concentrated until approximately 70 g/L of hexoses and fermented by Saccharomyces cerevisiae yeast. Fermentation results showed that sugar obtained from acid and neutral extractions were fermented to ethanol with maximum yields of 63% and 54% (22 g/L and 19 g/L), respectively. The conversion of wood hemicellulosic substrates to ethanol is feasible but the low ethanol yields obtained make the process not economically attractive and optimization of the process or alternatives uses for hemicelluloses should be evaluated.