The critical impact of rice straw extractives on biogas and bioethanol production.

Extractives are nonstructural constituents of lignocellulosic materials available in small portions; however, their influence on the bioconversion processes cannot be disregarded. This study evaluated the effect of various concentrations of rice straw water extractives (RWE) and ethanol extractives (REE) on enzymatic hydrolysis, anaerobic digestion, and simultaneous saccharification and fermentation productivity. By increasing the RWE or REE concentration, the glucose yield did not change after 72 h of enzymatic hydrolysis. The RWE increment enhanced ethanol yield to 95.6%. However, the REE increment decreased ethanol yield to 32.1%. Adding RWE caused a considerable reduction in the accumulated biogas and changed the composition of produced biogas from 74% methane to less than 1%. By increasing the REE concentration, the accumulated biogas increased from 167.9 to 524.4 ml/g VS. According to the gas chromatography-mass spectrometry (GC/MS) results, the most abundant RWE and REE components were 3-hydroxy-Spirost-8-en-11-one and guaiazulene, respectively.