Pretreatment and saccharification of cassava stems – effects of preparatory starch hydrolysis and comparison of pretreatment using sulfuric acid and ionic liquid

Because of their high carbohydrate content cassava stems are a potential non-food feedstock for ethanol production. In the current work, an analysis of the composition of cassava stems from different cultivars revealed a high starch content (up to 42% DM). A sample from the most starch-rich variety was subjected to an evaluation of several strategies for glucan saccharification. The investigation included pretreatment with sulfuric acid or with the ionic liquid 1-ethyl-3-methylimidazolium acetate, and enzymatic hydrolysis with amylases and cellulases. Cassava stems were pretreated either with or without a preparatory hydrolysis of starch, and afterwards the enzymatic saccharification of cellulose in the pretreated solids was evaluated. Cassava stems were treated with α-amylase and glucoamylase leading to an almost quantitative hydrolysis of starch, and the starch-free material was then subjected to pretreatment. Glucan recovery after acid pretreatment of starch-free cassava stems was higher than in the material that was pretreated without a preparatory starch hydrolysis step. The amount of hydrolysed glucan after enzymatic hydrolysis of cellulose was two-fold higher for the sample that was sequentially submitted to starch hydrolysis and acid pretreatment than for material that was pretreated directly without preparatory starch hydrolysis. The ionic liquid was more effective than sulfuric acid for the pretreatment as it led to a further increase of glucan conversion upon enzymatic hydrolysis. X-ray diffraction, Pyrolysis-GC/MS, FT-IR and CP/MAS ¹³C NMR analyses revealed major differences between the acid- and ionic liquid-pretreated materials.