Enhancing saccharification of cassava stems by starch hydrolysis prior to pretreatment

•Cassava stems are a glucan-rich feedstock with a high starch share (up to 42%).•Starch hydrolysis prior to pretreatment improves saccharification of cassava stems.•The hydrolysis of starch minimizes sugar degradation in acid pretreatment.•The starch hydrolysis leads to a two-fold increase of the overall hydrolyzed glucan.•Pretreating cassava stems with [Emim]OAc is more effective than with sulfuric acid. Chemical characterization of cassava stems from different origin revealed that glucans accounted for 54–63% of the dry weight, whereas 35–67% of these glucans consisted of starch. The cassava stems were subjected to a saccharification study including starch hydrolysis, pretreatment with either sulfuric acid or 1-ethyl-3-methylimidazolium acetate ([Emim]OAc), and enzymatic hydrolysis of cellulose. Starch hydrolysis prior to pretreatment decreased sugar degradation, improved enzymatic convertibility of cellulose, and increased overall glucan conversion. Glucan recovery after pretreatment of starch-free cassava stems (SFCS) was around 85%, but below 52% when the stems were pretreated under the same conditions without preparatory starch hydrolysis. The total amount of hydrolyzed glucan after cellulose hydrolysis was two-fold higher for pretreated SFCS than for directly pretreated stems. Pretreatment with [Emim]OAc resulted in 20% higher glucan conversion than pretreatment with acid. Pyrolysis-GC/MS, X-ray diffraction, CP/MAS 13C NMR and FTIR analyses revealed major differences between H2SO4- and [Emim]OAc-pretreated material.