Study On Efficient Hydrothermal Pretreatment And Enzymatic Saccharification Of Cassava Stalk

Objective:Hydrothermal pretreatment is a promising avenue to improve the efficiency of enzymatic hydrolysis and saccharification of lignocellulosic substrates.Compared with the herbs investigated in many studies,the efficient pretreatment and enzymatic hydrolysis of woody plant materials is a greater challenge.This work aims to construct an efficient hydrothermal pretreatment process assisted by organic acids and alkaline ethanol for woody plant cassava stalk,based on the previous studies on the hydrothermal pretreatment of Miscanthus and other herbs.The inherent mechanism of increasing sugar yield in the process will be illustrated,hence providing basis and technical support for the energy conversion of lignocellulosic biomass and its industrial application.Methods:First,the effect of organic acid-assisted hydrothermal pretreatment（OAHP）on cassava stalk was studied.The organic acid type,organic acid concentration and pretreatment temperature were chosen as the main factors.Yields of soluble sugars in the hydrothermal pretreatment step（YS_P）,the following enzymatic hydrolysis step(YS_EH)and the whole pretreatment-enzymatic hydrolysis process（YS_T）were chosen as the main indicators.Second,the effect of organic acid-alkali two-step pretreatment（OAHP-AHP）on sugar recovery during the enzymatic hydrolysis of cassava stalk pretreatment was studied.The mechanism of organic acid-alkali two-step pretreatment for the enhancement of enzymatic hydrolysis will be explored by detecting the changes in the composition,accessibility,enzymatic absorption ability and crystallinity of pretreated cassava stalks.Finally,the effects of alkaliðanol-assisted hydrothermal pretreatment（AEHP）on the composition and microstructure of cassava stalk,and subsequent enzymatic hydrolysis were investigated to enhance sugar recovery during pretreatment and enzymatic hydrolysis stages of cassava stalk.Results:（1）The OAHP process for improving YS_Pof cassava stalk has been established.The delignification ability of OAHP is not strong.However,OAHP can effectively dissolve and recover cellulosic components such as hemicellulose,and destroy the fiber structure.Operational parameters such as temperature,acid type and concentration are optimized.Under the optimal conditions（150℃,60 min,5%oxalic acid）,YS_P,YS_EH,and YS_Treached as high as 178.0 mg/g,91.2 mg/g,and 234.3 mg/g,respectively.A higher YS_Pwas observed in the OAHP process,but the improvement of YS_EHwas limited.（2）An OAHP-AHP two-step pretreatment process has been developed for improving both YS_Pand YS_EHof cassava stalk.OAHP can effectively recover easily hydrolyzed fiber components to obtain higher YS_P.Subsequent alkaline treatment（AHP）can result in efficient delignification,hence improving YS_EHof pretreated stalks.Under the optimal conditions(1^ststep OAHP:150°C,60 min,5%oxalic acid;2^ndstep AHP:180°C,60 min,3.0%Na OH),YS_EHand YS_Treached 206.6 mg/g and 273.2 mg/g,respectively.And the YS_EHwas 3-folds higher than that of the control.However,the YS_EHof the OAHP-AHP stalks is still not so attractive due to the recalcitrance of cassava stalk.（3）An AEHP one-step pretreatment process has been established to significantly improve the YS_EHof cassava stalk.The AHP,which is very efficient on herbaceous plant stalks（e.g.,corn stalks,106.9 vs.747.9 mg/g）,showed limited enzymatic hydrolysis enhancement on cassava stalk（92.3 vs.208.9 mg/g）.In contrast,the AEHP process produced stronger structural destruction,lignin removal and easily degradable hemicellulose retention.As a result,pretreated stalk accessibility and sugar yields were also higher.Under optimal conditions（180°C,60 min,3%alkali-ethanol）,YS_EHreached as high as 865.7 mg/g,which is significantly higher than those of the control and other pretreated stalks mentioned above.Conclusions:（1）OAHP assisted by oxalic acid enhances the dissolution of fiber components,structure destruction and recovery of dissolved components in the pretreatment process,and improves the YS_Pof cassava stalk.However,it fails to significantly improve the enzymatic hydrolysis efficiency.The OAHP-AHP two-step pretreatment further improve YS_EHby enhancing lignin removal and structural destruction to stalk residues from the first OAHP step.（2）AHP is very efficient for improving YS_EHof herbal materials,but it fails to achieve that for the case of woody plants（cassava stalk）.AEHP produces a stronger structural damage,higher lignin removal,accessibility,and easily degradable hemicellulose component retention,etc.,hence obtaining a significantly higher YS_EHthan that of raw cassava stalk.