| As a typical lignocellulose feedstocks,corn stover is cheap and widespread,one of its main application is converted into feed.However,the nutritional value of single corn stover was low,microbial fermentation of corn stover can improve protein and fat content,and further improve the utilization ratio and conversion rate,which shows good economic and ecological benefits.Pretreatment is necessary to break the highly recalcitrant cell wall structure of corn stover.In order to improve the efficiency of enzymatic hydrolysis and biological conversion of corn stover,a highly efficient pretreatment system by glycerol-steam explosion was built,which breaks the degradation resistance barrier of corn stover.The mechanism for breaking its firmness and production of high protein,high fat,high energy feed were investigated by solid state fermentation of pretreated corn stover in this study.The hydrolysis efficiency of corn stover was improved significantly by glycerol-steam explosion pretreatment.After a preliminary optimization,the corn stover was pretreated as follow: the mass ratio of corn stover to glycerol was 1:1,the pressure of steam explosion was 2.0 MPa and holding time was 150 s.Under the optimized conditions,the dissociations of both hemicellulose and lignin were improved: compared with separate steam explosion,the removal rate of hemicellulose and lignin were increased from 80.06% to 87.33% and 14.03% to 22.11%,respectively.Meanwhile,the retention rate of cellulose was increased from 82.8% to 95.5%.The cellulose digestibility of the pretreated material was up to 90% with enzyme loading of 20 FPU/g dry material.Several modern analytic techniques(SEM,AFM,FT-IR,XRD and CP/MAS13C-NMR)were used to characterize the structure changes of corn stover before and after glycerol-steam explosion pretreatment,the results demonstrated that this glycerol-steam explosion pretreatment could disrupt the complex and recalcitrant architecture of corn stover by three hierarchies:(1)For the composition of materials,some component barriers(i.e.,lignin,hemicellulose and acetyl group)was removed selectively,while the most of cellulose was retained;(2)For the physical structure,the specific surface area(especially the inner surface)and roughness were enlarged,which was beneficial to the adsorption of cellulase;(3)For the chemical structure,the key chemical bonds and functional groups(i.e.,β-ether bond,β-ester band and hydrogen bond)of inter-and intra-molecules was broke down,meanwhile,the pretreatment also has a certain effect on crystalline cellulose.These above characteristics fully clarify the mechanism of enzymatic hydrolysis efficient improvement for pretreated corn stover.A high protein,high fat and high energy feed production process from glycerol-steam explosion pretreated corn stover by solid state fermentation was explored in this study.Under the simultaneous saccharification and fermentation conditions(80% moisture content,the dosage of cellulase was 20 FPU/g dry material,10% inoculation amount and 28℃),the strain type,nitrogen source type and additive amount were optimized.The true protein content in the substrate inoculated with Geotrichum candidum was 5.02% in 72 h when 3% urea was added as nitrogen source,which increased 1.32 times compared with that before fermentation(2.16%).The crude fat content and true protein content of the substrate inoculated with Trichosporon rmentans 2.1368 was 7.21% and 5.68%,which increased 146% and 19.97% compared with that before fermentation.Finally,the strategy of mixed fermentation of wheat bran and pretreated corn stover was adopted.Under the condition of the mass ratio of wheat bran to stover was 4:6,the crude fat content and true protein content of the substrate inoculated with Trichosporon rmentans 2.1368 was 9.34% and 6.75%,which increased 41.30% and 74.42% compared with that before fermentation.In the substrate inoculated with Geotrichum candidum,the true protein content was 5.47%,which increased 53.37% compared with that before fermentation. |
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