Alkali-acid Pretreatments For High Ethanol Yield And Multiple Biochemical Products In Sweet Sorghum

Lignocellulose ethanol is an excellent additive to gasoline for less net carbon release and environmental cares.Sweet sorghum is a leading bioenergy crop for bioethanol production,due to much soluble sugars and high lignocellulose in the straw.In principle,lignocellulose ethanol process involves in three major steps:initial physical and chemical pretreatment,sequential enzymatic hydrolysis and final yeast fermentation.In this study,we performed alkali-acid two-step pretreatments in four representative sweet sorghum materials including two accessions（NS19 and NS143）and one mutant（11M-77）and its wild type（WT）.Using mild pretreatment and semi-simultaneous saccharification and fermentation（SSSF）of total sugars released from lignocellulose hydrolysis,together with soluble juice sugar from stems,this study obtained much high bioethanol yields and other biochemical products for agriculture and industry.The main results were described as follows:1.The fresh weights of NS19、NS143、11M-77 and WT varied from 27.66 to 108.9t/ha,but their dry bagasse yields ranged from 6.16 to 20.30 t/ha,confirming much high lignocellulose yields in sweet sorghum.2.NS19、NS143、11M-77 and WT showed juice soluble sugars in stem tissues varied from 11.61%to 23.02%（%dry bagasse）;and they had cellulose levels at 28.64%-31.75%,hemicellulose contents at 22.24%-30.59%and lignin levels at 13.15%-4.83%.3.Under 2%KOH-1%H₂SO₄ two-step pretreatments,four sweet sorghum samples showed biomass enzymatic saccharification varied from 80.90%to 91.71%（hexose yield against cellulose）,but their corresponding ethanol yields varied from 17.63%to 20.04%（%dry matter）.Total bioethanol yields reached to 21.96%-27.22%by yeast fermentation respectively using stem juice and sugars released from enzymatic hydrolysis of bagasse after pretreatments,equal to 1.15-5.53 t/ha.Hence,the sugar-ethanol conversion rates were 48.01%,45.47%,44.30%and 48.94%in four sweet sorghum samples,accounting for 94.14%,89.16%,86.86%and 95.96%of theoretical values,respectively.4.The 2%KOH pretreatment could dissolve 23.78%-37.02%of total hemicellulose and 34.17%-65.91%of total lignin,and the dissolved hemicellulose and lignin were sequentially precipitated by ethanol and acid.The precipitated hemicelluloses varied from16.41%to 17.70%,and the precipitated lignin ranged from 16.21%to 20.77%（%of total polymers）.The purity of precipitated hemicellulose and lignin were 67.59%-77.54%and83.09%-88.71%,respectively.5.Due to large amounts of pentoses in the fermented liquor,those pentoses could be used as raw material for xylitol production.After HCl precipitation of lignin in the alkali extracted liquor,the remaining supernatant contained high KCl level.Meanwhile,the remaining fermented liquor contained high concentration of K₂SO₄.Hence,both KCl and K₂SO₄ could be used as bio-fertilizers.6.The final fermented residues varied from 11.54%to 16.20%（%dry raw bagasse）in four sweet sorghum samples,which contained cellulose levels at 21.63%-26.05%（%final fermented dry residue）,hemicellulose contents at 10.70%-18.68%and lignin levels at 31.54%-43.45%.In conclusion,the two-step alkali-acid pretreatments are sufficient for high biomass enzymatic saccharification and bioethanol production using total sugars released from enzymatic hydrolysis and juices.It also generated multiple biochemical products including rich-xylose as raw material for sugar industry,KCl and K₂SO₄ as bio-fertilizers for farming,and lignin material for chemical industry,which provided additional economic benefits and reduced by-products and waste releases.