Enhancing Enzymatic Hydrolysis Of Lignocellulosic Biomass By Surfactant And Its Underlying Mechanism

With increasing global awareness of the adverse environmental impacts of the use of fossil fuel resources,transformation of the abundant and renewable biomass resources into clean and sustainable biofuels has drawn increased public attention.Furfural residues(FRs)are a potential raw material for bioethanol production due to their high cellulose content and easy availability.At present,most studies have focused on using various pretreatment technologies to remove the lignin and improve the enzymatic hydrolysis of FRs,but the problem of conversion cost still needs to be solved to achieve its commercial production.The highly complex internal structure of fibrous materials could be changed by effective pretreatment.Reduce the amount of chemicals and energy consumption required in the bioconversion process.High efficiency conversion of fiber raw materials to target products at low enzyme consumption should be achieved.In this study,FRs were pretreated by acid bisulfite pretreatment.The effects of pretreatment conditions on lignin removal,cellulose accessibility,interfacial properties of the hydrolysis system,and the relationship among them were investigated.Surfactants assisted enzymatic hydrolysis of FRs after acid bisulfite-pretreatment was studied.The feasibility of high-solids simultaneous saccharification and ethanol fermentation(SSF)in yeast was investigated using unwashed FRs with tea-seed cake(TSC).And the mechanism by TSC which enhanced the SSF of FRs was also investigated.Cassava residues(CR)were used as an ideal substrate for rhamnolipid production.Then the feasibility of rhamnolipid-assisted high-solids SSF of unwashed FRs and the process of rhamnolipid action was investigated.The main conclusion was shown as below:Acid bisulfite pretreatment followed by enzymatic hydrolysis of unwashed FRs is a promising technology for enhancing the efficiency of enzymatic hydrolysis.The results showed that the enzymatic digestibility of non-detoxified whole slurry,at a relatively low enzyme loading of 10 FPU/g-cellulose,increased from 71.02%to 93.82%,when FRs were pretreated with bisulfite at 100? for 3 h.Partial delignification caused by sulfonation during acid bisulfite pretreatment process produces lignosulfonate that could act as surfactant to enhance enzymatic saccharification due to its low affinity to cellulase.The surface tension of supernatants was decreased with the existence of lignosulfonate,resulting in an increased initial reaction rate and the decreased non-productive adsorption of cellulase on the substrate.Surfactant assisted high-solids enzymatic hydrolysis for acidic bisulfite-pretreated FRs was investigated.The results showed that the cellulose conversion of non-detoxified pretreated whole slurry of FRs,at a relatively low enzyme loading of 2.5 FPU/g-cellulose,increased from 32.73%to 59.39%,when Sapindus mukurossi saponin was added in the enzymatic hydrolysis process.Moreover,hydrolysis of FRs at 5 FPU/g-cellulose reached a maximum sugar concentration(88.85 g/L)and cellulose conversion of 91.38%when Sapindus mukurossi saponin was added at the dosage of 4 g/L.The surface tension(approximately 51.50 mN/m)of the supernatants from enzymatic system with surfactant was lower than that of other enzymatic systems(about 66.54 mN/m).Contact angle testing showed that the enzymatic system with the addition of surfactant(33.2°)had better wetting capacity than FRs(43.9°).Unwashed FRs are a suitable lignocellulosic feedstock for ethanol production with TSC addition.The tea saponin in TSC could effectively promote the conversion efficiency of unwashed FRs at high substrate concentration and low enzyme loading.The proteins in TSC could provide available nitrogen for yeast growth.Assay employing 15%(w/w)substrate concentration and 10 FPU/g-cellulose enzyme dosage obtained a final ethanol yield of 86.56%.Furthermore,fermentations in different media showed that the surface tension(49.21 mN/m)and contact angle(42.6°)of the fermentation system with TSC were lower than those in the other systems.This study found that the SSF process with TSC addition requires lower energy input compared with other bioconversion process involving pretreatment,which suggests that the capability of high-solids SSF can be assessed easily using a simple assay.CR was used as a carbon source for rhamnolipid production in four fermentation schemes using Pseudomonas aeruginosa ATCC 10145.The optimal process involved the direct use of the CR for SSF,and then,the SSF process parameters were optimized.A rhamnolipid concentration of 11.49 g/L was achieved from the SSF of 5%(w/v)CR with a low enzyme dosage(3.75 FPU cellulase/g-CR and 4.25 CBU ?-glucosidase/g-CR).The rhamnolipid produced from the CR exhibited a critical micelle concentration of 150 mg/L.The addition of rhamnolipid is an effective strategy to enhance ethanol production from FRs,even without pretreatment.Addition of 0.2 g/L rhamnolipid enhanced ethanol yield of unwashed FRs by 22.39%.SSF with 0.2 g/L rhamnolipid addition showed 82.38%cellulose conversion at 10 FPU/g-cellulose.Rhamnolipid influences the SSF process in the following ways:i)Adding rhamnolipid to the fermentation medium reduced the surface tension of the SSF system;ii)Rhamnolipid accumulated at the interface between the two phases and reduced the level of deactivation of cellulase;and iii)X-ray photoelectron spectroscopy spectra analyses revealed that rhamnolipid addition efficiently reduced the adsorption of the enzyme on fermentation residues.