Study On Extraction And Application Of Depolymerization Products Of Low Molecular Lignin From Chinese Quince Fruit

Chinese quinces(Chaenomeles sinensis;Rosaceae)are planted throughout China and eastern Asia.Lignin is one of the main constituents of Chinese quince fruits.Lignin has significant antioxidant activity,and it can be potentially used as a natural antioxidant in different products.Compared with the traditional lignocellulosic source materials,such as hard-woods,soft-woods and grasses,lignin from Chinese quince fruit were seldom studied.The antioxidant activity of lignin can be influenced by the high molecular weight,high heterogeneity and the presence of polysaccharides,among which molecular weight is a more important factor.The advantages of small molecular lignin include high oxidation resistance,low polydispersity,good solubility and less structural transformation cost.The purpose of this objective is to prepare the lignins with low molecular weight and high antioxidant activity from Chinese quince fruit.The main works and conclusions are as follows:1.The structural features and antioxidant activities of Brauns native lignin(BNL)isolated from Chinese quince fruit after oven drying(60°C,75°C,and 90°C),sun drying,freeze-drying,and subcritical dimethyl ether drying were assessed.The results showed that all lignin samples contained carbohydrates.Glucan and galacturonic acid are the main polysaccharides in BNL.?-O-4?,which account for 70-80%of all linkages,is the main linkage of BNL.The results of antioxidant assays showed that ODL₉₀had the highest antioxidant activity.These results indicate that BNL,especially BNL extracted from Chinese quince fruit dried at 90°C,is a potential natural antioxidant.However,the disadvantages of BNL are the high contents of carbohydrates and high polydispersity.2.The sequential fractionation with acetone solutions of different concentrations was used to improve the structural homogeneity and purity of BNL.After fractionation,the carbohydrate contents of the obtained BNL fractions were evidently decreased besides F_30%and F_40%.The molecular weights of BNL fractions were increased from F_30%to F_80%and the polydispersity of all the fractions were remarkably lower than BNL.The fractions with low molecular weights contain more?-O-4?linkages,while the fractions with high molecular weights contain more C-C linkages.The results of antioxidant assays showed that F_50%and F_60%had the higher antioxidant properties than BNL.The sequential fractionation with acetone solutions different concentrations is an effective method to prepare BNL fractions with low polydispersity and enhanced antioxidant property.3.Organosolv is an efficient process to produce a high-quality(free of sulfur and other inorganic impurities),low molecular weight(M_w)lignin.Therefore,the lignin was isolated from the residue solid by auto-catalyzed ethanol organosolv pretreatment.The effects of three processing conditions(temperature,time,and ethanol concentration)on yield,structural features and antioxidant activities of AEOL samples were assessed individually.Results showed the pretreatment temperature was the most significant factor;it affected the molecular weight,S/G ratio,number of?-O-4?linkages,thermal stability,and antioxidant activities of lignin samples.According to the GPC analyses,the molecular weight of lignin samples had a negative correlation with pretreatment temperature.The structural characterization indicated that the various pretreatment conditions affected the structures of organosolv lignin,which further resulted in differences in the antioxidant activities of the lignin samples.4.AEOL was sequentially fractionated with organic solvents(i.e.,dichloromethane,ethyl acetate,methanol,acetone,and dioxane/water)of enhancing dissolving capacity to prepare homogeneous lignin fractions.GPC results showed that the molecular weights of lignin fractions increased from dichloromethane fraction to dioxane/water fractions.Among the five fractions,the ethyl acetate fraction showed a lower proportion of?-O-4?linkages,the highest thermal stability,the highest phenolic OH content,and the highest DPPH radical scavenging index and reducing capacity.The high antioxidant performance of the ethyl acetate fraction implies that it can be used as a natural antioxidant.This study shows that sequential solvent fractionation of lignin can produce homogeneous fractions with enhanced antioxidant performance.5.Depolymerization of lignin is an effective technology to overcome the disadvantages,such as structural diversity,heterogeneity,and poor solubility,of lignin.The effects of lignin depolymerization products on oxidative stability of biodiesel were studied.13 phenolics were detected by GC/MS.After the addition of the three depolymerization products,the kinematic viscosity of the three biodiesels slightly increased.The depolymerization products can significantly extend the oxidation induction time of the three biodiesels.The results showed that the depolymerization products of Chinese quince fruit lignin is an effective antioxidant for vegetable biodiesels.