| Chinese quince（Chaenomeles sinensis）is a perennial deciduous shrub tree,and it belongs to the family of Rosaceae.The surface of the Chinese quince fruit is smooth and does not wrinkle when placed for a long time.Chinese quince fruit contains a large number of phenols,organic acids,lignin and other substances,which makes its flesh hard and astringent and cannot be eaten directly.Among them,lignin is widely found in Chinese quince,which together with cellulose,hemicellulose and pectin constitute the skeleton structure of plant cell wall.The structure with chemical relationship between lignin and carbohydrate such as cellulose,hemicellulose and pectin,is called lignin-carbohydrate complex（LCC）.The structure of LCC has an impact on the effective separation of lignin from lignocellulosic fibers,and also hinders the separation of components in the biorefinery process.Many LCC preparations can be used for structural identification,including LCC-Ac OH,Bj（?）rkman LCC,cellulolytic lignin（CEL）and mild wood lignin（MWL）.The samples can be divided into carbohydrate-rich LCC（including Bj（?）rkman LCC）and lignin-rich LCC（including LCC-Ac OH,CEL and MWL）.Generally speaking,MWL is usually used for the structural characterization of lignin in plant cell walls because it contains only a small amount of carbohydratesIn order to further understand the chemical bond connection information of cell wall skeleton structure,MWL and LCC in Chinese quince fruits under different growth stages and different subcritical water treatment conditions were studied.The results will help us to understand the changes of chemical composition and structure of cell wall in the process of lignification of Chinese quince fruits.At the same time,it will also provide theoretical support for us to realize the high value utilization of the fruit by using the subcritical extraction method to efficiently separate the effective components in the fruit.This study will also contribute to the further research on the structure interpretation of the MWL and LCC in Chinese quince fruits,which is of great significance for the study of the construction of plant cell wall.The main contents and conclusions of this thesis are as follows:1.MWL（M1,M2,M3,M4,M5 and M6）were isolated from Chinese quince fruits of each month from May to October 2019.The analysis results showed that galactose and glucose were the main monosaccharides in M1-M6 samples,and M6sample had the best homogeneity and thermal stability compared with MWL samples in other growth stages.NMR results showed that the lignin extracted from the fruit belonged to GSH type lignin,and the lignin in the fruit was mainlyβ-O-4′bond,and with the growth of the fruit,its chemical composition became more complex,and the content ofβ-O-4′bond was increasing.These results will provide some basis for the structural changes of lignin during the growth of Chinese quince fruit.2.Based on the study of structural changes of MWL in Chinese quince fruit at different growth stages,the LCC fragments（Bj（?）rkman LCC（L1-L6）and LCC-Aco H（LA1-LA6））in Chinese quince fruit at different growth stages were extracted and characterized,the changes of its structure in the whole growth cycle were observed.The results confirm that L1-L6 was a carbohydrate-rich LCC fragment and LA1-LA6was a more lignin-rich LCC fragment,the higher contents of rhamnose and galacturonic acid indicated the existence of lignin-pectin complex（LPC）.With the ripening of fruit,the molecular weight of Bj（?）rkman LCC and LCC-Aco H increased at first and then decreased,and the samples obtained from the mature fruit had the best thermal stability.All samples showed good antioxidant capacity,among which the DPPH free radical scavenging ability of L6 component and LA3 component was the most significant.This study provided a theoretical basis for the structural changes of LCC during the growth of Chinese quince fruit,which will help to understand the relationship between the chemical bonds of LCC macromolecules in plant cell wall.3.The raw materials of Chinese quince fruits were treated with subcritical water at different temperatures（130°C,150°C and 170°C）to study the structure change of MWL.The structural properties of these lignin samples（UL L130,L150 and L170）were studied using a series of modern instrumental analysis methods.The results show that subcritical water treatment is beneficial to the separation of MWL.The results of molecular weight and thermal analysis show that the MWL after pretreatment has higher thermal stability with the increase of molecular weight.The spectra of 2D NMR and 31P NMR showed that the chemical bonds of MWL were mainlyβ-O-4′ether bondβ-5′bond andβ-β′bond,the units were mainly G-S-H-type,and contained a small amount of ferulic acid.These results are consistent with the results of Py-GC/MS analysis.This study will help to understand the general mechanism of subcritical water extraction,and thus provide a theoretical basis for the development of subcritical water extraction technology.4.Based on the study of the effect of different subcritical water treatment on the structure change of MWL in Chinese quince fruits,we kept the same treatment conditions,and studied the structure change of lignin-carbohydrate complex（LCC）in the treated samples.The results showed that with the increase of subcritical water treatment temperature,the carbohydrate content in the sample decreased significantly,and the yield of Bjorkman LCC with high carbohydrate content decreased.In addition,the molecular weight size and polydispersity of LCC-Ac OH decreased gradually with the increase of subcritical water treatment temperature,while Bj（?）rkman LCC showed no significant difference.The subcritical water treatment cracked part of the chemical bonds between LCC components,and also caused some damage to the ether or ester bonds between coumaric acid and lignin as well as between carbohydrates.These findings are helpful to understand the properties of lignin and LCC in Chinese quince fruit and provide theoretical support for its potential application. |
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