Structural Characterization of Lignin in Fruits and Stalks of Chinese Quince

Chinese quince (Chaenomeles sinensis) is used in food and pharmaceutical products, but it is seldom eaten as a raw fruit due to its astringent, woody flesh. The structural characterization of lignin fractions from Chinese quince was very important to investigate the structure-activity relationships of lignin. In this investigation, to characterize the structure of lignin in Chinese quince fruits, the milled wood lignin sample was isolated from the fruits (FMWL) and the chemical structure of FMWL was investigated by sugar analysis, FT-IR, GPC, pyrolysis-GC/MS analysis, UV spectra analysis, thermogravimetric analysis (TGA), and advanced NMR spectroscopic techniques. In addition, the lignin fraction from the stalk of Chinese quince (SMWL) was also prepared for comparison to obtained more information of lignin structure in the fruits. The results showed that the two lignin fractions isolated from fruit and stalk of Chinese quince exhibited different structural features. The two MWL samples were mainly composed of β-O-4 ether bonds, β-5 and β-β′ carbon-carbon linkages in the lignin structural units. Compared to the SMWL, the FMWL fraction had the higher S/G ratio and more carbohydrates linkages. The predominant carbohydrates associated with FMWL and SMWL fractions were glucans-type hemicelluloses and xylan-type hemicelluloses, respectively. Understanding the structure of lignin could give insight into the properties of the lignin and enable the food processing industry to separate lignin more efficiently.