Abstract
Subcritical water treatment has received considerable attention due to its cost effectiveness and environmentally friendly properties. In this investigation, Chinese quince fruits were submitted to subcritical water treatment (130, 150, and 170 °C), and the influence of treatments on the structure of milled wood lignin (MWL) was evaluated. Structural properties of these lignin samples (UL, L130, L150, and L170) were investigated by high-performance anion exchange chromatography (HPAEC), FT-IR, gel permeation chromatography (GPC), TGA, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), 2D-Heteronculear Single Quantum Coherence (HSQC) -NMR, and 31P-NMR. The carbohydrate analysis showed that xylose in the samples increased significantly with higher temperature, and according to molecular weight and thermal analysis, the MWLs of the pretreated residues have higher thermal stability with increased molecular weight. The spectra of 2D-NMR and 31P-NMR demonstrated that the chemical linkages in the MWLs were mainly β-O-4′ ether bonds, β-5′ and β-β′, and the units were principally G- S- H- type with small amounts of ferulic acids; these results are consistent with the results of Py-GC/MS analysis. It is believed that understanding the structural changes in MWL caused by subcritical water treatment will contribute to understanding the mechanism of subcritical water extraction, which in turn will provide a theoretical basis for developing the technology of subcritical water extraction.
Highlights
Chinese quince (Chaenomeles sinensis; Rosaceae) is native to China and is widely planted in China, Korea, and Japan
Compared with the relatively tight and smooth surfaces of the untreated Chinese quince pulp residue samples, the surfaces of the treated residues were rougher and more severely disordered, and the color deepened with the increase of pretreatment temperature [19]
Shrunken, porous structures on the surfaces of samples became more and more prominent as the severity of the subcritical water treatment increased. This may be the consequence of changes in cell wall components during the subcritical water process under high heat [14,17]
Summary
Chinese quince (Chaenomeles sinensis; Rosaceae) is native to China and is widely planted in China, Korea, and Japan. Components extracted from Chinese quince fruits, such as polyphenols, flavonoids, and organic acids, have been found to have considerable antioxidant and anti-inflammatory properties [2,3]. Chinese quince fruit lignin, abundant, is not being used and has been rarely studied, especially in terms of its structural features [4]. The lignin in Chinese quince fruit cell wall, as a network of polymers, primarily consists of guaiacyl (G), sinapyl (S), and p-hydroxyphenyl (H) units, and these three units are linked by aryl ether and carbon–carbon bonds [5]. The complexity and lack of homogeneity of Chinese quince fruit components means that they are difficult to separate, which strongly restricts their applications. A more efficient technology for separating components from the cell wall of the fruits is an important prerequisite for developing the structural components of the fruits into valuable products [7]
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