Hemicellulose is the second abundant components of lignocellulose biomass. There are heterogeneous polysaccharides in the plant cell wall, which interconnected with cellulose by hydrogen bonds and linked to lignin by covalent bonds. (Sun, Wen, & Ma, 2013). Hemicellulose affects the strength properties and wettability of final paper due to the interactions between xylan and cellulose (Silva, Colodette, & Lucia, 2011). Hemicellulose is mainly composed of O-acetyl-(4-O-methylglucurono) xylan in Eucalyptus (Gullón, González-Munoz, & Gool, 2011). The backbone consists of xylan chain of approximately 200 linear (1,4)-linked-β-D-xylopyranosyl (Xyl) residues, and a (1,2)-linked (4-O-methyl)-α-d-glucopyranosyl uronic acid (MeGlcA) sidegroup linked with per ten xylopyranosyl residues, substituted at position C-2 position (Vignon & Gey, 1998), where is also a substitution by acetyl groups.
In the process of pulp and paper making, it is often necessary to remove lignin and part of hemicellulose to retain more cellulose. There are some studies considered that partial reservation of hemicellulose could increase the paper swelling and reduce the power consumption during pulp beating (Silva, Colodette, & Lucia, 2011). However, it is necessary to take into consideration that the formation of hexenuronic acid (HexA) from the MeGlcA side group of xylan chains in the cooking may consume more bleaching agent in the subsequent stages and do harm to pulp properties (Zhang, Nie, & Qin, 2019). Therefore, it is worth further studied about the hemicellulose changes pattern and how to effectively control the transformation of MeGlcA to HexA in the process of cooking, which will be helpful to reduce the degradation of polysaccharides and retain better paper properties. Marques, G. et al. (2010) isolated the heteroxylan by extraction of peracetic holocellulose and studied its behavior during soda/AQ pulping of sisal fibers, and the results showed that MeGlcA and glucuronosyl are mostly removed or converted to HexA. However, there is still 15% of the initially present MeGlcA was maintained intact upon cooking.
Nowadays, there are many studies on the extraction of hemicellulose from wood meal, but the extraction and separation of hemicellulose from pulp has not drawn wide attention (Hakala, Liitiä, & Suurnäkki, 2013). Compared with wood meal, the connections between hemicellulose and lignin closely on pulp have preliminarily been damaged (Borrega, Tolonen, & Bardot, 2013). The removal of lignin exposed more hemicellulose and increased the hemicellulose dissolution in alkaline extraction (Peng, Peng, & Bian, 2011). At present, there are many extraction methods for hemicellulose, such as acid-base extraction (Goldmann, Ahola, & Mikola, 2017), microwave assisted extraction (Yuan, Zou, & Zhou, 2019), organic solvent extraction (Valoppi, Lahtinen, & Bhattarai, 2019) and so on. Among these approaches, alkali extraction is widely used because the alkaline treatment could disrupt the cell wall and cleave the hydrogen bonds and covalent bonds like ester and ether linkages in the lignocellulose (Miller, & Fry, 2001), though hemicellulose extracted from aqueous alkali contains a small number of bound lignin (Wen, Sun, & Xu, 2010).
Compared with sodium hydroxide and lithium hydroxide, potassium acetate formed by potassium hydroxide during neutralizing alkali extract is more soluble in ethanol, so potassium hydroxide is more suitably selected for alkali treatment of lignocellulose materials (Sun, Wen, & Ma, 2013). The hemicellulose from sweet sorghum stem was obtained by extracted with 0.3%-2.0% potassium hydroxide aqueous solution followed with 60% ethanol containing 2.5% potassium hydroxide with the yield of 76.3% (Sun, Wen, & Ma, 2013). The solubility of hemicellulose extracted is different due to its various structural composition. Studies showed that the water-soluble polysaccharides consisted mainly of glucose units, while xylose, arabinose and glucuronic acid (UA) were the major glycosyl in alkali-soluble hemicelluloses (Peng, Peng, & Bian, 2011).
The purpose of this study is to extract and character water-soluble and -insoluble hemicellulose fractions from the pulps after different cooking degree with potassium hydroxide and precipitated with ethanol solution. The aim is to explore the changing patterns of glycosyl contents and distribution, molecular weight and structural characteristics of the above two components along with the deepening of cooking procedure, and hemicellulose degradation during the extraction process were considered by exploring the extraction temperature. The results would provide ideas for further utilization of hemicellulose and a hint to process optimization of pulping and papermaking for fiber recycling and paper aging.