Elsevier

Carbohydrate Research

Volume 346, Issue 1, 3 January 2011, Pages 111-120
Carbohydrate Research

Comparative study of alkali-soluble hemicelluloses isolated from bamboo (Bambusa rigida)

https://doi.org/10.1016/j.carres.2010.10.006Get rights and content

Abstract

The physicochemical properties and structural characteristics of seven alkali-soluble hemicellulosic preparations were determined. These were extracted from bamboo (Bambusa rigida) with 1 M NaOH, KOH, LiOH, NH3·H2O, (CH3CH2)3N, Ca(OH)2, Ba(OH)2, respectively, at 50 °C for 3 h, were comparatively studied. Sugar analysis showed that these hemicelluloses contained d-xylose as the major constituent, along with d-glucose and l-arabinose in noticeable amounts. Uronic acids, principally 4-O-methyl-d-glucuronic acid, occurred in a small amount. Furthermore, based on the sugar analysis and FTIR and NMR spectroscopy, it can be concluded that the hemicelluloses consist of a backbone of β-(1→4)-linked d-xylopyranosyl units having branches of arabinose and 4-O-methyl-d-glucuronic acid. Nitrobenzene oxidation revealed that the hemicelluloses obtained are mostly free of bound lignins. Moreover, it is noteworthy that hemicelluloses isolated with the different alkaline solutions presented different chemical compositions and slightly dissimilar structural features, indicating that alkalinity played an important role in cleaving the chemical linkages between the hemicelluloses and the lignins.

Introduction

The capture of solar energy through photosynthesis is a process that enables the storage of energy in the form of cell-wall polymers (that is, cellulose, hemicelluloses, and lignin). The energy stored in these polymers can be accessed in a variety of ways, ranging from simple burning to complex bioconversion processes.1 Currently and most importantly, the research activities worldwide focus on biomass utilization, mainly in developing renewable byproducts, such as bioethanol, xylitol, and a great number of chemicals from forest and agriculture residues.2

Bamboo is the common term for a number of large woody grasses of a particular taxonomic group (subfamily Bambusoideae, family Poaceae) that are widely distributed in the Asian countries, such as China, and other Southeast Asian countries. To date, bamboo is mainly used in construction and reinforcing fibers, in paper, textiles, and construction boards, as well as in the food industry and bioenergy applications.3 According to a research report by Scurlock et al.,3 the cellulose content of bamboo is about 40–48% of the total mass. A cellulose content in this range makes bamboo a useful feedstock for paper production and for processes that convert cellulose to fuels, chemicals, and other biobased materials. Similarly, the lignin content of bamboo is 25–30%, compared to 11–27% reported for non-woody biomass, which more closely resembles the ranges reported for softwoods (24–37%) and hardwoods (17–30%). However, hemicelluloses (pentosans that usually amount to 22–35% in dry bamboo) should not be ignored because they can also be used in chemical conversions. Tomalang et al.4 in their study also found that the main constituents of bamboo culms are holocellulose (60–70%), pentosans (20–25%), hemicelluloses and lignin (each of which amount to about 20–30%) and minor constituents like resins, tannins, waxes, and inorganic salts. Therefore, the large amounts of bamboo hemicelluloses are significant enough to be considered as a complementary source of raw material for different industries such as papermaking, baking, and food as well as non-food industries. Heretofore, there have been some studies that focused on the hemicelluloses of bamboos, yet the potential of this novel chemical has not completely been recognized due to its complex chemical structure.

Hemicelluloses, the second most abundant constituent of lignocellulosic biomass, are not chemically well-defined compounds but rather a family of polysaccharides composed of different five- and six-carbon monosaccharide units.1 Generally, hemicelluloses found in higher plants contain a basic backbone of d-xylopyranosyl residues linked together by β-(1→4)-glycosidic bonds. Substituents such as arabinosyl, glucuronic acid, and acetyl groups, as well as various oligosaccharides, can be attached to the main chain at the two free OH groups of carbons C-2 and C-3 of the xylopyranose residue. In addition, phenolic acids such as ferulic acid and p-coumaric acid have been found to be etherified or esterified to O-5 of some arbinofuranose residues in arabinoxylans (AX).5 Based on the hitherto reported review articles on the primary structure of xylans from various plant tissues, xylan-type polysaccharides can be divided into homoxylans and heteroxylans, which include glucuronoxylans, arabino (glucurono) xylans, glucurono (arabino) xylans, arabinoxylans, and complex heteroxylans.6 In addition, the degree, type, and distribution pattern of the substitutions along the xylan backbone largely determine the structural properties. Because of their structural varieties and diversities, they can be utilized in their native and modified forms in various areas, including food and non-food applications.7 It is for this reason that the industrial process for xylitol is by the hydrolysis of xylose-rich hemicelluloses to xylose, followed by hydrogenation of the xylose produced. Therefore, it appears especially important to study different plant hemicelluloses.

Based on studies in recent years, many methods have been developed to isolate hemicelluloses from the cell walls of agriculture residues. The hemicelluloses isolated from bamboo also have been investigated for decades.8 Generally, hemicelluloses isolated with alkalis are commonplace, a fact that can be explained in that alkali isolation usually can be easily performed and it also has considerable cost advantage. However, the extractability of hemicelluloses varies with the alkali type and isolation conditions in different plants. In general, the alkaline treatment of lignocellulosic substances disrupts the cell wall by dissolving hemicelluloses and lignin, hydrolyzing uronic and acetic esters, swelling the cellulose and decreasing its crystallinity, and cleaving the α-ether linkages between lignin and hemicelluloses, as well as the ester bonds between lignin and/or hemicelluloses and hydroxycinnamic acids, such as p-coumaric and ferulic acids. The cleavage of O-acetyl groups cannot be avoided because the pH of the solution is at pH ∼10, and all acetyl groups are hydrolyzed.9, 10 Although many literature reports address the different polysaccharides isolated with aqueous alkalis from various plants, little information about hemicelluloses of Bambusa rigida extracted with aqueous alkalis has been reported. Therefore, a comparative study of isolation and characterization of alkali-soluble hemicellulosic fractions of Bambusa rigida is of great importance in exploring and promoting the potential utilization of this biomass.

In the present study, the isolated bamboo hemicelluloses were studied by sugar analysis, alkaline nitrobenzene oxidation (NBO) of bound lignin, Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance spectroscopy (NMR), and gel-permeation chromatography (GPC), all of which provide significant evidence for the elucidation of the hemicellulosic structures of bamboo as well as the relationship between lignin and hemicelluloses.

Section snippets

Yield and chemical composition

The hemicellulosic polymer is a mixture of a number of different polysaccharides, and the yield and composition of the polymer can vary depending on the method of isolation. Generally, the yield may be affected by the alkalis used in the experiment. Experimental data showed that the use of strong alkalis resulted in a higher yield of hemicelluloses when performed at given temperature (50 °C), suggesting that linkages between hemicelluloses and lignin were significantly disrupted and more of the

Conclusions

The FTIR spectroscopy shows that alkali-soluble hemicellulosic fractions of Bambusa rigida belong to the xylan family. Sugar analysis and NMR spectroscopy further confirm that these hemicellulosic fractions contain branches which may be a single arabinose residue, 4-O-methyl-glucuronic acid residues or a short chain of sugar residues containing arabinose and galactose, belonging to an arabinoglucurnoxylan. GPC results also showed that the hemicelluloses extracted with strongly alkaline

Materials

Bambusa rigida was obtained from Si Chuan province, China. It was dried in an oven at 50 °C and then cut into small pieces. The cut Bambusa rigida was ground and screened to prepare 20–40 mesh (450–900 μm) particles. The dried bamboo particles were first extracted with toluene and EtOH (2:1, v/v) in a Soxhlet extractor for 6 h, and the de-waxed meal was allowed to dry in an oven at 60 °C for 16 h. All other chemicals were of analytical-reagent grade and purchased from Beijing Chemical Reagent

Acknowledgments

The authors are extremely grateful for the partial financial supports from Major State Basic Research Projects of China (973, 2010CB732204), National Natural Science Foundation of China (30930073, 30710103906), China Ministry of Education (111), and State Forestry Administration (200804015).

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