Xylans

High-potential, natural carbohydrate polymers for a wide range of applications

Biomass from plant materials represents an immense and widely spread renewable resource for the sustainable production of clean, cheap biofuels, and of high-value bio products for a variety of uses in medicine and pharma, food, cosmetics and other nonfood technologies. The cell wall of woody tissues of higher plants, in particular of hardwoods and softwoods is a natural composite of a complex chemical nature, mainly made of lignin, cellulose, and other plant polysaccharides, the so-called hemicelluloses.

Xylans (CAS No. 9014-63-5) are the most abundant hemicellulosic polymers, made up of xylose (a pentose sugar) units; they account for 25-35% of the dry biomass of woody tissues of dicotyl plants (hardwoods and herbaceous plants), while some tissues of monocotyl plants (cereals and grasses) contain up to 50%. Potential sources of xylans are forest and pulping waste products from hardwoods, but include also waste materials from various agricultural crops, such as straw, sorghum, sugar cane, corn stalks, cobs, grasses, cereals, and herbs.

Structure and isolation

The structure of xylans depends on the plant family they originate from. The most common xylans are made of a backbone of xylose by beta-1-->4 bonds, where the structural units are often substituted at positions C2 or C3 with arabinofuranosyl, 4-O-methylglucuronic acid, acetyl or phenolic substituents (1). In hardwoods, as represented by beech- and birch wood xylan (Cat. No.s X-6520, and X-6530, respectively) only the 4-O-methylglucuronoxylan type is found, where in softwoods the main xylan components are arabino-4-O-methylglucuronoxylans. Neutral arabinoxylans represent the main xylan component of cereal grains (e.g. oat xylan; Cat. No. X-6540). The degree of polymerization, i.e. the number of xylose-residues in the chain, depends on the origin and varies between less than 100 up to more than 500.

The isolation of xylans from plant materials, hardwoods in particular, is quite tedious and multi-step oxidative/alkaline delignification and extraction procedures with/without steaming have been proposed to yield xylan-rich polysaccharide fractions of variable purity.

Functional properties and applications

The functional properties of different xylans isolates affecting to great extent their potential uses comprise solubility, rheological behavior, surface active properties, and interactions with other biopolymers. For example, beech wood xylan shows good film forming, thickening, gelling, emulsifying and foam stabilizing properties, and has been linked with paper making, bread making, and textile printing applications. Native O-acetylated hardwood xylans (aspen, beech, birch) have been mixed with commercial cellulose esters and softeners as sorbitol or xylitol to produce transparent, biodegradable packaging films suited for foods. Similarly, biodegradable composite films with water transmission barrier properties have been produced using oat spelt xylan.

An example of a potential biomedical application has been provided recently by the synthesis of stable silver nanoparticles using wheat bran (WB) xylan as a reducing and stabilizing agent (2). The synthesized WB-xylan silver nanoparticles showed fibrinolytic activity in a fibrin plate assay as well as by dissolution of preformed blood clots.

It is known, that cereal xylans are components of dietary fibers that contribute to beneficial effects on biochemical and physiological processes in both human and animals. Nowadays, xylans from various plant origins have been recognized as source of xylooligosaccharides (XOS), xylose oligomers of variable short length, that are classified as active ingredients in functional foods. XOS are in fact non-digestible food ingredients, but cause prebiotic effects when ingested as part of the diet through the stimulation of growth of the beneficial gut microflora. These so-called probiotic lactobacilli and bifidobacteria play a critical role in maintaining host health. The production of XOS on industrial scale is done from xylan-rich lignocellulosic materials by chemical and enzymatic methods, the latter being the preferable route for food applications.


References:
(1) Ebringerova A, Heinze T. Xylan and xylan derivatives, biopolymers with valuable properties. 1. Naturally occurring xylans structures, isolation procedures and properties. Macromol Rapid Commun 2000;21:542–56
(2) Harish B S, Kiran Babu Uppuluri, Anbazhagan Veerappan. Synthesis of fibrinolytic active silver nanoparticle using wheat bran xylan as a reducing and stabilizing agent. Carbohydrate Polymers 2015;132:104-110



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