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Sequential extraction and compositional characterization of carbohydrates from selected agro-industrial wastes

Agro-industrial wastes comprise different types of plant residues that are generated as by-products on the field or in the factory. Such biomass is characterised by high carbohydrate content and nowadays is becoming an increasingly studied raw material for their isolation and re-utilisation within the sustainable production concepts in food technology and biotechnology. Due to the complexity of plant cell wall matrix, recovery of polysaccharides is a demanding task, whether the aim is obtaining satisfactory yields, preserving native structures or, often, both. The present study aimed to define a suitable procedure for sequential extraction of polysaccharides from sugar beet pulp (SBP), walnut shell (WS), cocoa bean husk (CBH), and pea pods (PP), as well as to establish a reliable method for compositional evaluation of the obtained fractions. Alcohol insoluble residue (AIR) was prepared by repeated extractions with 70% ethanol. AIR was subsequently subjected to hot diluted acid (HCl) extraction, delignification with NaClO2 and CH3COOH and finally alkaline extraction with NaOH. Yields of the obtained fractions were measured gravimetrically, while the composition using developed HPLC-UV technique with 3-methyl-1-phenyl-5-pyrazoline-2-one pre-column labeling, after complete hydrolysis. Three fractions were obtained, namely, pectic (P), hemicellulosic (H) and cellulosic (C), with fraction mass ratios P:H:C dependent on the sample and the employed extraction protocol. SBP exhibited the highest yield of pectins (18.24% dwb), while WS and PP of hemicelluloses (28.65% and 22.32% dwb). Monomeric composition evaluation revealed typical galacturonic acid dominance for pectic fractions of SBP and CBH, while their hemicelluloses were of very heterogeneous composition. The obtained hemicelluloses of WS and PP were xylan-type, with relative xylose content of nearly 90% and 80%, respectively. The established procedure is regarded as suitable for pectin and hemicellulose extraction due to high total xylan (71.53% for WS, 71.43% for PP) and galacturonan recoveries (88.84% for SBP) from the starting material.

Agro-industrial waste; Carbohydrate recovery; Monomeric composition; Polysaccharides; Sequential extraction

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