Agro-Processing Wastes as an Important Source of Bioactive Compounds
Agricultural and industrial residues are attractive sources of natural antioxidants (Volf and Popa 2004). Some studies have already been conducted on agricultural byproducts, which could be potential sources of bioactive compounds. Lignocellulosic waste comprises of rice straw, wheat straw, sugarcane bagasse, sweet sorghum bagasse, cotton stalk, etc., which is generated in huge quantum. Lignocellulosic waste, such as rice straw, is burnt in the fields after crop harvest in many Southeast Asian countries, resulting in smoke clouds causing loss of biomass and environmental pollution. A schematic flow diagram for extraction, isolation and characterization of bioactive compounds from Agricultural residues is presented in Fig. 11.2.
Fig. 11.2 Schematic diagram for extraction, isolation and characterization of bioactive compounds from agricultural residues
The agro-industrial residues mainly comprise of lignocellulosic materials and are poorly valorized or left to decay on land. Lignocellulosic materials (LCMs) are promising sources of antioxidant compounds (Domınguez et al. 2001). Lignocellulosic materials compose of cellulose, hemicellulose and lignin in varying concentrations depending on the nature of the biomass. The most important precursor of phenolic compounds from lignocellulosic waste is lignin. Lignin is a heterogeneous polymer of phenolic nature and is made up of three precursors: transconiferyl, trans-sinapyl and trans-p-coumaryl alcohols. Gymnosperm lignins show predominance of guaiacyl groups, woody angiosperms lignins contain guaiacyl–syringyl groups, and lignins from grasses contain guaiacyl–syringyl-phydroxyphenyl groups. Partial depolymerization of lignin and lignin–hemicellulose linkages occurs during the hydrolytic processing of LCM (Ando et al. 2000). Ferulic and p-coumaric acids (the most abundant hydroxycinnamic acids) are linked to arabinoxylans or pectins through ester bonds. In hardwoods, condensed tannins (proanthocyanidins) and hydrolyzable ellagitannins make also part of the phenolic fraction (Cadahıa et al. 2001), whereas acids (gallic, vanillic and ellagic) and aldehydes (syringaldehyde and sinapaldehyde) have also been detected (Conde et al. 1995). A selective recovery of the phenolic compounds from hydrolysates can be achieved by extraction with solvents such as ethyl acetate or diethyl ether. Ethyl acetate removes water-soluble phenolics and hemicellulose degradation products, whereas lignin–carbohydrate complexes remain in the aqueous phase (Bouchard et al. 1991). Phenolic compounds extracted from various LCMs are presented in Table 11.3.
A number of technologies are available for the mild hydrolysis of LCMs for production of various bioactive compounds (Amendola et al. 2012). Among them, the simplest one is autohydrolysis, wherein, the LCM is treated with water or steam. Related processes include the utilization of additional reagents such as mineral acids (prehydrolysis), SO2 or oxygen (wet oxidation). Autohydrolysis of LCM is an environment-friendly process in which the hydronium ions from water autoionization and from organic acids generated in the reaction promote the hydrolytic degradation of cell wall components. A variety of compounds appear in the liquors obtained by these technologies, including sugar oligomers, monomeric sugars, sugar degradation products (furfural and hydroxymethylfurfural), organic acids (citric and malic acid coming from the cells of the biomass, formic and levulinic acid from sugar degradation products, acetic acid from acetyl groups), extractives and phenolics.