Microwave Heating for Solubilization of Polysaccharide and Polyphenol from Soybean Residue (Okara)

Abstract

Okara, a soybean by-product of tofu production, was treated with microwave irradiation in water for solubilization of its components. The solubilization rate increased with increasing temperature and reached higher than 70% around 200℃ with a heating time of 7 min. The main solubilized components were neutral carbohydrates consisting of arabinose and galactose, while the residues were mainly composed of cellulose. LV-SEM images confirmed solubilization of cell wall components. Production of new polyphenolic compounds that have antioxidant activity was observed above 180℃. The heating temperature and heating time were optimized for neutral carbohydrate solubilization using the response surface methodology. The optimized condition was heating at 196℃ for 2 min, indicating that a short heating time is effective for solubilization of carbohydrates to prevent secondary decomposition.Keywords: soybean residue, Okara, microwave heating, response surface methodology *To whom correspondence should be addressed. E-mail: stsubaki@kais.kyoto-u.ac.jp IntroductionOkara (soybean residue) produced following extraction of soymilk from soybean is mainly constituted with dietary fibers accompanied by other substances such as protein and lipids (O'Tool, 1999). As it amounts to 7.0 x 10 5 tons per year in Japan (O'Tool, 1999) and is mostly burnt or dumped into landfills, establishing a utilization process is necessary to prevent possible global warming and environmental pollution. Extraction of its components is one utilization scheme for recycling purposes. Several studies have reported on the extraction of these components, including pectic polysaccharide (Yamaguchi et al., 1996), oil components using supercritical carbon dioxide (Quitain et al., 2006), oligosaccharides by enzymatic digestion (Kasai et al., 2004) and mainly carbohydrates by subcritical-water treatment (Harada et al., 2004). Several treatments have been previously developed for the extraction of components from biomass, such as physical (mechanical comminution and pyrolysis), physico-chemical (steam explosion, ammonia fiber explosion and CO 2 explosion), chemical (ozonolysis, acid hydrolysis, alkaline hydrolysis, oxidative delignification, solvolysis process) and biological treatments (wood rotting fungi) (Sun and Cheng, 2002;Mosier et al., 2005). Subcritical-water is one of the tools for hydrolysis of biomass (Yu et al., 2008;Akiya and Savage, 2002), as it has a lower dielectric constant and higher ion product than water under ambient conditions, leading to hydrolysis of polymers in biomass without a catalyst.Microwave heating is now attracted much attention as an alternative heating source (Adam, 2003) for its environment friendliness, known as green chemistry. Microwaves enable rapid and uniform heating of polar substances by direct and internal heating generated by friction of dipole rotations. Microwave heating has been widely applied in organic chemistry (Perreux and Loupy, 2001;Lidström et al., 2001), extraction methods (microwave-assisted extraction; MAE) ...

Isolation and Structural Characterisation of Okara Polysaccharides

“…Okara polysaccharides also contain a considerable amount of galacturonic acid. Mateos-Aparicio et al and Tsubaki et al also reported galactose, arabinose and galacturonic acid as the major sugars [ 11 , 13 ]. However, in the study of Mateos-Aparicio et al , a maximum amount of arabinose was present in the cellulose-rich residue (equivalent to OP5), and more xylose existed in the 4 M NaOH fraction [ 11 ].…”

Application of Microwave Technology for Utilization of Recalcitrant Biomass

“…with 1 : 20 of solid : liquid ratio resulted in approximately 70% of solubilization (Tsubaki et al, 2009) (Fig.14). Due to high content of pectic arabinogalactan in soybean residue, the maximum carbohydrate production (24.6% at initial weight basis) was achieved at 160°C and 7 min.…”

Valorizing okara waste into nutritionally rich polysaccharide/protein-extracts for co-encapsulation of β-carotene and ferrous sulphate as a potential approach to tackle micronutrient malnutrition

“…Okara polysaccharide and protein were extracted using a previously reported microwave-assisted water extraction technique ( Tsubaki et al, 2009 ). Briefly, raw okara was defatted using hexane and dried overnight.…”