SE–SAS pretreatment of orange peel was an efficient method for improving the SDF yield and physicochemical properties under optimal condition (0.8% H2SO4 concentration, 0.8 MPa steam pressure and residence time 7 min). The extraction yield of SDF could be significantly (p < 0.05) increased by the SE–SAS treatment. And the physicochemical properties of SDF from SE–SAS treated orange peel were dramatically improved and exhibited higher binding capacity for three toxic cations (Pb, As and Cu), in comparison with SDF from untreated orange peel. The main weight-average molecular weight (Mw) of SDF from SE–SAS treated orange peel (174 kDa) was significantly (p < 0.05) smaller than that of untreated SDF (470 kDa). These results of SEM and thermal analyses revealed a high concentration of short-chain SDF with a tough, irregular surface and high thermal stability (up to 200 C). Therefore, SE–SAS technology would be a preferable choice for the production of active SDF from orange peel.
The detailed mechanism that SE significantly enhanced the extraction yield of SDF from orange peel was unclear. However, during SE treatment, the shear stress through thermal explosion decomposition and the flow of treated slurry created tensile forces that broke the orange peel and resulted in further substantial breakdown of the lignocellulosic structure.
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