3.6. Microstructural analysis
To better understand the influence of different extraction methods on the characteristics of RBO, microstructure of native and samples after extraction were investigated by SEM. The images showed that the internal tissue of the rice bran seems to undergo a process of destructing and restructuring. In Fig. 3a, natural rice bran had a relatively complete structure, regular or dense shape and smooth surface, meanwhile, the intact oil cells on the surface of bran tissues before extraction were observed. Fig. 3b showed the expansion process caused the disruption of cell walls and loose cell arrangement of expanded rice bran, making conventional extraction or enzymatic extraction more accessible to organelle. After AEE, the oil was almost extracted together with aleurone grains and the substances in the cell gradually were decreased, therefore, the structure of rice bran cell was further destroyed (Fig. 3c). In Fig. 3d, the surface of rice bran treated with n-hexane became more fragmented, dried and cracked, and some lipids and starches were dragged away (Alcázaralay, Cardenastoro, Santos & Meireles, 2015). In the process of SE, the chemical affinity between organic solvent and oils affected the mass transfer process which caused a high oil extraction yield, but it could also damage the protein and starch in the extraction process. The process of enzymatic extraction was based on the insolubility of oils in water and the efficiency was lower than that of SE. However, AEE could simultaneously separate solid phase (mainly starches and proteins) and an oil phase, so the development of these by-products could also make full use of rice bran.