Effect of steam explosion on dietary fiber, polysaccharide, protein and physicochemical properties of okara

Okara is the byproduct of tofu and soymilk industries. A large amount of okara is produced annually in the world, but only a small amount is fully utilized. The effect of steam explosion (SE) treatment on dietary fiber, protein and physiochemical properties of okara was investigated in this paper. The results showed that untreated okara contained 76.38% total dietary fiber (TDF) and 18.10% protein, but the content of soluble dietary fiber (SDF) was only 1.34%. After SE treatment, SDF content increased significantly. When the SE strength was 1.5 MPa for 30s, SDF content increased to 36.28%, higher by 26 times compared to the control okara. However, the contents of TDF and SDF both decreased when the SE strength enhanced further. Gel filtration chromatography showed that molecular weight of okara polysaccharides were distributed from 55 KDa to 2087 KDa. With the increase of SE strength, the ratio of low molecular weight polysaccharides increased and the molecular weight range of polysaccharides became narrower. Okara protein showed two molecular weight peaks. After SE treatment, the area of high molecular weight peak decreased and low molecular weight peak increased. Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that okara protein appeared in five bands: 73.7 KDa, 55 KDa, 41.1 KDa, 32.7 KDa and 20.2 KDa. After SE treatment, the bands of 73.7 KDa, 55 KDa and 41.1 KDa weakened markedly, indicating high SE strength can dissociate macromolecular protein aggregates. Water solubility of okara increased significantly, while swelling capacity, oil and water holding capacities decreased after SE treatment. Scanning electron microscopy (SEM) demonstrated that the loose sheet structure of okara surface was destroyed into small fragments after SE treatment and the internal nucleated structure was disintegrated into small particles at high SE strength. The change in morphological structure also produced important influences on physiochemical properties of okara. This study suggested that SE treatment is an effective method for improving SDF content and quality of okara, so as to expand its development and utilization.