Effect Of Steam Explosion Treatment On Physicochemical Property Of Okara And Its Application In Biscuits

Okara is the byproduct of tofu and soymilk industries.For tofu processing,1.2 tons of okara is produced from one ton of soybean.China produces about 20 million tons of okara annually,but most of them are used as feed or waste.Okara contains rich dietary fiber and protein,plus saponine,isoflavone and other nutritional and bioactive substances.However,the high IDF content and rough taste influence the utilization in food.Steam explosion(SE)is a method of treating a sample in a high-pressure steam and then releasing the pressure instantaneously.In this process,the compact structure of the fiber is destroyed,the content of soluble dietary fiber(SDF)increased,and the physicochemical properties of sample changed greatly.Compared with other methods of improving okara's quality,the SE treatment has the advantages of fast,low cost and effective.In this paper,the effect of SE treatment on the composition and content of dietary fiber,the molecular structure of dietary fiber and protein,and the physiochemical properties of okara were investigated and the application of SE okara in the production of tough biscuit was studied.The results were as follows:(1)The okara used in this paper contained 76.38% dietary fiber and 18.10% protein,but the SDF content was only 1.34%.After SE treatment SDF content increased distinctly.When the SE strength was 1.5 MPa for 30 s,SDF content increased to 36.28%,higher 26 times than the control,and SDF/IDF was 50.93%.However,the contents of TDF and SDF decreased when the SE strength improved more.The reason was probably that the fiber structure of okara became loose after the SE treatment,and some substances which are bound to the fiber were released at the same time.The degradation of macromolecular polysaccharide and the decrease of molecular weight led to the increase of SDF content.When SE strength was too high,the monosaccharides and smaller molecular weight of oligosaccharides formed from excessive degradation of polysaccharides cannot be precipitated by 75% ethanol in the determination of dietary fiber,so the contents of TDF and SDF were decreased.(2)Gel filtration chromatography showed that the molecular weight distribution of okara polysaccharides was 55 KDa~2087 KDa.With the increase of SE intensity,the proportion of low molecular weight polysaccharide increased,and the molecular weight distribution narrowed.In 2.0 MPa for 60 s and 120 s,the okara polysaccharide was symmetrical peak and the molecular weight was about 1.5 KDa.The results showed that SE treatment make the polysaccharides of okara degraded into small molecular weight polysaccharides and oligosaccharides,so the SDF content increased.Moreover,the higher SE intensity was,the deeper polysaccharides degraded.(3)Okara protein showed two primary molecular weight peaks and their peak areas were almost similar in gel filtration chromatography.After SE treatment,the area of high molecular weight peak decreased and low molecular weight peak increased.The high molecular weight peak almost disappeared when SE pressure exceeded 1.5 MPa.Sodium dodecyl sulfate polyacrylamide gel electrophoresis(SDS-PAGE)showed that okara proteins appeared 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.When SE pressure was 2.0 MPa,only band of 20.2 KDa was left.The results showed that SE treatment can depolymerize or disintegrate okara protein,resulted in the decrease of its molecular weight.(4)Water solubility of okara increased significantly while expansibility,oil and water holding capacity decreased after SE treatment.The decline became flat after 1.0 MPa for 60 s and 120 s.Environmental scanning electron microscope(ESEM)observed that the loose sheet structure was divided into small slices during SE treatment.The inner nuclear structure was also collapsed into small granules.The change of morphological structure produced important influence on physiochemical properties of okara.The increase of small fragments and particles improved the water solubility of okara.While the destruction of surface loose structure and inner holes decreased the expansibility,oil and water holding capacity.Furthermore,the increased water solubility of okara was also related to the expose of more hydrophilic groups from the degradation of polysaccharides and protein.The decreased expansibility,oil and water holding capacity were related to the decrease of molecular weight of polysaccharides and proteins.(5)The optimal process conditions of okara hard biscuit was that: okara replacing 5% plain flour,the ratio of palm oil to white sugar was 1:1.5,the ratio of sugar and flour was 1:2.25,raising agent sodium bicarbonate,ammonium bicarbonate,gluconic acid-?-lactone of the lactone as 2:1:2.The baking condition was up fire temperature 180?,under fire temperature 160?,and baking time for 10 min.The addition of SE okara can improve the quality of okara biscuit.When the addition amount of okara was 10%,the quality of biscuit was better than that of the control group.The SE strengths of 0.5 MPa for 60 s,0.5 MPa for 120 s,1.0 MPa for 30 s and 1.0 MPa for 60 s were best for okara biscuit preparaition.After SE treatment,the content of SDF increased distinctly while polysaccharides and proteins molecular weight were both decreased,water solubility of okara increasing significantly,improve the quality of okara biscuit.The addition of SE okara can improve the quality of okara biscuit.