Study On Process And Mechanism Of Steam Flash Explosion-assisted Extraction Of Protein From Heat-denatured Soybean Meal

High-temperature denatured soybean meal (HTDSBM) is by-product of soybean oilsolvent extraction, which is desolventized by means of high-temperature desolventizing. InChina, annually about forty million tons of HTDSBM have been produced. HTDSBM, whichcontains approximately50%protein, is an excellent source of protein. The thermal-processingresulting in the heat-denaturation of protein renders HTDSBM low solubility of protein andother poor functional properties, which limit its application in food processing. If theheat-denatured protein in HTDSBM could be refunctionalized, the more benefits ofHTDSBM can be realized. In this study, steam àash-explosion (SFE) was applied in proteinextraction of HTDSBM, meanwhile the functional properties of protein and the relevantmechanism was investigated. Main result as follow:1. The process of protein extraction of HTDSBM assisted by SFE was studied. Theresearch result showed that the particle size of HTDSBM significantly affected the proteinextraction of HTDSBM. When the particle size of HTDSBM was more than100-mesh screen,the HTDSBM particle would form caking, which impeded the penetration of steam intoHTDSBM resulting in the decline in nitrogen solubility index (NSI). Therefore theoptimum particle size for HTDSBM was between20-mesh screen and80-mesh screen. Thechanges of NSI of HTDSBM treated under steam pressure from1.3MPa to2.4MPa and theresidence time of180s were studied. The result showed that when SFE treatment conditionwas1.8MPa for180s èthe NSI of SFE-treated HTDSBM was significantly improved by2.1times than that of untreated HTDSBM. However, the steam pressure would be controlledwithin1.8MPa, because the higher steam pressure can damage the functional properties ofprotein. The SFE treatment can significantly improve the extraction yield of protein. Underthe SFE treatment condition of1.3MPa for180s,1.8MPa for120s and1.8MPa for180s,the extraction yield of protein of SFE-treated HTDSBM were increased from50.5%to62.5%,64.6%and65.7%, compared with untreated HTDSBM.2. The effect of acid soaking pretreatment on extraction of protein from soybean mealassisted by SFE was studied. When HTDSBM was soaked in0.9%(w/v) and1.2%(w/v)sulfuric acid at80for2h and then was treated by SFE, the extraction yield of protein wasimproved to70.0%and77.1%. The analysis of gel filtration chromatography (GFC) indicatedthat1.2%sulfuric acid can raise acid hydrolysis of protein, so the sulfuric acid concentrationwas controlled within0.9%. When soybean meal was pretreated by SFE at1.8MPa,2.0MPa,2.2MPa for8min after soaked in0.9%sulfuric-acid, the extraction yield of protein wereincreased to67.7%è69.5%and70.5%. The protein content of soy protein isolate (SPI) from acid soaking-SFE pretreated HTDSBM was about81%, which was3%higher than that of SPIfrom untreated HTDSBM. Scanning electron micrograph of pretreated samples showed thestructural disruption of HTDSBM to promoting the protein extraction.3. The effect of SFE treatment on functional properties of protein was studied. Thefunctional properties of SPI prepared from HTDSBM treated by single SFE and acidsoaking-SFE were all improved compared with SPI prepared from untreated HTDSBM.Especially acid soaking-SFE treatment more significantly improved the functional propertiesof SPI compared with single SFE treatment. The functional properties of SPI prepared fromsoaking-SFE treated HTDSBM were significantly superior to those of SPI from white flakes(WFs), in addition to the foaming stability. After acid soaking-SFE treatment, the emulsifyingactivity index (EAI), emulsion stability index (ESI), foaming capacity (FC), foaming stability(FS) and fat-binding capacity (FBC) of SPI were56.44m2/g,58.83min,47.50%,47.22%,and814%; and those of SPI from WFs were21.95m2/g,14.86min,31.50%,81.05%and500%; while those of SPI from untreated HTDSBM were32.90m2/g,16.55min,17.50%,29.16%,754%.4. The mechanism of single SFE and acid soaking-SFE treatment on improvement offunctional properties of soy protein was studied. The high performance size exclusionchromatography (HPSEC) and native-polyacrylamide gel electrophoresis (native-PAGE)indicated that after single SFE and acid soaking-SFE treatment the molecular weight (MW)distribution became broad. It is concluded that during single SFE and acid soaking-SFEtreatment the protein aggregates were dissociated and new aggregates were formed. Reducingand Non-reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS–PAGE)indicated that the bands densities corresponding to subunits of SPI were decreased, while thebands between20.1kDa and29.0kDa, and the bands of less than20.1kDa were increased.The protein aggregation with covalent bound of on-disulfide bond appeared at the beginningof stacking gel and separating gel after single SFE and acid soaking-SFE treatment. Analysisof carbohydrate stain of SDS-PAGE, fourier transformed infrared spectroscopy (FT-IR), totalsugar in SPI and free amino group content of SPI indicated that protein-saccharide conjugatewas formed during single SFE and acid soaking-SFE treatment. After single SFE and acidsoaking-SFE treatment, the decrease of the surface hydrophobicity of SPI, compared to SPIprepared from untreated SBM, was partly attributed to the glycation of SPI. In addition, thechanges of emission flourescence spectra of glycated SPI further prove that the saccharidecovalently coupled to protein. According to the analysis circular dichroism (CD) spectrum,ater acid soaking and acid soaking-SFE treatmen, the content of-strand and turns increased.The surface charge of the protein increased after acid soaking-SFE treatment. The changes of protein structure significantly affect the functional properties of soy protein.