The Effect Of Glycinin, β-conglycinin, Soy Protein Isolate On Dough Properties And Steamed Bread Quality

With the improvement of living standards, people require more and more flour products which are not only to provide abundant nutrition, but also varieties of functional properties. According to amino acids complementary principle, the content of protein increased with the addition of soy protein to flour product, and the nutritional quality of protein were also enhanced. The effect of soy β-conglycinin(7S), globulin(11S) and isolated soy protein(SPI) on dough rheological property, quality of steamed bread and the wheat protein were studied. This study provided the theoretical basis of improving processing quality of flour products through taking advantage of soybean protein and enlarging the application of soybean protein in flour food processing.The results were as follows:(1) Purification and identification of 7S or 11 S from defatted soy flour.The optimization of the SE- HPLC chromatographic conditions were carried out, and the optimal conditions were: ratio of eluant(water/ acetonitrile) 70/30, flow rate 1.0 ml/min, injection volume 10 μL. The parameters of 7S or 11 S purification were determined based on extraction ratio and purity(11S: sedimentation pH 6.4 and standing time 12 h, 7S: sedimentation pH 5.4); under this condition the extraction ratio and purity of 7S or 11 S were 20.9%, 86.3% and 17.3%, 92.0% respectively. The SE-HPLC method was set up to analyze molecular weight of 7S or 11 S, and the standard curve equation of molecular weight was Log(M)=-0.2221x+3.8789, and the linear correlation coefficient(R2) was = 0.9969. In addition, the SDS-PAGE method was adopted to verify the subunit molecular weight and purity of 7S or 11 S.(2) The effect of 7S, 11 S soy globulin and isolated soy protein on dough property and quality of steamed bread.First, the soybean protein isolate(SPI) was added to two kinds of wheat flour. The effect of SPI on dough property and quality of steamed bread was investigated. It indicated that wet gluten content took on a downtrend and gluten index was on rise with the SPI addition. Moreover, the differences between groups were significant(P<0.05). The water absorption, development time, stability time and farinograph quality number of mixed dough were gradually increased with the addition of SPI, but softness of mixed dough were gradually decreased. When the content of SPI was 7%, the tread would be leveling off. The tensile energy and extensibility of mixed dough were gradually brought down, and extension resistance and ratio were gradually increased. Compared to CK, with the increased of the SPI, the peak viscosity and setback of mixed dough were decreased. The hardness and chewiness of steamed bread made by mixed dough showed an increasing trend; the elasticity and reversion increased slightly, but the cohesion showed decreasing trend. Through contrast test of two kinds of wheat flour, the effect of SPI on gluten, farinograph quality, tensile property, gelatinization properties of mixed dough and texture properties of steamed bread had nothing to do with species of wheat flour. Considering the sensory test of SPI-wheat flour steamed bread and the nutritional value, the content of SPI should be less than 7%, and the SPI might slow down retrogradation of flour products.The correlation between SPI-dough property and quality of steamed bread was studied. It demonstrated that there was a negative correlation between specific volume of steamed bread and most of the dough properties. Moreover, there was a positive correlation between specific volume of steamed bread and tensile energy, extension of mixed dough. Also there was a non-significant negative correlation between specific volume of steamed bread and stretching resistance, stretching ratio of mixed dough. There was a significant correlation between hardness, chewiness, specific volume of steamed bread and most farinograph factors. The effect of the SPI on steamed bread quality was investigated through the standardized partial regression coefficient of the instrument quality indicators. The quality indicators of mixed dough were utilized to evaluate steamed bread quality, both development time and water absorption played an important role in specific volume and grade of steamed bread.In addition, the differences of purified 7S, 11 S on dough property and steamed bread quality were analyzed with the addition of purified 7S, 11 S. Both 7S or 11 S had weakening action on farinograph characteristics of dough, and weakening degree 7S>11S. The decreased degree of 7S on the peak viscosity and setback was less than 11 S. Meanwhile, the elastic modulus and viscous modulus of 7S-wheat flour were less than 11 S. According to the texture, sensory characteristics and microstructures of steamed bread made by 7S or 11 S and wheat flour, although the farinograph characteristics tensile properties of dough decreased with 7S or 11 S addition, the mixture was suitable for making good quality steamed bread, and 11 S was better than 7S.(3) The mechanism of 7S, 11 S soy globulin and SPI on wheat proteinThe nuclear magnetic resonance(NMR) method was carried out to discuss the influence of moisture migration features. The T2(1), T2(2) and total proton signal amplitude were increased with the increased of SPI. The results showed that the addition of SPI could enhance the water-holding capacity of dough and slow down retrogradation flour products. The water holding capacity of 7S was greater than 11 S through analyzing the effect of 7S, 11 S on proton signal amplitude.There was a new absorption peak in ultraviolet spectrum when the SPI reached at the level more 5%. There was a new peak(NP) in the SE-HPLC chromatograms of 3% CSP, 4% CSP, 7S or 11 S mixed dough, and all values of NP were increased with the increase of CSP, 7S or 11 S. The new peaks of 7S, 11 S and CSP were different. The mixed dough of 11S-wheat flour had more –S-S-, and it showed that a part of sulfhydryl groups changed into disulfide bonds. The mixed dough of 11S-wheat flour had more insoluble protein in SDS, and the UPP% was higher. In summary, the interaction of soybean protein, 7S or 11 S with wheat protein were verified through ultraviolet absorption and SE-PAGE methods.The mechanism of soybean protein, 7S or 11 S with wheat protein was further discussed through the SDS-PAEG method. There were some more than 200 kDa molecular weight bands in all of the electrophoretograms of soy- wheat gluten under the non-reducing condition, but there was no these bands in the electrophoretograms of wheat gluten, CSP, 7S or 11 S, it turned out that it formed the polymer between soy protein and wheat protein. There was a more than 97 kDa molecular weight band in the electrophoretograms of CSP and 7S- wheat gluten under the reducing condition, and the band was more significant with the increase of CSP and 7S, but it was not appearance in the electrophoretograms of wheat gluten and 11S-wheat gluten. In conclusion, it showed that the interaction between 11 S and wheat protein formed the polymer only through disulfide bond, but the interaction between 7S and wheat protein formed the polymer by non-disulfide bond.