Mechanism Of Soy Protein Fractionation And Separation

There are two major storage protein components in soybean seed, the 7S and 11S globulins, which have different functional and nutritional properties. It is important of separation of the 7S and 11S globulins in industry-scale for utilization of soy protein resources. However, current fractionating methods are only used in bench-scale because of their demanding operations and low recoveries. In addition, Soybean seeds also contain 1.5-2.2% phytate and other minerals such as Ca2+ and Mg2+, which have significantly effects on the functional and nutritional properties of soy protein products. In this thesis, the effect of pH, ionic strength and Ca2+/Mg2+ concentrations on soy protein fractionation was studied. We had developed a new method of fractionating soy 7S and 11S globulins from defated soy flour. In addition, the interaction among divalent cations, protein and phytate during fractionation was studied. The effect of Ca2+/Mg2+ concentrations on the surface charge characteristic (ζ-potentials) of protein molecular of the recovered fractions and the aggregation kinetics of soy protein during fractionation were investigated. Effect of removal of phytate on the functional properties of soybean proteins were investigated, with an emphasis on the rheological behaviors ofβ-conglycinin globulins after phytate removal. The effect of cationic polyelectrolyte on the surface charge characteristic and aggregation properties of soybean protein were futher analyzed. The following conclusions were drawn from this essay:(1)Effects of Ca2+/Mg2+ on soy protein fractionation were systematically compared. The interaction among Ca2+/Mg2+, phytate and soybean protein was investigated and the fractionating procedure was optimized. The results show that, based on the yields, purities and phytate contents, Mg2+ was more suitable to fractionating soy protein than Ca2+. Solubility of phytate-Mg2+ was higher than that of phytate-Ca2+. Aboundants of phytate incorporated into protein aggregation induced by Ca2+, which significantly affected fractionation by two kinds of effect: the increasing Ca2+ binding sites of globulin induced more amounts of protein aggregation and precipitation, and more linkages between 11S and 7S globulins through the formation of mineral briage, which decreased the efficiency of fractionation. These effects do not accure in case of Mg2+, due to phytate do not incorpreate into protein aggregates when using Mg2+ as pricipitate agent. Therefore, much lower amount of phytate was recovered by the glycinin-rich fraction using Mg2+ than Ca2+. A new simple method for fractionating soy protein was developed by using Mg2+ instead of Ca2+. The yields of the two fractions separated by the new method were higher than that of Deak'method using Ca2+, and also yielding the 7S-rich fraction with higher purities. In addition, comparing with Deak'method, much lower phytate was contained in our 11S-rich fractions.(2)Effects of Ca2+/Mg2+ concentrations on the thermal properties and solubility of 11S and 7S fractions were investigated.The thermal stability of the 11S-rich fraction changed more sensitively with separating conditions than that of the 7S-rich fraction. Compared with MgCl2, CaCl2 had a more significant contribution to the thermal stabilities of the recovered fractions, especially that of the 11S-rich fraction. With Ca2+/Mg2+ concentrations increased, solubility of 11S fractions decreased significantly, in neutral and moderly acidic conditions, and the effect of Ca2+ was higher than Mg2+. The protein fractions with low phytate contents was more soluble at acidic conditions, dispite of rotein composition.(3)Effects of Ca2+/Mg2+ on soy protein fractionation were further studied in two aspects, including the surface charge chariteristic (ζ-potential) of soy protein fractions and aggregation kinitics during fractionation. With Ca2+/Mg2+ concentrations increased, theζ-potential titration curve of 11S fractions shifted toword 0 potential and lower pH value, and the reduced effect inζ-potential by Ca2+ was more significant than by Mg2+. Effect of Ca2+/Mg2+ concentrations on theζ-potential of 7S globulin was less signiaficant. The different effect on theζ-potential of protein between Ca2+ and Mg2+ was due to their different precipitate effects on phytate. the results also show that, two stages in 11S globulin aggregation and precipitation can be observed. First, aboundant primary particles with average chorld length of 3.6μm were formed immediately, when the pH was adjusted to the value for 11S globulin precipitation, then, most of the primary particles aggregate into more larger secondary particles, which have average chorld length of 60-80μm. The addition of Ca2+/Mg2+increased the amounts of 11S globulins aggregated particles and alter its aggregation kinictics model. The results show that, the complex actions of Ca2+/Mg2+ and phytate caused the decrease of suface charge of 11S globulins and protein stability, resulting more proteins aggregated and precipitated.(4)Low phytate soybean protein were prepared by different methods including dialysis and phytase treatment. Effects of phytate removal on the rheological behavior of 7S globulin and functional properties of soy protein isolateds (SPI) were studied. Removal of phytate and salt from 7S globulin induced the formation of weak gel network in 7S globulin aqueous solution at 25℃. Enhanced solubility and decreased aggregation were observed for SPI after treated by phytase. Removal of phytate by phytase decreased binding sites of protein to phytate.(5)The effects of cationic polyelectrolyte on the surface charge characteristic and aggregation properties of soybean protein were futher analyzed. The surface charge characteristic of 11S and 7S globulins were drmaticly modified by cationic polyelectrolyte, through the formation of biomolecular complexes. As a result, isoelectric points (IEP) of soy protein increased significantly and proteina aggregation was inhibited at both atmospheric temperature and heating conditions. Inhibited effects to protein aggregation was more obvious by heating at pH beyong IEP than at IEP. Inhibited effect to 11S globulin aggregation was more dramastic than to 7S globulin aggregation by cationic polyelectrolyte.The results showed that the surface potential and aggregated behavior of biomolecular mixed system was correlated closely with the composition of the mixed system and the manner of thermal treatments.