Study On The Fractionation And Properties Of Soy Protein Isolate

Soy proteins have been widely applied in food products due to their nutritive value. Butthe soy protein isolate, which was obtained with traditional method (acid precipitated), havean undesirable off-flavor and poor funditional properties which diminish their application as afood material. The main reason was that there was lipophilic protein (LP) existence. Hence, itis necessary to follow the fate of LP during the precipitation of acid-precipitated proteins.However, current fractionating methods are not suitable in industrialization productionbecause of their demanding operations. In this thesis, a simple, rapid and effective method forfractionating Less-LP SPI was established. In addition, the effects of LP and phospholipids onthe gelation of soy protein were studied. The following conclusions were drawn from thisessay:（1） The thesis studied the effect of pH，different divalent cations on soy proteinisolate yields, recoveries, contents and lipid contents. Results indicated that Ca(Ⅱ)had bestadvnatge to remove LP, but Zn(Ⅱ) had worst effect. Combining SDS-PAGE and TLCconfirmed that the optimized fractionating procedure was as follpws: The dispersion wasadded10mM CaCl2at pH7.0for1h, and then heated in water bath at50℃for1h.Centrifugation was performed after the previous steps to remove the insoluble residue. Thesupernatant was adjusted to pH4.5, after another centrifugation the Less-LP SPI wasrecovered as the precipitated curd. This new method decreased the yields of Less-LP SPI by1/3compared with the acid-precipitated protein (APP), but the protent content of the SPIincreased from (87.61±0.81)%to (94.23±0.80)%(w/w) and lipid content was decreased by45%(w/w). The gas chromatography-mass spectrum(GC-MS) indicated that the total contentof the compounds in the Less-LP SPI was decreased by32%compared with APP.（2） Different among Less-LP SPI and other SPIs obtained from traditionalfractionation or Samoto method were detected in solubility, emulsification activity andgelling. Results indicated that Less-LP SPI had lower solubility (pH≤10) and emulsificationactivity than that of APP. whereas the Less-LP SPI had superior emulsification stability; After7%samples were induced by MTGase for4h, Less-LP SPI showed the earliest gel point and had the same final elastic moduli with7S:11S, which more than that of APP.（3） Effects of LP on other soy protein gels were studied. The results showed LP gelhad the lowest gel strength,11S gel with the strongest. The presence of LP led to a largedecrease the strength of11S and7S gels. There were large different among gelsmicrostructure. The appearance of the11S gel had larger pores and is thicker, coarser thanthat of the7S and LP, respectively. In contrast, the7S gel had small pores distributeduniformly in a dense.（4） Effects of phospholipids on the11S cold gels were investigated systematically.The results showed that the elastic moduli of11S gel increased and then dcreased with theincreased of the phospholipid concentrations. The presence of3%phospholipid resultedenhancement the gel strength and let to the11S gel point ahead of time. The phenomenon ofthe effect of phospholipids on the11S gels is related with the change of hydrophilic andZeta-potential of11S after phospholipids added. The scanning electron microscopic (SEM)showed that the11S gels mocrostructure with3%phospholipid concentration showed moreuniformly and thicker structure.