| Soybean isolates proteins (SPI), due to high nutritional value and good solubility and emulsibility, is widely used in food industry. But the natural SPI cannot meet the food processing requirements simultaneously, because the functional properties are incompatible. So the natural SPI need to be modified. Lecithin (Lee), as a high nutritional emulsifier, can combine with SPI through the hydrophobic interaction and form a complex which will affect the food quality seriously. Therefore the purpose of this research is exploring the combination mechanism of SPI and Lee by the2D-and3D-fluorescence speetrophotometry and analyzing the physicochemical, functional properties and the rheological property of the complex.This research has found that the fluorescence quenching mechanism of Lee with SPI was mainly a static quenching. And the thermodynamic parameter value confirmed the interaction force between the two substances was hydrophobic interaction. Meanwhile,3D-fluorescence spectrophotometry indicated the combination of Lec led to more Tyr and Tip be buried in the hydrophobic region and the extended of peptide chains. And the maximum combination weight ratio of Lee and SPI was1:1.5which was calculated by the fluorescence quenching curve.Further research demonstrated that the combination of SPI and Lee was affect by the relative content of11S and7S and the combination capacity of Lec with11S was higher than with7S. SPI which was undergone by heat treatment (70℃), microwave treatment (100W, lmin) and ultrasonic wave treatment (100W,10min) all would be beneficial to combine with Lee; the first two treatment would led to the change of protein slobility and structure. But the ultrasonic wave treatment was just facilitating to solubilization; Moreover, when solution pH was6the combination extent was largest which was depended on the structure change. And when the ion strength was0.05, the soluble protein content was the largest and the combine ablity was the strongest.The analysis of physicochemical and rheological property was showed that the enhanced negative charge of protein particle, greater particle size and improvement of soluble protein were occurred with the increasing of the combined lecithin content. The rheological characterization data displayed that the corresponding protein concentration reduced gradually when the n value, which indicates the fluid type, began to less than0with the increasing of lecithin combined content. This change explained that the complex of lecithin on soybean protein could lower the inflection point of protein concentration which indicated the change of fluid type. And then the protein viscosity was reduced by the effect of moderate lecithin content was concluded.Subesquently, three Lec-SPI complex samples were prepared and the natural SPI powder properties and functional properties were compared with three complex samples. The results showed that the sample with1/100Lec would improve the protein solubility significantly, and the powder wetting time was the shortest and disperse rate was the fastest. After dissolved the soluble protein was improved; and at the scope of1/500-1/300, the emulsifying activity was the optimal; the complex of Lee had no influence on the foaming properties. And foaming stability was better for the sample with 1/300Lec and the water binding capacity for this sample was strongest. The oil binding capacity was bring down with the increase of Lee; after formed the gel the complex and the control was analyzed by TPA. Data indicated that the Lec will play an important role in the process of gel formation and the gel hardness, cohesiveness, and elasticity would be affect in different extent, but the water holding capacity keep undifferentiated.At last, the solubility, emulsibility and gel texture properties of complex in different storage time was investigated by the oxidative stability experiment which was carried out by the Schaal oven method. The results testified that in the long time storage lecithin could improve the solubility, but could not change the reduced trend of solubility in storage time; The emulsifying activity index and emulsifying stability index of complex were all over more than the control without lecithin, and both of which were not decrease in the storage period; The dynamic viscoelasticity of soybean protein heat gel showed that in the short storage period, the gel formation temperature (Tgel) was reduced and the storage modulus G’ was increased by the addition of lecithin. But the complex of lecithin could not affect the change trend of G’in the storage period. And the frequency sweeping results indicated that lecithin could weaken the viscosity behavior of heat gel, but could not affect the change trend of gel hardness in the storage period. And the gel TPA analysis testified that the protein gel gumminess reduced significantly after the complex of lecithin, cohesiveness lowered slowly, but the elasticity enhanced gradually. And in the storage period, the cohesiveness have not changed much, the elasticity slightly reduced, and the gumminess reduced significantly. |
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