Studies Of Salts And Sugars On The Papain-induced Soybean Protein Isolate Gels

To improve the gel strength of enzyme induced-soybean protein gel, soybean proteinisolate (SPI)(the protein concentration of SPI dispersion was4%) was used as raw materialand papain as coagulant. Adding salts and sugars into SPI, respectively, physicochemicalproperties and gel strength were studied before or after the coagulation of papain-induced SPIgel. The results were as follows:1. The clotting time was shortened and gel strength was increased, both remarkably afteradding univalernt and divalent salts including NaCl, KCl, CaCl₂, MgCl₂and MgSO₄. TheCa²⁺and Mg²⁺ions were more effective than Na⁺and K⁺on both clotting time and gelstrength under the same ionic strength. The pH value of the SPI dispersion was lowered afteradding salts. After the enzyme was added, the pH values decreased again and continuously toa steady value after a certain time of incubation. There was little effect for proteindegradation of papain-induced SPI gel after adding different kinds and various concentrattionof salts. The gel strength increased60%when the amount of papain was reduced from0.15%to0.1%(m/m) and adding CaCl₂to the ion strength of15(the amount of substanceconcentration was5mmol/L).2. It was proved that the effects of salts on the clotting time of papain-induced SPI gelwas in accord with Arrhenuis plot. According to Arrhenuis plot, activation energy wascalculated of papain-induced SPI gels under different ionic strength. It was found that addingsalts lowered activation energy of papain-induced SPI gels. Activation energy lowered withthe ionic strength increased.3. To optimize the effect of salts on gel strength of papin-induced SPI gel, CaCl₂wasselected as typical. Basis on single factor tests and according to the Box-Benhnkencenter-united experimental design principles, the method of response surface analysis with3factors and3levels was adopted. The model which could predict the gel strength was buildfor papain-induced SPI gels. The results showed that the optimum condition ofpapain-induced SPI gel was: the concentration of CaCl₂7.45mmol/L, coagulationtemperature49.7℃, and the amount of papain0.06525%, the maxium gel firmness was 61.754g, which increased by165%than the blank gel firmness of23,3g.4. Reducing sugars (glucose lactose and dextran) as well as non reducing sugar (sucrose)were freezen drying with SPI. Then place the mixture in a dry heat condition to make mallardreaction. It was found that reducing sugars lead to mallard reaction with SPI. Howerwe, nonreducing sugar could not. Mallard reaction could be shown from SDS-PAGE with Coomassiebrilliant blue R250staining and Periodic acid-Schiff staining. With the mallard reaction wenton, the degree of glycation got larger, the color of SPI-sugar got browner. In contrary, thecontent of free amnio group decreased, so as the solubility of SPI.1hour of dry heat reactiondid't cause solubility decrease and it could make a certain degree of glycation, so1hour wasselected for the time of dry heat reaction.After1hour of dry heat reaction, spare reducing sugar was removed from the SPI-sugarmixture to make pure glycated SPI. Compraed with the blank and untreated SPI-sugarmixture, papain was added to make papain-induced glycated SPI gel. It was found that addingsugars increased gel strength, glycoslation decreased the strength of papain-induced SPI gel.Therefore we conjecture: deglycosylation of the SPI could improve the gel strength ofpapain-induced SPI coagulation.5. From the test, two methods could be taken to improve the gel strength ofpapain-induced SPI coagulation. The first was to add a suitable concentration of salt under adesirable condition, e.g.: temperature of about50centigrade, the concertation CaCl₂of7.45mmol/L and papain of0.06525%. The second method was deglycosylation of the SPIusing certein technology.