| Soybean meal is an important by-product in the process of soybean oil extraction.It is mainly used in animal feed,human food and tissue products,and has broad development space.At present,the studies on extracted oil soybean meal by pressing method mainly focus on the evaluation of oil yield and quality by different degreasing methods,but the research on protein structure and functional characteristics of soybean meal is less.In this paper,the effects of continuous secondary cold pressing on the structure and properties of soybean protein were studied,and its application in chiba tofu was discussed.The main research results of this paper are as follows:（1）Cold pressing had a certain effect on the functional characteristics of soybean protein.Compared with natural soybean protein,the protein solubility after the first and second cold presses decreased by 5.97%and 59.58%,respectively.Cold pressing made protein form aggregates and reduced the solubility of soybean protein,and the change of solubility of soybean protein was more significant after cold pressing.Reduced solubility changes the functional properties of proteins.Compared with the soybean protein without cold pressing,the emulsification and emulsification stability of soybean protein decreased by 10.53%,17.58%and 7.20%,80.83%respectively after the first and second cold pressing,water and oil holding decreased by 16.77%,34.6%and 28.20%,51.28%respectively.In the process of physical pressing,the material was affected by shear force,and the anti-nutrient factors were passivated,which reduced urease activity by 36.78%.The scavenging rate of DPPH free radical,ABTS+free radical and superoxide anion free radical decreased by 22.22%,10.35%and 10.83%,respectively,and the reducing power decreased by 25.34%,while hydroxyl free radical scavenging rate had no significant change.Heat treatment can make the cold pressed soybean powder solution form elastic gel（storage modulus is greater than loss modulus）.Changing the heat treatment conditions will have a certain effect on the properties of cold pressed soybean powder gel.Rheological results showed that the viscosity,storage modulus and loss modulus of cold pressed soybean powder gel increased with the increase of heating temperature and heating time.The gel strength and water holding capacity also increased,and reached the maximum（130.57±0.02 g,0.99 g/g）at 100℃ for 90 min,respectively.（2）The influence of different times of cold pressing on soybean protein structure was investigated by Fourier transform infrared spectroscopy,UV-Visible spectroscopy,endogenous fluorescence spectroscopy,sulfhydryl group,disulfide bond,chemical crosslinking,particle size,Zeta potential and other indicators.The results showed that the α-helix structure of soybean protein increased by 54.65%and the random curl structure decreased by 43.34%after the second cold pressing.The ordered structure of soybean protein was increased by the second cold pressing and the structure was closer.By comparing the r value corresponding to the second derivative UV spectrum of soybean protein under different times of cold pressing,it can be seen that the structure of soybean protein is closer under the action of shear force after the second cold pressing.The maximum absorption wavelength of soybean protein was blue shifted from 342 nm to 341 nm by cold pressing.Cold pressing destroys the conformation force of soybean protein,forming aggregates,increasing particle size and decreasing absolute value of Zeta potential.Sds-page analysis showed that after cold pressing,the color of 7S and 11S was deeper,and the protein aggregates were formed.The degree of aggregation and crosslinking of proteins increased,and the changes of protein structure influenced their functional properties.（3）Using cold-pressed soybean powder as raw material,soy protein isolate（SPI）,ice water and Transglutama-minase（TG）as auxiliary materials,single factor experiment and response surface optimization were used to optimize the processing technology of cold-pressed soybean powder based thousand-page tofu.According to sensory evaluation and texture analysis,the optimum technological conditions of cold-pressed bean curd were 0.22%TG,80%ice water,5.78%SPI and 14%cold pressed bean curd.The hardness and elasticity of the optimized cold-pressed bean curd were 2271.24g and 0.95,and the sensory score was 89.88.At-18℃ and-45℃,the T21 value decreases with the extension of freezing time,the relaxation time is shortened,and the flow of water in the gel network is restricted.Lowering the freezing temperature shortens the time when the water is confined,resulting in reduced water mobility.The value of T22 increases first and then decreases with the increase of freezing time,and reaches the maximum value at 7 and 3 days,respectively.Freezing will make more bound water in the internal structure of cold-pressed soybean-powder-based thousand-page tofu transfer to free water,so that the structure of cold-pressed soybean-powder-based thousandpage tofu becomes loose and the pore size becomes larger.In conclusion,cold pressing changes the structure of soybean protein,and the second cold pressing has a greater impact on protein structure and further affects its functional characteristics.It is of great significance to study the changes of functional characteristics of cold-pressed soybean meal protein for its further processing and utilization. |
