1) Water retention
In addition to the absorption of water, soy protein isolate also has the ability to retain water during processing. Its water retention is related to viscosity, pH, ionization strength and temperature. Salt can enhance the water absorption of protein, but it weakens water retention. The highest water retention capacity is 14g water/g protein at pH7 and 35～55℃.
Emulsification refers to the ability of mixing oil and water to form an emulsion. The emulsion of fat and water is not easy to stabilize because of the positive free energy generated by the tension on the interface. Protein has the characteristic structure of affinity of emulsifier, and contains both hydrophilic group and lipophilic group in protein molecule. In the oil and water mixture, the tendency of protein, oil and water interface. Therefore, when soybean protein is used in food processing, it accumulates at the oil-water interface to reduce its surface tension and promote the formation of oil and water emulsions. After forming an emulsion. The emulsified oil is stabilized by the protein accumulated on its surface, forming a protective layer. This protective layer can prevent oil accumulation and destruction of the emulsified state. This shows that soy protein not only has emulsifying properties, but also has strong stability. Soy protein isolate is a surface active agent, it can reduce the surface tension of water and oil, but also can reduce the surface tension of water and air, so it is easy to form a more stable emulsion, and the emulsified oil is accumulated by the protein on the surface of the oil. Stable to form a protective layer. In the production of grilled food, frozen food and soup food, there have been research reports on adding a large amount of soy protein isolate as an emulsifier to stabilize the product state.
3) Oil absorption
The oil absorption of soy protein isolate is manifested in two aspects: (1) It promotes fat absorption. The role of soy protein isolate in absorbing fat is emulsification. When protein isolate is added to meat products, it can form an emulsion gel to prevent fat from moving to the surface, thereby promoting fat absorption and fat binding, thereby reducing meat product processing The loss of fat and juice during the process helps maintain the stability of the appearance. Oil absorption increases with the increase of protein content, and decreases with the increase of pH. (2) Control fat absorption. Soy protein isolate can also play a role in controlling fat absorption under different processing conditions, such as preventing excessive absorption of fat during frying. This is because the protein is denatured by heat and forms an oil layer on the surface of fried noodles.
Foaming refers to the increase in volume of soy protein during processing. Foam is made up of air dispersed in liquid phase and semi-solid. It is composed of many groups of air wrapped in a layer of soluble protein film activated on the liquid surface, which reduces the surface tension of air and water. Bubbles are caused by the separation of the elastic liquid film or semi-solid film to prevent the merger of the bubbles. Utilizing the foaming properties of soy protein can give food a loose structure and good taste.
The viscosity of protein refers to the internal friction that appears when the liquid flows, also known as fluidity. It is important to adjust the physical properties of food. The viscosity of protein solution is comprehensively affected by various factors such as protein molecular weight, friction coefficient, temperature, pH, ionic strength, processing conditions and so on. These factors can change the morphological structure, state of association, hydration, swelling and viscosity of protein molecules. After soy protein isolate undergoes alkali, acid or heat treatment, its swelling degree and viscosity increase. The surface viscosity of the soy protein solution increases exponentially with the increase of the protein concentration, and is related to the swelling degree of the sample. When the protein is heated to 80℃, the viscosity will increase, and the viscosity will decrease when it exceeds 90~C. When the pH is 6 to 8, the protein structure is the most stable and the viscosity is the largest; when the pH exceeds 11, the viscosity decreases sharply because the protein binding is destroyed.
Some of the polar parts of the soybean protein molecule can be ionized, such as amino and hydroxyl groups, so that the polarity and solubility can be changed by changing the pH value. When the pH value is 0.5, about 50% of the protein is dissolved; when the pH value of the system reaches 2.0, about 80% of the protein is dissolved. As the pH increases, the solubility of the protein decreases, until the pH value is within the isoelectric point range of 4 to 5, the protein solubility tends to the minimum, about 10%. Then, as the pH value gradually increased, the solubility of the protein increased rapidly again. When the pH value is 5.6, the protein solubility can reach more than 80%, and when the pH value is 12, the maximum solubility can reach more than 90%. According to the dissolving properties of soy protein, the isolated soy protein can be added to the salted salt water through injection and rolling to make the salt water evenly diffuse into the muscle tissue and cooperate with the salt-soluble meat protein to maintain large pieces of meat such as ham and corned beef. The integrity of the product improves the yield.
Soy protein isolate (SPI) has a broad application prospect in the field of food processing. The main components of soy protein isolate are 7S and 11S globulins. Therefore, the solubility of soy protein isolate is very low, especially in the isoelectric point region of soy protein at pH 4~5, which limits the range of use of soy protein isolate. Therefore, the solubility of soy protein isolate is improved, and it becomes soy. Problems that need to be solved urgently in the practical application of protein isolates.
When heated, soy protein isolate has the ability to form a gel, and the gel-forming ability is one of the functional characteristics of soy protein isolate. Gelability refers to the ability of proteins to form limb-like structures. It makes the soy protein isolate have higher viscosity, plasticity and elasticity. It can be used as a carrier for water as well as a carrier for flavors, sugars and other complexes. This feature is extremely beneficial to food processing. Soy protein dispersion material can be heated, cooled, dialysis and alkali treatment to obtain gel. Its formation is affected by the concentration of solids, temperature and heating time, refrigeration, the presence or absence of salts, sulfhydryl compounds, sulfites or lipids. The higher the protein content, the easier it is to make a firm, tough, elastic hard High-quality gel, and the protein content is less than 7%, can only be made into soft and fragile gel. The protein dispersion should be at least more than 8% to form a gel. The gel made of llS globulin is firmer than the gel made of 7S globulin and is easier to restore to its original state, because their globulins are different in sensitivity to heat denaturation.
8) Water attached
Soy protein contains many polar groups along its peptide chain backbone. Due to the attraction between these polar groups and water molecules, hydration is prone to occur when protein molecules come into contact with water molecules. The shape of protein molecules is not regular, and the distribution of polar groups on the surface is also difficult to be uniform. Therefore, the hydration film on the surface of protein molecules is not uniform, and more water molecules are adsorbed on the surface where the polar groups are concentrated. ; On the contrary, there will be less adsorption of water molecules. When adding soy protein isolate to meat products, the water absorption and water retention of the protein becomes an important issue. It can retain moisture even when heated, which is essential for meat products. Due to these characteristics of soy protein isolate, it is currently widely used in the production of meat products.