Emulsification refers to the performance of mixing oil and water to form an emulsion. Soy protein isolate is a surfactant, which can reduce the surface tension of water and oil as well as the surface tension of water and air. Easily form a stable emulsion. The emulsified oil droplets are stabilized by the protein accumulated on the surface of the oil droplets, forming a protective layer. This protection can prevent the accumulation of oil droplets and the destruction of the emulsified state, and promote the stability of the emulsification performance. In the production of baked foods, frozen foods and soups, adding large protein isolate as emulsifier can stabilize the product state.
The emulsifying ability of protein isolate is often affected by pH and ionization strength, and alkaline conditions are the most favorable. For example, when the pH is 7 and the ionization strength is 0.05, the emulsifying ability is 5ml oil/mg protein; when the pH is 7, the ionization strength is 0.03, the emulsifying ability is 3.5ml oil/mg protein.
Soy protein isolate contains many polar groups along its peptide chain backbone, so it has water absorption, water retention and swelling properties.
2.1 Water absorption
Generally refers to the adsorption capacity of protein to water, which is closely related to Aw (ie water activity), pH, depth of protein particle size, particle structure, particle surface activity, and so on. With the increase of Aw, the water absorption occurs fast-slowly. The pH value is directly proportional to the water absorption capacity, the higher the pH value, the stronger the water absorption capacity. The concentration (content) of protein has a great influence on its water absorption. The water absorption of protein isolate is much stronger than that of concentrated protein, and the former is hardly affected by temperature.
2.2 Water retention
In addition to water adsorption, soybean protein has the ability to retain water during processing, and its water retention is related to viscosity, pH, ionization strength and temperature. Salts can enhance protein water absorption but weaken the water retention of protein isolates. The highest water retention capacity is 14g water/g protein at pH=7 and temperature 35~55°C.
Swelling is the expansion effect of protein, which means that the protein will swell after absorbing water. It is significantly affected by temperature, pH and salt. Heat treatment increases the swelling of soy protein. 809C is the best, and the swelling between 70~100°C is basically close. Salts (sodium chloride 0.1~0.4m1/L) can significantly reduce the swelling rate of the isolated protein (about 60%). The swelling rate also increases with the increase of pH, such as pH from 5 to 9, the swelling rate increases by 2 times.
- Oil absorption
3.1 Promote fat absorption
The function of protein isolate to absorb fat is another form of emulsification. When protein isolate is added to meat products, it can form emulsion and gel. When protein isolate is added to meat products, it can form a combination of emulsion and gel or fat, which can reduce the loss of fat and juice during the processing of meat products and help maintain the stability of the appearance. The oil absorption increases with the increase of protein content and decreases with the increase of pH.
3.2 Control fat absorption
Protein isolate can also play a role in controlling fat absorption under different processing conditions. For example, it can prevent excessive absorption of fat during frying, and it can prevent excessive absorption of fat during frying. This is because The protein denatures when heated, forming an oil layer on the surface of the fried noodles.
- Gelation (also known as gelation)
It refers to the ability of protein to form a colloidal structure. It makes the isolated protein have higher viscosity, plasticity and elasticity. It can be used as a carrier for water, as well as a carrier for flavor sugars and other complexes, which is extremely beneficial to food processing.
The dispersed substance of soybean protein can be gelled by heating, cooling, dialysis and alkali treatment. The formation is affected by the presence or absence of salt sulfhydryl compounds, sulfites or lipids by the solids concentration, speed, temperature and heating time. The higher the white content, the easier it is to make a firm, tough and elastic hard gel, while the protein content Less than 70% can only be made into soft and fragile gel. The white matter dispersion should be at least 8% higher to form a gel. The temperature will increase with the increase in concentration to achieve the ideal gel performance. 11S (S is the degree of polymerization) gel made of globulin is better than gel made of 7S globulin. They are firmer and easier to restore to their original state, because their globulins are different in sensitivity to heat denaturation.
It refers to the ability of protein to dissolve in aqueous solution or salt solution. The degree of its solubility is called solubility. Solubility usually refers to water solubility. A protein with good solubility must have good functionality, good gelling, emulsifying foaming and lipoxygenase activity. It is easy to process and utilize food. It is easier to blend into food. The solubility of soybean protein is greatly affected by the heating treatment of the raw materials, the amount of water added during dissolution, pH, and coexisting salts.
Heating will cause the denaturation of soy protein and reduce the solubility, so the heating temperature should not be too high when processing the raw materials, or use dry heating (that is, the raw soybeans are not high in moisture content and heated at high temperatures when there is no water vapor). The liquid ratio has a greater influence on the solubility of soybean protein. When the liquid ratio is less than 5 times, the protein leaching rate drops sharply, and the protein molecules easily react with each other. The solubility is reduced. Generally, the liquid ratio is about 1:10. pH has a great influence on globulin. At pH 4.2~4.6, globulin is almost insoluble. Coexisting salts also have an impact on solubility. If sodium chloride and calcium chloride are present, even in the range of isoelectric point
It can also dissolve in the pH 4.2~4.6. On the other hand, some salts (such as gypsum powder) can reduce protein solubility and can be used as precipitating agents.
Foaming property refers to the increase rate of the volume of soy protein in processing, which can play a role in crisping. Foam is made up of air dispersed in liquid or semi-solid phase. It is composed of many air droplets wrapped in a layer of liquid surface-activated soluble protein film, which reduces the surface tension of air and water. Bubbles are due to elasticity. The liquid film or semi-solid film is separated to prevent the merger of bubbles.
Utilizing the foaming properties of soy protein can give foods a loose structure and good taste. The degrading agent can be used to improve the foaming property to degrade the soybean protein to a certain extent. The lower the degree of polymerization, the better the foaming property. In addition, the foamability of soy protein is also related to the leaching solvent, solution concentration, temperature and pH. Foaming is best when low fat, high concentration, 30~35°C, and pH above 10.
The viscosity of protein refers to the internal friction that appears when the liquid flows, also known as fluidity. It is important in adjusting the physical properties of food. The viscosity of protein solution is affected by the protein molecular weight, friction coefficient, temperature, pH, ionic strength, processing conditions and other factors. These factors can change the morphological structure, association state, hydration degree, swelling degree and viscosity of protein molecules. After soy protein isolate undergoes alkaline acid or heat treatment, its swelling degree increases and its viscosity increases. The apparent viscosity of the soy protein solution increases exponentially with the increase of protein concentration, and is related to the swelling degree of the sample. When the protein is heated to 80°C, the protein will dissociate or dissociate, the specific volume of the molecule will increase, and the viscosity will increase, and the viscosity will decrease if it exceeds 90°C. When the pH is 6~8, 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 association is destroyed.
It means that when soy protein isolate is mixed with a certain amount of water, it can be made into a dough-like substance. This property can be used in the processing of flour products, such as bread, cakes, etc., to increase the protein content of the product and improve its performance. Soy protein dough has low elasticity and cohesiveness. 50% of water is appropriate, less than 50% is fragile; if too much water is added, the food will become soft or even syrupy.
It refers to the process in which soy protein is processed, its protein molecules rearrange and combine to have a directional structure, and form a fibrous protein similar to meat after solidification. The protein isolate itself does not have the chewiness similar to livestock meat and fish meat, and only after proper processing can it have the properties similar to livestock meat and fish meat.
There are many methods to organize soybean protein, such as spinning method, extrusion cooking method, wet heating method, freezing method, gelatinization method, etc., among which the extrusion method is the most widely used.
It means that after soy protein and water form a dough, after high-pressure cooking, a film is formed on the surface. This film is a barrier between water and water-containing agents. This is a feature of soy protein suitable for intestinal processing.
After the meat is chopped, it is coated on the fiber surface with a mixture of protein isolate and egg protein to form a film. It is easy to dry, can prevent the odor from being lost, is beneficial to the rehydration process, and provides a reasonable structure for the rehydration product.