Extraction method of soybean protein isolate

 Soybeans are high in protein content and rich in nutrients, generally containing 30%-50% protein. Soy protein contains 8 kinds of essential amino acids for human body, and the ratio is reasonable, but lysine is relatively higher, and the content of methionine and cysteine is lower. At present, soy protein has become an important protein resource. In particular, soy protein isolate contains more than 90% protein and is an excellent food material. 

Soy protein isolate is mainly composed of 11S globulin (Glycinin) and 7S globulin (β-con-glycinin), which accounts for about 70% of the whole soybean grain storage protein. The composition, structure and conformation of these two globulins are different, and the functional properties of soy protein isolate are also different. Soy protein isolates will undergo physical and chemical changes during extraction, processing, storage and transportation. These appropriate changes can improve the functional properties of soy protein in food applications. This article reviews the extraction and modification methods of soy protein isolate. 

1. Extraction method of soy protein isolate 

1.1 　 alkaline extraction and acid precipitation method 

The traditional extraction method of soy protein isolate is alkaline extraction and acid precipitation. Mix the defatted soybean meal with distilled water in a ratio of 1:10, adjust the pH of the mixture to 7-9 with NaOH, fully stir and extract the alkali-soluble soybean protein, centrifuge, and adjust the pH of the supernatant to 4.5 with dilute HCI. —4.8, the protein is precipitated, centrifuged, the precipitate is re-dissolved in a pH 7.0-8.0 NaOH solution, sprayed or freeze-dried to obtain soybean protein isolate, the protein content can reach more than 90%, and the yield is 24%—— 38%. 

1.2　membrane separation method 

Nie Youhua et al. studied the preparation of soy protein isolate using membrane separation technology. First use Ca(OH)2 dilute solution to extract defatted soybean meal, and the protein extraction rate can reach about 80%. The extract is separated by circulating ultrafiltration, and the concentration of the retentate can reach about 13%. The retentate is sprayed or freeze-dried to obtain the soybean protein isolate product, the protein content of which can reach more than 95%. Compared with the traditional alkaline extraction and acid precipitation method, the product yield is high, the quality is good, the energy consumption is low, and the waste water pollution is solved to a certain extent. 

1.3　Bipolar Membrane Electroacidification 

This method is developed on the basis of electrodialysis. The bipolar membrane is composed of 3 layers: anion exchange membrane and cation exchange membrane and a hydrophilic layer in the middle of the anion and cation exchange membrane. Under the action of electric current, water molecules ionize into H+ and OH- on the bipolar membrane. As the membrane selectively permeates anions or cations, the pH value of the solution decreases, reaching the isoelectric point of soybean protein and causing protein precipitation. This method does not need to add acid or alkali to adjust the pH value of the protein solution, avoid mixing salt ions in the separated soybean protein, and can protect the functionality of the soybean protein. 

1.4　 Bubble method 

Foam separation technology is a new separation technology developed in the past ten years. It is a means to separate and purify substances based on differences in surface activity. It is widely used in many ways such as environmental protection, bioengineering, metallurgical industry and pharmaceutical industry. This technology is also an effective way to separate and concentrate proteins and enzymes. Xie Jihong and others studied the separation of soy protein in yellow pulp discharged from soy product factories. In this method, the separation of soybean protein is completed in a continuous foam distillation tower. Nitrogen is introduced into the pool liquid from the bottom of the tower, the raw material liquid enters the tower from the foam interface, and the foam is led out from the top of the tower and broken into foam. Liquid, foam liquid is the separated soybean protein. 

2. Modification method of soy protein isolate 

2.1　 heat treatment method 

Sorgentini et al. treated 5%, 8%, 11%, 13%, and 15% of soy protein isolate at 80°C or 100°C for 30 minutes, and then cooled at 4°C overnight. When the concentration was greater than or equal to 8%, a gel was formed. At the same temperature, the higher the concentration, the more insoluble components; when the concentration of the protein solution is the same, there are more insoluble components treated at 100°C than at 80°C. As measured by DSC, the protein was partially denatured when treated at 80°C, while the protein was completely denatured when treated at 100°C. After heat treatment, the surface hydrophilicity (S0) of the soluble and insoluble components of the protein changes. When the concentration is low, the internal hydrophilic groups are exposed due to thermal denaturation, and S0 increases. In the non-thermally denatured protein, the soluble and insoluble components have the same water binding capacity (WIC). After thermal denaturation, the WIC of both the soluble and insoluble components increase, especially at the highest concentration of 8%. After heat treatment, the emulsification performance of protein South is improved. 

L0pezde Ogara et al. In the process of alkaline extraction and acid precipitation to prepare soy protein isolate, the pH value of the solution was adjusted to the isoelectric point and then treated in an aqueous solution of 50°C, 60°C, 65°C and 70°C for 30 minutes. Protein solubility (NSI) decreases, water absorption (WAC) increases rapidly when the temperature rises from 50°C to 60°C, and decreases slightly when the temperature continues to increase. After heat treatment, the oil absorption is slightly increased. The gel obtained from the protein obtained at 60°C has the highest viscosity and the lowest water loss. 

2.2　acid treatment 

Wagner et al. treated soy protein isolate and Glycinin with acid. Only the protein extracted from the soybean meal stored for a long time, the solubility will be reduced after acid treatment, the change of pH has little effect on the solubility and water binding capacity (WIC), and the foaming and foam stability are very large. improve. During acid treatment, the pH value of 11S globulin has a strong denaturation effect. 

2.3 　 Enzyme treatment 

Wu et al. used papain to hydrolyze soy protein isolate and ultrafiltration to separate the hydrolysate. The 11S protein subunit is more sensitive to papain than the 7S globulin. Papain significantly improves the surface water absorption, solubility and emulsification of soy protein isolate. 

QI et al. studied the hydrolysis of soy protein isolate by trypsin. Trypsin includes trypsin and chymotrypsin, which can hydrolyze the two basic subunits of 11S soy glycinin. After the soy protein is hydrolyzed by trypsin, the surface water absorption (S0) is significantly improved, the solubility changes little, the protein and oil content of the emulsion increases, the emulsifying ability increases, but the emulsifying stability decreases.