Basic characteristics of 1 plant protein
According to the source of intake, protein can be divided into two types: animal protein and plant protein. Animal protein mainly comes from eggs, milk and meat of poultry, livestock and fish. Mainly casein, characterized by high absorption and utilization rate; Plant protein, as its name implies, is extracted from plants, and its nutritional composition is similar to that of animal protein. However, the protein of plants is wrapped by fiber membrane, which makes the protein of plants more difficult to digest than the protein of animals. Therefore, the absorption and utilization rate of plant protein is lower than that of animal protein, but because plant protein contains almost no cholesterol and saturated fatty acids, processed plant protein is not only easier to be absorbed by human body, but also healthier than animal protein.
In terms of nutritional components, protein is mainly composed of various amino acids. Protein is degraded into various amino acids by various enzymes and absorbed by human body. The amino acid composition of animal protein is relatively comprehensive, while the types of amino acids in plant protein are not as many as those in animal protein, and the contents of lysine, threonine, tryptophan and methionine are relatively insufficient. Therefore, protein can be divided into complete proteins and incomplete proteins, and most plant proteins belong to incomplete proteins. For example, grain protein contains about 10% of protein, but the content of protein is not high, which is the main source of protein in human diet, and grain protein generally lacks lysine; Oil protein is mainly methionine deficiency. For example, wheat protein is mainly lysine and threonine; Protein of maize is mainly deficient in tryptophan and lysine; Cottonseed protein is mainly methionine deficiency; Peanut protein mainly lacks methionine; Beans are rich in protein, especially soybeans. The protein content is as high as 36% ~ 40%, and the amino acid composition is reasonable. Except methionine and cysteine, soybean protein has basically balanced amino acid composition and high utilization rate in vivo, which is a very good source of protein in plant protein. The content of methionine in sunflower seed protein is high. If sunflower seed protein with high methionine content is mixed with soybean protein, the deficiency of methionine in soybean protein can be supplemented. Therefore, mixing various plant proteins to make food has a great market prospect [2]. On the other hand, excessive intake of animal protein will lead to excessive intake of relative cholesterol and saturated fatty acids, which will lead to various "rich diseases" such as hypertension, hyperlipidemia and obesity. The reasonable collocation of various plant proteins can not only provide various essential amino acids for human body, reduce the incidence of various diseases, but also improve immunity and fight cancer. Therefore, people pay more and more attention to the role of plant protein in establishing a healthy diet structure.
Protein has good processing characteristics. After processing, it has water retention and shape retention, which makes its products have good economic quality such as storage resistance. Protein can be made into various foods alone, or combined with other foods such as vegetables and meat. Today, in the pursuit of nutrition, health and safe diet, processed vegetable protein drinks and protein powder are also favored by more and more people. These economic, nutritional and functional advantages of plant protein make the extraction and processing of plant protein a hot industry in the world, with great development potential and broad market prospects.
Classification and Extraction of Plant Protein
According to the different contents and sources of various components in plants, plant proteins can be divided into four types of proteins, namely oilseed protein, legume protein, cereal protein and spirulina protein which appeared in recent years. Different physical forms of plants have different contents of protein components, and the corresponding extraction methods are also different.
2. 1 oilseed protein
Oilseeds mainly include peanuts, rapeseed, sunflower, sesame and so on. Their protein species are mainly globulin. Among them, the content of protein in peanuts is 26% ~ 29%, and the content of globulin can reach 90%. After processing, it has high solubility and low viscosity, and can be used to make bread and drinks. Sunflower is an important source of oil raw materials, which contains high globulin, but its lysine content is limited. The rapeseed yield is very high, with protein content of 25% in rapeseed and protein content of 35% ~ 45% in rapeseed meal after deoiling [3]. In protein, rapeseed protein has the highest nutritional value, and there are no restrictive amino acids, especially many sulfur-containing amino acids that are lacking in soybeans. Protein concentrate can be processed from defatted rapeseed. In the process of extraction and separation, protein is easily affected by denaturation due to heat, which reduces the solubility of protein and cannot form colloid, while oilseed protein has good water and oil retention performance. In addition, the isolated protein with less denaturation has good foaming, emulsifying and gelling properties.
At present, there are two main methods to extract protein from oil: alkali dissolution and acid precipitation and reverse micelle extraction [4]. Among them, the alkali dissolution and acid precipitation method has a large amount of acid and alkali, which is serious to the environment. Therefore, the reverse micelle extraction method with mild extraction conditions and difficult inactivation of protein is generally selected, which also has the economic advantages of recycling solvent and low cost. The main function principle is to dissolve surfactant in organic solvent, add a certain amount of water to form reverse micelle solution, and extract oil and protein from vegetable oil at the same time. Extracting and dissolving oil in organic solvent, adding protein or chlorogenic acid and extracting into polar core of reverse micelle. After chlorogenic acid was extracted, protein was extracted, and finally back-extracted with strong ionic strength solution. After desalting and drying, protein products were obtained. However, this method also has some limitations. Because there are many solvents used in the extraction process, which are easy to remain in protein products, the reverse micelle extraction method is not suitable for extracting protein with relatively large molecular weight.
2.2 protein of beans
Protein is rich in beans and mainly exists in protein Formation. The protein content of beans is as high as 40%, and the content of protein group is as high as 80%. Generally speaking, protein contains less basic amino acids, but more acidic amino acids such as glutamic acid and aspartic acid, among which globulin is the main component, and it is also rich in unsaturated fatty acids, calcium, phosphorus, iron and dietary fiber. It contains no cholesterol and has high nutritional value. Studies by modern nutritionists have confirmed that soy protein can reduce hypertension, cardiovascular diseases, nutrition absorption and blood lipid. Not only that, beans also contain active ingredients such as saponins and isoflavones, which have the functions of anti-aging, improving immunity and promoting calcium absorption [5]. In the production of bean products, the extraction rate of protein is generally low. Taking soybean extraction as an example, the extraction rate of soybean protein is mostly below 60% [6]. Most of soybean protein is soluble in water, so there is great potential to improve the extraction rate of soybean protein. Soybean protein can be divided into albumin and globulin according to the solubility characteristics of protein [7]; According to the different centrifugal separation coefficient (i.e. sedimentation coefficient), soy protein isolate can be divided into four components, namely 2S, 7S, 1 1S and 15S [8]. According to the separation degree of protein, the separation methods of protein can be divided into two categories. One is to use low denatured soybean meal as raw material, leach protein with weak alkaline solution, separate sugar and insoluble matter with high-speed centrifuge, adjust pH value to 4.5 ~ 4.6 with acid for extraction, precipitate protein from the solution, neutralize it with alkaline water, then send it to a heater for rapid sterilization and spray drying to get the finished product. At this time, the protein product contains a small amount of soluble sugar, ash and other trace components [9- 10]. The other is concentrated protein (SPC) obtained by removing soluble sugar, ash and trace elements from protein, which improves the content of protein. Weeks [1 1] By adjusting the process parameters, choosing the pH value of 7.0, soaking soybeans in water for 15 h, and then grinding at 70℃ according to the ratio of soybeans to water, the extraction rate of soybean protein reached nearly 80. With the development and utilization of new technology, the extraction technology of soybean protein is constantly innovating. For example, various imitation meat products appearing in the market at present are that protein separated from soybeans is neutralized with alkali, and then put into acetic acid solution through a diaphragm with thousands of holes, so that protein is solidified and precipitated, and protein molecules are arranged to a certain extent to form a textured soybean protein product [12].
2.3 Grain protein
Cereals mainly include corn, wheat and rye. Protein in cereals is insoluble in water or salt solution, and its main components are glutenin soluble in alkali solution and gliadin soluble in alcohol. Corn contains more gliadin, while wheat contains 13% protein. The contents of glutenin and gliadin are basically the same, ranging from 30% to 50%. Gluten and gluten, which constitute gluten, are the main protein in wheat grains, and they together constitute gluten in flour. Glutenin and gliadin are difficult to separate, slightly soluble in hot dilute ethanol, but become flocculent precipitate after cooling. Only the newly prepared and undried gluten is easily soluble in weak base and weak acid, and will precipitate again during neutralization. Except leucine, methionine, cystine and tryptophan, the content of protein in wheat can't meet the standards recommended by the World Health Organization (WHO), and lysine is seriously lacking, so wheat flour protein belongs to incomplete protein.
Wheat germ also contains some protein, with protein content as high as 30%. Wheat germ protein is a complete protein, which contains 8 amino acids and 2 semi-essential amino acids, accounting for 34.7% of the total amino acids [13], and is easily absorbed by human body. In the composition of wheat germ protein, albumin accounts for 30.2%, and three globulins, α, γ and δ, account for 13. Therefore, it is more beneficial to human health to reasonably mix all kinds of grains into staple food.
2.4 Spirulina protein
Spirulina is a spiral single-celled aquatic plant with a blue-green appearance. It is a source of protein that has attracted more attention in food industry recently. The main protein in Spirulina is phycocyanin (PC), which mainly exists in the form of phycobiliprotein, and its protein content is as high as 70%. Phycobiliproteins are composed of various phycobiliproteins and connective proteins or polypeptides [15], which contain threonine and lysine necessary for human body. At the same time, spirulina protein is easily absorbed and utilized by human body, and has high nutritional value.
Spirulina protein has high nutritional value and broad market prospect. Can be use as raw materials of food and medicine, and has high economic value. At present, there are many methods to extract phycocyanin. Among them, phycocyanin was separated and purified from fresh algae filaments by graded gradient salting out, and the purity of PC extracted by hydroxyapatite chromatography was higher than the recognized standard [16]. Another simple method is designed by Ganapathi Patil et al. [17], which mainly includes two steps: aqueous two-phase extraction and ion exchange chromatography. The method comprises the following steps: obtaining a crude extract of phycocyanin from spirulina, then improving the purity through an aqueous two-phase extraction step, and further purifying protein through ion exchange chromatography.
3 the application value of plant protein
Protein is a protein rich in amino acids. Because of its rich nutrition and many excellent functional characteristics, it is widely used in various foods, such as meat, baked goods, dairy products, beverages and so on. Protein, a plant, generally contains no or only a small amount of cholesterol, oil and so on. , and is favored by many obese people, hypertension, hyperlipidemia and beauty lovers. Not only in diet, but also in medical treatment. For example, phycocyanin has antioxidant and anti-inflammatory effects, and can also be used to treat some neurodegenerative diseases induced by oxidative stress, such as Alzheimer's disease and Parkinson's disease [15, 18], and to promote immune system function and inhibit hemolysis. Protein-based plant protein industry has already reached a considerable scale in China. Using vegetable protein as auxiliary raw material in surimi products further reduces the cost of surimi products [19]. Bean cake and soybean meal are the best cakes in various feeds, which are widely used in compound feed for pigs and chickens. In recent years, vegetable protein drinks are also deeply loved by people, such as peanut milk and almond dew, which can be seen everywhere in daily life.
4 conclusion
The appearance of more and more plant protein products shows that plant protein has become an indispensable protein in life, and its huge development prospect will be recognized by people with the improvement of production technology and equipment. Plant protein can provide nutritious and cheap protein, and countries all over the world are actively developing plant protein resources to solve the problem of resource shortage in protein. At present, the development of plant protein resources is mainly through high-tech research on the application of plant protein resources and transformation of traditional products. However, the development of plant protein extraction and processing is still limited to some extent, mainly due to the loss of nutrients during processing and the treatment of waste after processing. This still needs to be solved step by step, making full use of plant protein sources, improving its utilization efficiency and making it better used by people.