What Is Rice Protein Powder?

Rice is one of the world's staple foods, and more than half of the world's population and more than two-thirds of China's population rely on rice as their main food source. Therefore, rice protein is an important source of protein in people's diets. China has a large area of paddy fields, and the annual output of rice is 180 billion kilograms. In addition to supplying people's daily dietary needs, the rice processed from these paddy fields is also used as a raw material for monosodium glutamate fermentation and starch sugar production.

These processing links produce a large amount of by-products such as rice bran and rice residue. Rice bran is rich in nutrients, with a protein content of about 12%, and the protein content of defatted rice bran can be as high as 18%. The protein content of rice residue is above 40%, commonly known as rice protein powder and rice protein concentrate (RPC). They are all valuable protein resources. Foreign countries attach great importance to the development and utilization of rice and rice bran, and have produced nutritious health foods and cosmetics with high added value. In the past, they were used as animal feed in China, and the resources were not used rationally. In recent years, the country has attached great importance to this, and some scientific research institutions and enterprises have increased their research and development efforts. This paper introduces the latest research progress on rice and rice bran protein at home and abroad in recent years from the perspective of development and utilization.

1 Structure, composition and properties of rice protein Powder

Understanding the structure, composition and properties of rice protein powder is the basis for its development and utilization. There are many types of rice protein, which are generally classified according to their solubility characteristics. The protein fraction obtained by first extracting the protein from rice or rice bran with water is called albumin; the protein fraction obtained by extracting the residue with a dilute salt solution is globulin; the fraction extracted with 75% ethanol is alcohol-soluble protein, and finally the protein in the residue can only be dissolved with acid or alkali, which are collectively referred to as acid-soluble protein and alkali-soluble protein, respectively.

Gluten and alcohol-soluble proteins are also called storage proteins. They are the main protein components in rice, with gluten accounting for more than 80% of the total protein and alcohol-soluble proteins accounting for about 10%. The content of albumins and globulins is extremely low. They are the physiologically active proteins in rice and play an important physiological role during the early stages of rice germination.

Different proteins have different amino acid compositions. The content of uncharged hydrophobic amino acids in albumin is high, while the content of acidic amino acids is low. Globulin has a high content of basic amino acids, accounting for more than 15%, while the content of basic amino acids in alcohol-soluble protein is only about half of that in globulin, but its hydrophobic amino acids are much higher than those in other types of protein [1].

The solubility of protein is not only related to its amino acid composition, but also to its state of existence. Studies have shown that in the endosperm, protein mainly exists in two aggregate forms, namely PB-I and PB-II. Electron microscopy shows that PB-I aggregates have a lamellar structure, with dense particles measuring 0.5–2 μ m, and the alcohol-soluble protein is present in PB-I; PB-II is ellipsoidal, not stratified, with a uniform texture and a particle diameter of about 4μm. Its outer membrane is not obvious, and glutenin and globulin are present in PB-II. The two aggregates often exist together [2-3].

During the germination of rice, the two protein aggregates disintegrate, but the digestibility of the two is significantly different. PB-II is more easily digested and hydrolyzed because it does not have a dense hard core, while PB-I maintains a lamellar structure even 9 days after germination. Studies using SDS-PAGE technology have shown that PB-II continuously produces new electrophoretic bands, i.e., new protein components appear, while the components of PB-I are stable [4]. This indicates that there are differences in the metabolism of the two protein molecules.

Rice protein powder has a higher cysteine content and contains more -S-S-bonds. These intra- or inter-chain -S-S-bonds cause the protein polypeptide chains to aggregate into dense molecules, which may be an important reason for the formation of protein aggregates. Polyacrylamide gel electrophoresis (PAGE) analysis results show that the proteins in PB-II aggregates contain components with molecular weights of 64, 140, 240, 320, 380 and 500 KDa, and even more than 2000 KDa [5]. Molecular biology studies have shown that when the gene for rice storage protein is expressed, the first protein molecule synthesized is one with a molecular weight of 57 KDa, which is then cleaved into two subunits of 22 KDa and 37 KDa. The protein molecules of different sizes in glutenin are assembled from these two subunits through —S—S—[6]. SDS can break the —S—S—linkage. By changing the amount of SDS used, the components with molecular weights of 22–23 KDa and 37–39 KDa can be found to exist. Therefore, these two components are actually the basic units of macromolecular aggregates [5].

There are also protein components with a molecular weight of up to 100 KDa in albumin, but since albumin has a very low cysteine content, it is not easy to form –S–S– bonds, so albumin is more soluble in water. This shows that the presence of disulfide bonds is very important for stabilizing protein aggregates.

After the protein is extracted, its amino acid composition is analyzed and it is found that some proteins in rice are not simple proteins composed entirely of amino acids, but rather are bound proteins containing sugars (rhamnose) or lipids [7]. These non-amino acid components not only affect the properties of the protein, but also give the protein a special physiological function.

In addition, a large number of studies have shown that the type of protein in rice is not fixed. During the aging process of rice, although the total protein content remains the same, its structure and type change, which in turn affects the rheological properties of the rice. The prominent changes are an increase in the number of disulfide bonds, an increase in the molecular weight of the protein, denser protein aggregates, and a denser network structure of the protein and starch after cooking, which limits the swelling and softening of the starch granules during soaking.

If a suitable amount of reducing agent is added at this time to break the disulfide bonds, the stickiness of the rice will increase [8–11]. Ren Shuncheng et al. also used SDS-PAGE to demonstrate this change in the molecular weight of rice proteins before and after aging [12]. Teo et al. also demonstrated that changes in the proteins in rice are an important factor in the changes in the rheological properties of rice [13]. These experiments all show the importance of the -S-S-bond to the properties of the protein.

Not only does rice protein form larger molecules during aging, but it also undergoes significant protein molecule aggregation when heated. Mujoo pointed out that when stir-frying rice, molecules with molecular weights of 24, 34, and 68 KDa can aggregate into extremely large aggregates of 4 × 104 KDa, but the alcohol-soluble protein with a molecular weight of 13–16 KDa does not participate in the formation of such protein bodies [14].

It can be seen that the development and utilization of rice proteins should pay particular attention to the effects of rice aging, heating and the oxidation and reduction of disulfide bonds on the properties of the proteins.

The content of the four types of proteins in rice bran is significantly different from that in rice.

The content of clear, spherical, alcohol-soluble, acid-soluble and alkali-soluble proteins obtained by sequential extraction with water, salt, alcohol, acid and alkali solutions is 34%, 15%, 6%, 11% and 32% respectively. Among them, acid-soluble and alkali-soluble proteins are both gluten proteins, which means that the water-soluble protein content in rice bran is very high. Chromatographic analysis showed that the molecular weights of the first four proteins ranged from 10 to 100 KDa, 10 to 150 KDa, 33 to 150 KDa and 25 to 100 KDa, respectively. The molecular weight of the main components of the alkali-soluble protein, which has had its disulfide bonds broken during the extraction process, is still distributed between 45 and 150 KDa. All such gluten proteins have a higher molecular weight and are more difficult to dissolve in water. However, if the disulfide bonds are broken, more than 98% of the rice bran protein can be dissolved [15]. It should be noted that the content of various protein components in rice bran changes greatly before and after stabilization treatment (usually heat inactivation of enzymes), mainly manifested as a decrease in the content of albumin (due to denaturation) and a significant increase in the content of gluten [16].

2 Nutritional value of rice protein powder

Rice protein powder is recognized as high-quality edible protein, mainly because its amino acid composition is balanced and reasonable, in line with the ideal model recommended by the WHO/FAO. Among them, the methionine content is relatively high, which is unmatched by other plant proteins. The biological value of rice protein and rice bran protein is very high, and their nutritional value is comparable to that of eggs and milk.

In addition, rice protein powder is hypoallergenic and does not cause allergic reactions, which is very beneficial for the production of baby food. Infant rice protein nutrition powder is sold in many countries around the world. Many plant proteins contain anti-nutritional factors, such as trypsin inhibitors and lectins in soy protein and peanut protein, a type of albumin in wheat, and bromelain in pineapple, which often cause immune responses that make consumers develop allergic or toxic reactions. Some allergenic factors are also found in animal foods, such as lactoglobulin in milk and ovalbumin in egg white. These factors are most likely to cause allergic reactions in infants and young children. In contrast, rice protein is the safest, and rice is the only cereal that can be exempted from allergy testing [17]. With the gradual improvement of research techniques on rice protein, rice protein-fortified foods for infants and the elderly are becoming more popular in the market.

In addition to its basic nutritional functions, rice protein also has other health benefits. Morita's experiments with rice protein isolate (RPI) and casein in mice showed that RPI significantly reduced the concentration of cholesterol, triglycerides and phospholipids in the blood serum, and the weight of the mouse liver was also lower than that of the test group fed casein [18].

Dimethylbenzanthrene (DMBA) is a mutagenic agent for breast cancer. Mice were fed 30 mg DMBA/kg body weight, and the protein in the basal diet was RPI, soy protein isolate (SPI) and casein, respectively. The results showed that the tumor weight of mice fed RPI was lower than that of mice fed casein, and there was no significant difference in the activity of phenol hydroxylase in the serum of mice in each group at 7 days. This indicates that RPI has the effect of resisting DMBA-induced carcinogenesis [19]. RPI extracted from rice bran also exhibits the same effect [20]. Further analysis of the components of RPI using high-performance liquid chromatography revealed the presence of triterpenoid alcohols, ferulic acid and other components [21], indicating that RPI is a binding protein. The special effect of protein may be due to the participation of these non-amino acid components.

The experiments of Neriega are also very interesting. He compared the tolerance of sub-threshold physical training in people who consumed rice and bread, and found that those who consumed rice had stronger endurance and lower lactic acid levels in the blood [22].

Rice bran also has an anti-diabetic effect. Streptozotocin (STZ) is a diabetes-inducing agent. In a functional test of rice bran, it was found that feeding rats with rice bran for two months significantly reduced the symptoms of STZ-induced diabetes. The levels of glycerol and cholesterol in the serum of the test rats were lower than those in the control group, and the symptoms of polyuria were also improved. It can be inferred that the protein in rice bran plays an important role [23].

The above studies show that rice protein not only has unique nutritional functions, but also many potential health care effects. This is one of the important reasons why foreign countries attach great importance to the research, development and utilization of rice protein. However, there has been relatively little research on the functionality of rice protein in China.

3 Development and utilization of rice protein powder

The main component of rice is starch, and the protein content is only about 9%. It is obviously not economical to extract protein directly from rice. The protein content of rice starch sugar and the waste material (i.e. rice residue) in the production of monosodium glutamate is 40% to 65%. It can also be called rice protein concentrate, which is the main raw material for the development and utilization of rice protein. This is a valuable resource in large quantities. In the past, it was mainly used as protein feed for animals, but from the perspective of resource utilization, this is not economical. With the recognition of the value of rice protein, more and more rice proteins are being developed into raw materials and additives for food production with high added value. High-protein nutritious rice flour is sold on the market, but it still has starch as the main ingredient and a very low protein content. The potential for development and utilization as a protein resource has not been fully utilized.

3.1 Rice protein isolate (RPI)

The protein content of rice protein concentrate (RPC) is already over 40%, but many of its functions are not yet ideal. Rice protein isolate (RPI) with a protein content of over 90% can be obtained by removing the carbohydrates in it by chemical or biochemical methods. RPI can be modified by hydrolysis or biochemical methods to produce various edible protein supplements. Since RPC is mostly made up of water-insoluble proteins, the traditional method of extracting it is to use an alkaline solution and an acid to precipitate it. This method can produce RPI with a high degree of purity, but it has significant disadvantages, such as the product being dark in color, the lysine in the protein being damaged, side reactions that produce bitter, harmful substances, and a low protein recovery rate.

Based on the characteristics of RPC, in which the protein is insoluble in water and the non-protein components are mainly carbohydrates, the extracted protein should be further purified. It can also be treated with cellulase, pectinase and amylase to promote the dissolution of more carbohydrates. This method is used in the production of rice starch sugar, which can not only increase the yield of starch sugar, but also obtain RPI with high purity, and the recovery rate of protein can also reach a satisfactory level [24-26].

The protein content of rice bran is 10% to 12%, and as mentioned above, about 35% of it is water-soluble. However, there is a lot of fiber in rice bran, and most of it has been stabilized. Heating greatly changes the solubility of the protein, making effective extraction difficult. Current research on this problem focuses on the homogenization of rice bran and the application of enzyme technology. The particle size of the rice bran has a significant effect on the solubility of the protein, especially for rice bran that has not been heat-treated. Some studies have shown that milling and homogenization can dissolve 38% of the protein, which is 75% higher than the original solubility, and the molecular weight of the dissolved components varies greatly [27].

The use of biological enzymes for the extraction of rice bran protein is even more effective. The enzymes that can be used include cellulase, ligninase, protease and phytase. Cellulase and ligninase can break the binding of rice bran cellulose to protein, so that the protein content in the extract can reach more than 50% [28-29]. If defatted rice bran is treated with a combination of phytase, cellulase, ligninase, etc., rice bran protein isolate (RBPI) with a protein content of up to 92% can be obtained, and the yield can reach 74.6% [30].

The application of proteases can also achieve good results. Hamada et al. used proteases to treat rice bran to achieve a protein hydrolysis degree (DH) of 10%, and the protein extraction rate was 92%. If Na2SO3, SDS, etc. are used to break the disulfide bonds of proteins, even if the degree of hydrolysis is only 2%, the protein recovery rate can reach 84% [31]. Two or more proteases with different hydrolysis sites should be used in the extraction process, and the physicochemical properties of the resulting protein hydrolysate are better than those of a single enzyme [32].

The above experiments all increase the solubility of the protein to improve the extraction effect, and the foaming and emulsifying properties of the protein obtained are also improved to a certain extent. This is obviously different from the technical direction and product properties of the rice proteinase method.

3.2 Rice protein foaming powder

More than ten years ago, the emergence of rice protein foaming powder provided an option for the large-scale application of rice protein in food production. However, this foaming powder is made from concentrated rice protein, a product of limited hydrolysis of the protein with NaOH. The product is dark in color, has a high pH value and a bitter taste. The disadvantages mentioned above can be overcome by hydrolyzing rice protein with protease. Rice protein has a high molecular weight and contains a relatively high proportion of hydrophobic amino acids, which results in poor solubility and prevents it from exhibiting physicochemical functionality.

After proper hydrolysis with protease, more —COOH and —NH2 are released, increasing the polarity of the protein molecule. While increasing the solubility of the protein, the colloidal properties of the solution are also enhanced, exhibiting certain emulsifying and foaming abilities. It can be widely used as a raw material for food processing and imparts certain processing properties to foods. At present, more research in China is focused on the hydrolysis of soy protein and wheat gluten protein. Wang Zhangcun et al. obtained good results by hydrolyzing soy protein isolate with protease [33]. In recent years, there have also been reports in China on the research of preparing edible foaming powder by enzymatic hydrolysis of rice protein [34]. It is believed that with the improvement of technology, enzymatic rice protein foaming powder will be widely used in food production.

3.3 Protein hydrolysates

It uses rice protein powder as raw material, and through different degrees of hydrolysis, protein hydrolysates with different uses can be obtained. Most of them can be used as protein nutritional enhancers for instant drinks, while some contain special flavors or health functions.

The preparation of amino acid nutrient solutions is a traditional method of using vegetable protein. The domestic research and use of this method is mostly the acid hydrolysis method. The so-called chemical soy sauce is based on this method, but due to environmental protection and safety issues, it should be eliminated. If protease hydrolysis is used, due to the limitation of enzyme specificity, no enzyme can completely hydrolyze the protein, and the application of multiple enzymes is not economical.

In fact, nutritional products designed to supplement amino acids do not need to completely hydrolyze the protein, just hydrolyze it into small peptides. Current nutritional research shows that small peptide molecules are more easily absorbed and utilized by the small intestine than amino acids. Peptide absorption is achieved through the active transport mechanism of peptide carriers present in the intestinal mucosal border, using a proton gradient. Small peptides have a lower osmotic pressure, do not cause dysentery or allergic reactions after consumption, and have a better sensory effect than amino acids. They can be used as protein nutritional supplements. NutriBi- otics rice protein powder, which is currently well-known in the United States, is one such product.

What is more interesting is that many small peptide molecules have important physiological functions, such as immune regulation, anti-oxidation, anti-cholesterol, anti-thrombosis, anti-diabetes, etc., also known as active peptides. At present, research on the hydrolysis of animal proteins to produce bioactive peptides has become a worldwide trend, and many active peptides with potential application value have been discovered [35]. However, there has been relatively little research on active peptides derived from rice. One of the more reported active peptides from rice is the Gly-Tyr-Pro-Met-TYR-Pro-Leu-Arg peptide molecule, named Oryzatensin. Tests on guinea pigs have shown that it has the effects of causing contraction of the ileum, resisting morphine and regulating immunity. It mainly causes contraction by activating phospholipase to hydrolyze lysophosphatidic acid and release arachidonic acid [36].

In addition, hydrolysis of rice protein can also produce certain flavor peptides. Modern instrumental analysis shows that these flavor peptides are high in glutamic acid, which combines with salt to form monosodium glutamate and imparts a savory taste. When this product produced by enzymatic hydrolysis of rice protein is mixed with dextrin and spray dried, a commercially available food flavor enhancer is obtained [37].

3.4 Chemical modification of rice protein

Natural plant proteins generally have poor physicochemical functionality. Scientific researchers hope to improve the properties of proteins through chemical means and increase their use in foods. This will not only meet the needs of food processing performance, but also improve the nutritional value of foods. At present, there has been a lot of research on the modification of soy protein. The main methods are the introduction of phosphate groups and acetyl groups, or the removal of amide groups such as glutamyl amide and asparagine in proteins. These measures are safe and effective. However, there have been no reports on the chemical modification of rice isolated protein.

In summary, rice protein is a valuable protein resource that needs to be vigorously developed. It is a protein polymer molecule connected by more disulfide bonds. Rice protein and its hydrolysates not only have important nutritional functions, but also have potential health care effects. The enzymatic hydrolysis and chemical modification of rice protein can improve its physicochemical functional properties. These products have broad application prospects. There has been a lot of research on rice protein abroad, and some results have been achieved. It is believed that there will be significant progress in the research and development of rice protein in China.

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