What Is Brown Rice Protein Powder?

Brown rice protein contains high-quality lysine and is hypoallergenic, making it very suitable for developing infant foods [1]. The amino acid composition of brown rice protein is superior to that of casein and soy protein isolate, and can meet the amino acid needs of children aged 2 to 5 [2]. In addition, brown rice protein can be processed into soy sauce, high-protein powder, protein drinks, peptone and protein foaming powder. If it is degraded into short peptides or amino acids, it can be made into amino acid nutrient solutions with extremely high nutritional value, which can be used in health drinks, condiments, food additives, etc.

The global demand for protein has been growing, especially for protein that can add value to food. Rice bran is a major by-product of rice processing. After rice bran oil is extracted, the defatted rice bran contains up to 15.4% protein[3]. The protein extracted from it can be used as a nutritional food ingredient or made into high value-added foods. However, in China, rice bran is currently mainly used as feed, and the added value it provides is not fully utilized. Therefore, it is meaningful to develop rice protein. This paper reviews the structure, properties, nutritional and health functions, development and utilization status, and research methods of brown rice protein.

1 Composition and structure of brown rice protein

1.1 Composition of brown rice protein

Brown rice protein is mainly composed of four types of protein: albumin, globulin, alcohol-soluble protein and glutenin, of which glutenin and globulin are the main components, accounting for 80% and 12% respectively, and alcohol-soluble protein accounting for 3%. Globulin and albumin are physiologically active proteins in rice endosperm, and there are many types, with relative molecular masses of 10–200 KDa and 16–130 KDa. Alcohol-soluble protein and glutenin are storage proteins in rice. The content of alcohol-soluble protein is not high, but it is closely related to the morphology of endosperm protein bodies. Rice bran is mainly composed of endosperm proteins, which are composed of albumin (4% to 9%), salt-soluble globulin (10% to 11%), alcohol-soluble glutenin (3%), and alkali-soluble glutenin (66% to 78%).

1.2 Structure of brown rice protein

Brown rice protein mainly exists in the form of two protein bodies (PB), namely PB-I and PB-II. Electron microscopy shows that PB-I protein bodies have a lamellar structure, with dense particles measuring 0.5–2 μm in diameter. Alcohol-soluble proteins are found in PB-I. PB-II is ellipsoidal, not stratified, and uniform in texture, with particles measuring about 4 μm in diameter. Its outer membrane is not obvious, and glutenin and globulin are found in PB-II. The two types of protein bodies often exist together [4-5]. Table 1 shows a comparison of the rice protein content in different raw materials.

2 Properties of brown rice protein

The properties of brown rice protein include nutritional properties and functional properties. Its functional properties include solubility, emulsifying properties, foaming properties, water retention, oil retention, gelling properties, and flavor binding properties. Among these properties, solubility often affects other functional properties such as thickening, foaming, emulsification and gelling. Wang Zhangcun [6] found that measures to increase the solubility of rice protein are beneficial to improving the emulsifying properties of rice protein. The emulsifying properties of brown rice protein are greatly affected by pH. At pH = 5, the emulsifying activity of rice protein is the lowest, and then increases with increasing pH, which is consistent with the trend of changes in their solubility. Related studies have found that although brown rice protein extracted from rice bran has poorer foaming properties than casein, it has better emulsifying properties and maintains stable emulsifying properties in the presence of high concentrations of salt and sugar [7].

Cross-linking and aggregation between the molecules of brown rice protein can easily form deposits and denature the protein. Proteins can be modified using acids, alkalis or enzymes. Studies have found that modifying brown rice protein with the alkaline protease Alcalase can significantly improve its solubility, emulsification and foaming properties [8]. A study was conducted on the deamidation modification of brown rice protein using hydrochloric acid of different concentrations. It was found that within the range of 0–66.2% deamidation, the solubility of rice protein increased linearly with the degree of deamidation, and both water retention and oil retention were improved. The emulsifying properties of brown rice protein are best in the range of 25.1% to 33.4%; the foaming properties are ideal at 59.5%; and the nutritional value and digestibility of modified rice protein are both improved [9].

3 Nutritional and health-promoting functions of brown rice protein

3.1 Nutritional value

Brown rice protein is rich in the amino acids the body needs, including methionine, arginine and lysine, which are higher in content than in other cereals. It has high bioavailability, is hypoallergenic, has a balanced amino acid composition and is very close to the ideal model recommended by WHO/FAO.

3.2 Anti-hypertension and cholesterol

The effect of brown rice protein isolate on the expression of cyp4a and cyp2c in the kidneys of young rats can improve arachidonic acid metabolism and can be used as an anti-hypertension ingredient. Studies have found that rice protein isolate (RPI) can increase the amount of messenger ribonucleic acid (mRNAs), which are responsible for the synthesis of two important proteins in the kidney, CYP2C11 and CYP2C23. These two proteins play an important role in the metabolism of fatty acids, arachidonic acid and 20-HETE (20-hydroxyeicosatetraenoic acid), which is important in regulating blood pressure [10]. Clinical studies have found that rice protein isolate can lower cholesterol. Rice contains many chemicals related to its protein composition, including vitamin E derivatives, tocotrienols and oryzanol, which have a certain effect on lowering cholesterol [11].

3.3 Prevention of chronic diseases

A reasonable nutritious diet can prevent some chronic diseases, such as heart disease and cancer. Asians have a lower risk of heart disease than Europeans, which may be related to the fact that Asians mainly eat rice. Studies have found that brown rice protein isolate has a certain inhibitory effect on atherosclerosis in a mouse model with hereditary high cholesterol. It can reduce the damaging effect of atherosclerosis on the arteries. The mechanism of action is not yet clear, and experiments have also shown that dietary rice can reduce the incidence of heart disease [12].

3.4 Anti-cancer

Molita et al. showed that feeding brown rice protein isolate (RPI) to dimethylbenzanthrene (DMBA)-induced female mice resulted in lower tumor weights than feeding casein to mice, and that RPI had an anti-cancer effect against DMBA-induced carcinogenesis [13]. In addition, brown rice protein isolate has a preventive effect on chemically induced breast cancer in rats.

4 Development and utilization of brown rice protein

4.1 Brown rice protein bioactive peptides

Takahashi et al. isolated and purified an active peptide called oryzatensin from the tryptic hydrolysate of rice albumin. Its amino acid sequence is Gly-Tyr-Rro-Met-Tyr-Pro-Leu-Pro-Arg, which can cause contraction of the guinea pig ileum, resist morphine and have immunomodulatory activity [14]. Brown rice protein contains a large amount of hydrophobic amino acids. Enzymatic hydrolysis at specific sites by a suitable protease yields peptide fragments with a C-terminal hydrophobic amino acid, i.e. the ACE (angiotensin-converting enzyme) active peptide fragment. ACE inhibitors are a major class of drugs used to treat hypertension. Brown rice protein hydrolysate is a safe, non-toxic peptide that can be used to develop ACE-inhibiting peptides.

4.2 Nutritional supplements

Rice protein is widely used in the development of high-protein, hypoallergenic infant formula rice cereals due to its low antigenicity and high nutritional value. Rice protein is gluten-free and suitable for people with wheat intolerance, allergies or celiac disease. Rice protein concentrate can also be used as a supplementary food for patients with reduced normal protein intake or impaired protein digestion to maintain nitrogen balance. It can be used as adjuvant therapy for peptic ulcers, osmotic diuretics and trauma.

4.3 Food additives

Enzyme-treated rice bran protein has significantly improved solubility, foaming properties, emulsifying properties, etc. In particular, moderate hydrolysis and deamination of rice bran protein with endopeptidase and exopeptidase has obtained a protein hydrolysate with moderate peptide chain length and functional properties, which improves the solubility of rice bran protein and improves other functional properties. It can be used as a food emulsifier, foaming agent, and nutritional fortifier.

4.4 Edible film

The Southern Research Center in Louisiana, USA, has developed an edible film with a certain tensile strength and water vapor resistance using a combination of rice protein concentrate and pullulan polysaccharide. It can be used as a carrier for flavoring substances and nutritional additives or as a barrier for separation, protection and preservation.

4.5 Rice protein feed

A by-product of rice starch can be used as feed-grade rice protein powder. Its rich nutritional composition has a function of promoting disease resistance in livestock and poultry, and can also improve the utilization rate of feed. It is an excellent additive for the animal husbandry and feed industries.

5 Extraction technology of brown rice protein

5.1 Alkali extraction

More than 80% of rice protein is alkali-soluble rice gluten. Dilute alkali can loosen the compact starch structure in brown rice, and alkali has a degrading effect on the macromolecular rice gluten, thereby dissolving the protein in the rice starch granules and separating it. Sun Qingjie et al. [15] studied the optimal process for extracting rice protein using sodium hydroxide (NaOH). When the concentration of NaOH is 0.09 mol/L, the extraction rate of rice protein reaches 90.1%. As the concentration of NaOH increases, the extraction rate of rice protein increases, but if the concentration is too high, the starch will gelatinize. The alkaline method for extracting rice protein is simple to operate, but the yield of protein is generally low due to degradation under high alkaline conditions, which can cause cross-linking and rearrangement between molecules, resulting in a decrease in the nutritional value of the protein and the possible formation of toxic substances such as lysine alanine [16]. The alkaline extraction process is as follows: rice flour or bran → add alkali → centrifugal separation → protein solution → acid precipitation → centrifugal separation → water washing → acid neutralization → drying → rice protein.

5.2 Enzyme extraction

Enzyme extraction uses the degradation and modification of brown rice protein by proteases to make it soluble and extractable as peptides. Currently, microbial proteases used to extract rice protein include acid proteases, alkaline proteases, neutral proteases and complex proteases. Ge Na et al. [17] found that the extraction rate of rice protein by acid protease was the highest, followed by alkaline protease, and the extraction effect of flavor protease and neutral protease was the worst. The reason may be that acid protease can better interact with the macromolecular rice gluten at the interface, while loosening the starch structure, so that the protease can diffuse into the interior of the starch to promote the degradation and dissolution of the protein, achieving a better extraction effect. Usually, the effect of a single enzyme is not as good as that of a compound enzyme. Qian Ying et al. [18] used a new type of raw material compound hydrolytic enzyme to treat rice at low temperatures to obtain high-purity rice protein with a protein content of more than 75%. The protein extracted by enzymatic method has better functional properties and a higher protein digestibility, but the extraction time is longer and the cost is higher. The process is as follows: rice flour or bran → protease hydrolysis → centrifugation → protein solution → ultrafiltration → drying → rice protein.

5.3 Stepwise hydrolysis method

The stepwise hydrolysis extraction method is a combination of alkali and enzyme extraction in steps. Wang Yalin et al. [19] first extracted some of the proteins by alkali solubilization, and then slightly hydrolyzed the residue with alkaline protease to improve the solubility of the protein and perform a secondary protein extraction, which gave better results. Chi Mingmei et al. [20] used an enzyme-alkali two-step method to extract rice protein. First, α-amylase was used to partially enzymolyze the starch to loosen the binding of the starch to the rice protein. Then, the enzymolyzed product was extracted with alkali and acid precipitation to extract the protein. The purity of the rice protein obtained was 85.1%, but the dividing point of the stepwise hydrolysis method is not easy to control and requires further research.

5.4 New extraction methods

Foreign research has found that the combination of physical treatments such as ultrasound, freeze-thaw, high pressure and high-speed homogenization with enzymatic treatments is more effective for extracting proteins from rice bran. I Sereewatthanawut et al. [21] carried out research on the extraction of rice proteins and amino acids from defatted rice bran using subcritical water hydrolysis. The results showed that subcritical water at a temperature of 200 °C and a reaction time of 30 min was effective in extracting rice proteins and amino acids from defatted rice bran. The protein yield was higher than that obtained by the traditional alkaline hydrolysis method. At the same time, as the temperature increases, the protein extraction rate increases, which is because the solubility of the protein increases at high temperatures. However, the main reason is that as the temperature increases, the ionization constant increases. Under the conditions of the presence of hydrated ions and hydroxyl ions, peptide bonds break to form small molecule soluble proteins and amino acids.

6 Conclusion

Extracting brown rice protein from rice bran is currently the direction of comprehensive utilization of rice. On this basis, there is great potential for developing high value-added brown rice protein powder and active peptides. China is a major rice producing country with abundant rice protein resources. The research and development of brown rice protein is not only beneficial to the comprehensive utilization of deep processing of rice and improving economic efficiency, but also provides more protein ingredients and protein supplements for the food industry and people's nutritional health care, with broad application prospects.

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