What Are the Uses of Rice Protein Powder?

Rice protein (RP) is a general term for proteins derived from rice, and is generally found in rice processing by-products. Compared with wheat and corn proteins, rice proteins are hypoallergenic and easily digested. They are recognized as high-quality dietary proteins with broad application prospects in infant foods and high-end foods [1]. Rice proteins produced by different processing methods differ in composition and functional properties. This article introduces the main sources, functional properties and current processing and utilization status of rice proteins to provide a reference for their subsequent development and utilization.

1. Sources of rice protein powder

The protein content of intact rice is about 8%, mainly composed of albumen (2% to 5%), globulin (2% to 10%), glutenin (66% to 78%) and alcohol-soluble protein (1% to 5%). Among them, the water-insoluble glutenin and alcohol-soluble protein account for more than 80% [2]. Therefore, the content and quality of rice protein produced by different processing methods vary greatly. Rice protein has the highest biological value of all grains (77) [3]. In addition, rice protein is a hypoallergenic vegetable protein. The digestibility, biological value and net protein utilization rate in young children are 88.8%, 90% and 79.9% respectively, making it suitable for use in foods for infants, young children and the elderly [4].

1.1 By-products of rice starch processing

Starch is the main component of rice (80%), followed by protein (8%) [5]. Rice starch is a typical small particle starch (2μm to 8μm) with uniform particles. Gelatinized rice starch absorbs water quickly, has a very smooth texture, is similar to butter, has a fatty mouthfeel, and spreads easily. Therefore, it can be used as a fat analogue in foods [6]. In addition, rice starch has good freeze-thaw stability, which can prevent food from dehydrating and shrinking during the freezing process [7]. At present, the annual output of rice starch in China is about 40,000 tons, and it is mainly used as a thickener, filler, shaping agent and functional factor in the food and pharmaceutical industries [8].

Rice protein is the main by-product of rice starch processing. The main methods of rice starch processing are: the alkali solution method, the enzyme method, and the mechanical method. The alkali solution method uses an alkali solution to remove the protein and some lipids, and then rinses and centrifuges to obtain the starch. The protein in the alkali solution can be recovered by acid precipitation [9-10].

This method causes a lot of environmental pollution and is rarely used nowadays. The enzymatic method is a more environmentally friendly and efficient method. Protease is used to hydrolyze the insoluble proteins in the rice into soluble proteins, and then centrifugation or filtration is used to obtain rice starch and proteins. The disadvantage of the enzymatic method is that the rice starch obtained has a relatively high residual protein and lipid content [11]. The mechanical method is a relatively new method for separating rice starch and protein. This method uses a special high-pressure homogenizer to break up the aggregated starch particles and proteins in the rice and separate them. The physical separation of starch and protein is achieved by exploiting the difference in density between the two. This method retains the original quality of rice starch and protein, and the product quality is better [12].

1.2 By-products of rice starch syrup processing

Rice starch syrup is generally produced using broken rice, which is a residue produced during the rice processing process, accounting for 15% to 20% of the residue [13]. The residue is rich in rice proteins (50% to 70%), lipids (3% to 8%) and ash (2% to 3%), and is the main source of rice protein [14]. The quality of broken rice and the processing technology of the syrup directly affect the protein and lipid content of the residue. The high lipid content makes rice residue extremely susceptible to lipid oxidation and rancidity even after drying, which produces an off-flavor and discoloration that directly affects the functional properties of the protein. After impurity removal, drying and degreasing, the residue can contain more than 80% protein, which can be directly used for food additives or further processing.

At present, the main source of rice protein is the by-product of rice starch and rice starch syrup processing. However, the resulting rice protein is insoluble in water and was previously used mostly as feed. In recent years, due to the increasing market demand for rice protein powder, rice residue has become the main raw material for producing edible rice protein powder.

2. Modification of rice protein

Rice protein is a rigid globular structural protein. The subunits form a dense molecular aggregate through intramolecular and intermolecular disulfide bonds and hydrophobic interactions, and its poor solubility limits its application in the food industry [15]. Therefore, the modification of rice protein by physical, chemical and enzymatic methods to change its spatial structure and physicochemical properties and improve its functional properties is currently a research hotspot.

2.1 Physical modification

Physical modification refers to the use of methods such as heat, electromagnetic fields, mechanical force, high pressure and microwave radiation to improve the functional properties of proteins. Physically modified products have few toxic side effects and are low in cost, but the modification effect is often not significant, and there are relatively few related research reports. Studies have shown that when rice protein is homogenized under pressure of 0–120 MPa, the solubility of the protein increases significantly with increasing pressure (p < 0.01), and the emulsifying activity also improves significantly [16]. In addition, it has been reported in the literature that the solubility of rice protein can be increased to more than 98% by repeated alkali dissolution, freezing, crushing and centrifugation, and the emulsifying and foaming properties are also significantly improved [17].

2.2 Chemical modification

Chemical modification refers to the use of chemical methods to modify groups such as hydroxyl, carboxyl, and amino groups on the side chains of polypeptides or introduce other functional groups to change the molecular structure of proteins and improve their functional properties. At present, the main chemical modifications of rice protein are: deamidation, acylation, glycosylation, phosphorylation, and alkylation.

There are various methods for protein deamidation, among which the acid method is widely used. Jiang Tianyan et al. [18] found that the solubility of rice protein is directly proportional to the degree of deamidation. When the degree of deamidation is 64.5%, the solubility reaches 96.6% and the foaming property is 27%. The emulsifying property of rice protein is better when the degree of deamidation is 19.6% to 35.7%. The solubility of rice protein can also be improved by acylation modification [19]. In addition, the solubility, emulsifying properties and foaming properties of rice protein are significantly improved after glycosylation modification [20]. The above studies show that chemical modification has a significant effect on improving the functional properties of rice protein, but considering nutrition and safety, more consideration needs to be given to the choice of methods, control of conditions and the environment.

2.3 Enzyme modification

Enzyme modification mainly involves the use of enzymes to act on the peptide bonds and side chain groups in protein molecules to change their structure and function. There are many different ways to do this, including enzymatic hydrolysis, proteinoid reactions, deamidation, phosphorylation, etc., the most common of which is protease hydrolysis. Currently, there are many different proteases used for the enzymatic modification of dietary proteins, including those from different sources such as microorganisms, plants and animals. Because different proteases have different enzymatic sites, their product molecular structures are different, and their physicochemical functional properties are also different.

Due to the strong hydrophobicity of rice proteins and the fact that they have been denatured during processing, the effect of complex enzymatic hydrolysis is often better than that of single enzyme hydrolysis during enzymatic modification. Some studies have shown that alkaline protease can increase the nitrogen solubility index (NSI) of rice protein to 95%, emulsifying power to 55%, and foaming power to 70% when hydrolyzed under neutral conditions [21]. Liu Hongfu et al. [22] used Alcalase and Flavourzyme enzymes to hydrolyze rice protein to 10.26%.

The functional properties and nutritional value of rice protein can also be improved by using a protein-like reaction to recombine the original amino acid sequence. Yang Qian et al. [23] used an alkaline protease hydrolysate of rice protein as a raw material, and used pepsin to catalyze a protein-like reaction. The essential amino acid content of the obtained rice protein, such as Thr, Ile, Phe, and Lys, was significantly increased. In addition, enzymatic deamidation of rice protein can also significantly improve its functional properties. Some studies have shown that after treatment with glutamine synthetase, the solubility of rice protein can reach 96.99%, the water retention capacity can be increased by 1.75 to 2.03 times, and the oil retention capacity can be increased by 1.58 to 1.94 times [24].

Compared with chemical modification, the modification conditions of the rice proteinase method are milder, more specific, safe and environmentally friendly, and it is currently the most researched and applied modification method.

2.4 Composite modification

In order to further improve the efficiency of rice protein modification and reduce costs, it is sometimes possible to combine two or more modification methods, or combine with other techniques, such as ultrasound and irradiation. Pan Zheng et al. [25] showed that ultrasonic-assisted alkaline treatment can improve the solubility of rice protein in rice residue (19.99 mg/mL).

Some studies have shown that alkaline protease hydrolysis assisted by electron beam irradiation (EBI) technology can increase the degree of rice protein hydrolysis by up to 19.02%, and the polypeptide yield can reach 13.50% [26]. EBI treatment is beneficial to the stretching of the molecular structure of rice protein, which is conducive to the action of protease.

In short, with the emergence and application of some new technologies, combined with traditional protein modification methods, it is beneficial to obtain modification effects. There is much room for research in this area, such as ultra-high pressure technology [27], extrusion technology [28], pulsed electric fields (PEF) [29], etc.

3 Development and utilization of rice protein

As a dietary protein, rice protein is increasingly being used in baby food due to its high biological value and hypoallergenic properties. In addition, rice protein is also attracting increasing attention for use in food additives and functional foods.

3.1 Food additives

Rice protein itself is not highly soluble, and its associated functional properties such as emulsifying, foaming, gelling, water retention and oil retention are not ideal. However, after moderate hydrolysis, its solubility improves, and its functional properties also improve significantly. Wu Yujing [30] used food-grade rice residue as a raw material, and obtained a protein foaming powder with no odor, high protein content, and good foaming properties after operations such as de-saccharification, neutral protease modification, decolorization, and drying. This protein foaming powder can be used to stabilize and thicken liquid foods and to foam baked goods [31].

3.2 Edible rice protein

Rice bran, a by-product of the production process for rice starch or rice starch syrup, can be used to process edible rice protein powder. Rice bran produced during the processing of starch syrup generally contains 40% to 60% protein, 6% to 12% fat, 3% to 5% minerals, and 15% to 25% total carbohydrates [32]. The quality of rice bran is greatly affected by the quality of the raw rice (or broken rice) and the processing technology of the syrup [33-34]. Fat is the main factor affecting the stability of rice bran. After degreasing, impurity removal, drying and crushing, the protein content in rice bran can reach more than 80%, which can be used as edible rice protein powder.

Studies have shown that rice protein is rich in nutritional value and has significant effects in regulating blood lipids and cholesterol metabolism [35-36].

At present, the international market demand for high-quality rice protein powder is growing rapidly, and there is a large market gap. The market demand for high-end edible rice protein powder is for rice protein content of 80% or more, lipid content of less than 1%, heavy metal content of less than 10mg/kg, and no detectable residues of more than 270 pesticides [37].

Therefore, in the processing of syrup, in order to improve the quality of rice residue protein, it is necessary to improve the original process from multiple aspects such as raw materials, syrup processing, degreasing, and removal of heavy metals. For example, it is necessary to select organic broken rice and low-pollution broken rice as raw materials. Impurity treatment technology for broken rice: remove stones, metal scraps, as well as rice bran and husk. For rice bran with high fat content, degreasing treatment must be carried out, which can effectively reduce the generation of odor and discoloration of rice protein during storage [38]. For rice residues with excessive heavy metals, methods are also needed to remove heavy metal ions such as cadmium and lead [39-40].

3.3 Instant protein powder

The poor water solubility of rice protein, especially rice bran protein, limits its application in beverages and nutritional protein powders. Solubilization modification is required to improve the solubility of rice protein. Rice protein solubilization modification refers to the increase of solubility of rice protein under limited hydrolysis. The main difference between it and rice peptide is that the degree of hydrolysis is kept low so that the protein is not excessively hydrolyzed.

At present, the commonly used solubilization modification methods are glycosylation and enzymatic methods. Lu Qian et al. [41] used pumpkin polysaccharides and dextran to glycosylate and modify rice proteins, respectively, which increased the solubility of rice proteins by 32.27% and 41.75%, respectively. Wang Zhangcun et al. [42] used Alcalase to hydrolyze rice protein, and the solubility, foaming properties and emulsifying properties of the enzymatic products obtained were up to 50.2%, 50mL/g and 73.6mL/g, respectively. However, the degree of hydrolysis of the rice protein was not stated in the paper.

Cui Shasha et al. [43] treated rice protein with alkaline protease and analyzed and compared the physicochemical functional properties of rice protein with a hydrolysis degree of 1% to 5%. It was found that the solubility of rice protein with a hydrolysis degree of 5% was the highest, reaching 65.93%. The above research shows that limited enzymatic hydrolysis can significantly improve the solubility of rice protein. Under the condition of maintaining a relatively low degree of hydrolysis, rice protein still has good functional properties of macromolecules, such as emulsifying, foaming, water absorption and oil absorption [44]. This is the essential difference between the enzymatic modification of rice protein for solubilization and the enzymatic preparation of rice peptides.

3.4 Rice peptides

Rice peptides are small molecule peptides produced by hydrolyzing rice protein molecules using methods such as acid, alkali and protease. Currently, acid and alkali methods are no longer used due to environmental pollution and safety issues, and the enzymatic method is the most common method for processing rice peptides.

The enzyme preparations used in enzymatic processing of peptides are derived from plants, animals and microorganisms, such as papain, bromelain, fig protease, pepsin, trypsin, neutral protease, alkaline protease, acid protease and flavor protease. At present, there are three main raw materials for preparing rice peptides: rice protein, rice bran protein and rice residue protein. Different raw materials have different amino acid compositions and sequences, while different proteases have different enzymatic cleavage sites, which will produce rice peptides with different molecular structures and activities [45].

Rice peptides have a variety of physiological health benefits, such as antioxidant, anti-aging, blood pressure lowering, blood fat lowering, immune regulation and flavor peptides [46-49]. Rice peptides, which are obtained by alkaline protease hydrolysis of rice protein and purified by membrane filtration, have very good scavenging activities for hydroxyl radicals, superoxide anion radicals and DPPH radicals. Adding them to cosmetics has a significant anti-skin aging effect [46].

In order to increase the hydrolysis degree of rice protein, a combination of proteolytic enzymes is a good choice. Cai Jun et al. [47] successively used alkaline protease, neutral protease and trypsin to hydrolyze rice protein, and the hydrolysis degree of rice protein could reach up to 15.9%. The rice peptides obtained showed good in vitro antioxidant activity. Wang Shen et al. [48] used rice as a raw material and a stepwise enzymatic method with alkaline protease and trypsin to obtain a rice peptide with an absolute molecular weight of 549 to 1158, an essential amino acid content of 35.61%, a hydrophobic amino acid content of 45.2%, and the highest content of glutamic acid, which was 16.1%. This rice peptide has strong antioxidant activity and also shows high ACE inhibitory activity, with an IC50 of 0.057 mg/mL.

In addition, rice-derived peptides also have the effect of treating type II diabetes. It is believed that human α-glucosidase and dipeptidyl peptidase IV (EC 3.4.14.5, DPP-IV) play an important role in the development of type II diabetes [49]. DPP- IV is an intracellular enzyme that breaks down a hormone secreted by intestinal cells (GLP-1), whose main function is to lower blood sugar by stimulating insulin, inhibiting glucagon, inhibiting gastric emptying, and regenerating islet cells. Inhibiting or inactivating DPP-IV activity can reduce the breakdown of GLP-1 and thus lower blood sugar.

In recent years, DPP-IV inhibitors have become one of the mainstays of diabetes treatment. Hatanaka T et al. [50] used sake lees, rice and rice bran as raw materials and found that rice protein hydrolysates had inhibitory activity against dipeptidyl peptidase IV (EC3.4.14.5, DPP-IV) (IC50=1.45±0.13 mg/mL). The peptides with DPP-IV activity inhibition that have been identified from rice protein hydrolysates are four dipeptides, the molecular structures of which are: Ile-Pro, Met-Pro, Val-Pro and Leu-Pro, respectively. The concentrations of these dipeptides in rice protein hydrolysates are 1.22 μg/mg, 0 .23 μg/mg, 1.59 μg/mg and 1.94 μg/mg, respectively. Pooja K et al. [51] used computer simulation to predict the molecular structure, sensory and toxicological properties of some DPP-IV active inhibitory peptides derived from rice bran. The results showed that rice bran protease hydrolysate is one of the better sources of DPP-IV active inhibitory peptides.

5 Conclusion 

Rice protein has the characteristics of a balanced amino acid composition, being easily digested and absorbed, and being hypoallergenic. It is a high-quality dietary protein. However, the poor water solubility of rice protein directly limits its application in foods. Therefore, at present, research and utilization of rice protein mostly focuses on its solubilization and modification, with enzymatic solubilization and modification being the main method. In addition, the enzymatic preparation of rice peptides from rice protein, especially the preparation of high-purity active peptides, is the main direction of deep processing of rice protein.

Studies have shown that rice-derived active peptides have various functions such as treating type II diabetes, anti-oxidation, anti-aging, lowering blood pressure, lowering blood lipids, and immune regulation. However, current research on rice peptides requires further research and improvement in the separation and purification, drying, and stabilization techniques of active peptides, especially in the industrial processing equipment for high-purity rice active peptides.

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