Nutritional value and utilization of rice protein

As the largest grain variety in China, the annual output of rice has been more than 180 million t since 2007, accounting for 42% of the total national grain output and about 34% of the global total rice output (Wang, Kai-Li et al., 2012; Shen, Hong-Yuan, 2011). The rice protein produced after the production of sugar from broken rice is a kind of feed protein resource with great development value. In this paper, the composition, nutritional value, preparation method and application of rice protein in livestock and poultry production are summarized to provide reference for the development and utilization of rice protein resources.

 

1 Composition and structure of rice protein

Rice protein is a major source of plant proteins in the human diet, and its content in rice is about 8% (Mar- shall and Wadsworth, 1994). According to the Osborne classification, rice proteins are classified into four categories: water-soluble albumin, globulin dissolved in 0.5 mol/L NaCl, prolamin dissolved in 70%-80% ethanol, and glutelin dissolved in dilute acid or alkali (Chen, J. & Sun, Z. G., 2008); Deng Xiao et al. 2007), each accounting for 2%-5%, 2%-10%, 1%-5% and ≥80% of the total protein. Among them, glutenins and alcohol soluble proteins are storage proteins that are the main components of rice protein, while serum proteins and globulins are physiologically active proteins in rice with lower content.

 

Rice proteins mainly exist in the form of PB-I and PB-II proteasomes. As observed by electron microscopy, PB-I has a lamellar structure, dense particles with a diameter of 0.5-2 μm, stable structural components, and is the site of alcohol soluble proteins; while PB-II has an ellipsoidal shape, is not layered, has a uniform texture, and has a diameter of about 4 μm, with an inconspicuous peripheral membrane, and unstable fractions of gluten and globulin in it (Collier et al., 1998). The contents of the four types of proteins are shown in Table 1, from which it can be seen that the relatively high content of glutenin in rice reflects, to some extent, the instability of the protein structure of rice (Li et al., 2006).

Table 1 Main components of selected cereal proteins %

2 Nutritional value of rice protein

2.1 Low antigenicity Proteins are the material basis of life activities and play an important role in regulating the physiology and metabolism of the animal body, and their sources include plant proteins and animal proteins (Ye Jingjing et al., 2011). Low antigenic activity is a prerequisite for the efficient utilization of protein nutrition in the animal body. It is well known that many plant proteins contain antinutritional factors, such as trypsin inhibitory factor (TIF), flatulence factor (FF) in soybeans and lectins in peanuts, which greatly limit their use in production (Yin et al., 2012; Jiang et al., 2012). However, relevant studies have shown that rice protein is highly nutritious, gentle and non-irritating, with low antigenic activity, and does not produce allergic reactions (Guo Yan and Zeng Li, 2010; Hettiarachchy and Rath, 2001), which makes it a plant protein resource with great potential for development. In addition, some animal protein raw materials contain antinutritional factors, such as lactoglobulin and some ovalbumin, which may produce allergic or toxic reactions in the animal body, but rice protein does not contain similar allergenic factors, and its application is safe and reliable (Wang Zhangcun et al., 2004).

2.2 Amino acid composition balance    

The protein content of rice protein is as high as 40% to 70%, with high absorption and utilization efficiency in animals, biological efficiency as high as 77, and nutritional value is much higher than that of plant proteins such as corn and wheat (Li Ming et al., 2006). The amino acid composition of rice protein is balanced and rich in essential amino acids, which is very close to the ideal model recommended by WHO/FAO (1973). Analysis of its composition revealed that rice protein is high in lysine and contains more than 80% of alkaline soluble gluten (Cuiping Yi and Huiyuan Yao, 2003). Table 2 lists the amino acid composition of rice, wheat and maize proteins and the optimal amino acid pattern of proteins recommended by WHO (Yan Guo and Li Zeng, 2010). Table 2 shows that rice protein contains slightly lower levels of lysine, cystine, and threonine than ideal protein, slightly lower levels of leucine and threonine than maize protein, and higher levels of all essential amino acids than wheat protein, making it a protein with good amino acid nutritional value.

 

Table 2 Amino acid composition of rice, wheat and maize proteins in relation to the WHO recommended optimal amino acid pattern for proteins (%).

 

In addition, combining with Table 3, it can be seen that under the same conditions of feed crude protein level, compared with the standard requirement, except lysine is slightly lower, the other three amino acids can satisfy the growth needs of pigs at all stages; compared with soybean meal, lysine and threonine are lower, while methionine + cystine and isoleucine are higher, so it can be seen that rice protein is a more ideal protein feed for pigs.

 

 3 Preparation of rice protein

The purpose of rice protein extraction is to obtain high-purity rice protein products, which are generally categorized into rice protein concentrate (RPC, with a protein content of 50-89%) and rice protein isolate (RPI, with a protein content of 90% or more). Broken rice, rice dregs, rice bran, etc. can be used as raw materials for the preparation of rice protein. At present, the main methods of rice protein extraction in China include alkali method, enzyme method and combined extraction method (Table 4). From Table 4, it can be seen that the extraction rate of rice protein varies greatly among different extraction methods, among which the extraction rate of compound extraction method is higher, that of enzyme extraction method is relatively lower, and the difference in the extraction rate of other methods fluctuates greatly.

 

4 Application of rice protein in livestock and poultry production

As a kind of plant protein, rice protein is rich in various amino acids and has a balanced composition, similar to Peruvian fishmeal. Nutrient analysis found that the crude protein content of rice protein is ≥60%, crude fat accounts for 8% to 9.5%, digestible protein is 56%, and lysine content is extremely rich, ranking first among cereal foods. In addition, rice protein contains a variety of trace elements, biologically active substances and microbial enzymes, making it physiologically regulating (Luan Hui, 2011; Yang Lin et al., 2010). Relevant studies have shown that adding rice protein products to feed can improve the growth performance and immunity of livestock and poultry, and improve the environment of livestock and poultry houses, which is a protein feed resource with broad application prospects.

Table 4 Extraction methods of rice protein

 

 

4.1 Application in pig production    

Wu Xin et al. (2008) used rice protein powder to replace whey protein concentrate in the diets of weaned piglets, and the results showed that the differences in ADFI, ADG and F/G of weaned piglets were not significant (P ＞ 0.05) among the experimental groups, and there was a tendency to increase the ADG and decrease the F/G of the piglets in the experimental groups during the period of 8-14 d (P ＞ 0.05); the blood tests showed that there was no significant difference (P ＞ 0.05) between the groups in terms of red blood cells, white blood cells and platelets, Blood test showed that the differences of red blood cells, white blood cells and platelets in the blood of piglets in each group were not significant (P＞0.05); organ index test also showed that the differences of immune function in each group were not significant (P＞0.05), which indicated that rice protein powder could be used to replace a certain proportion of whey protein powder for early weaned piglets.

 

Liu Weidong et al. (2011) showed that when 35-day-old piglets were fed diets containing 1.5%-2% rice peptides, the average daily weight gain, energy metabolism rate, protein metabolism rate, and feed efficiency of the pigs were significantly increased (P＜0.05), the total protein, blood glucose, and alanine transferase content in the blood were all increased to a certain extent (P＞0.05), and the rate of diarrhea, urea nitrogen in the blood, and the concentration of lactate dehydrogenase were all significantly reduced (P＜0.05). Diarrhea rate, blood urea nitrogen and lactate dehydrogenase concentration were all significantly reduced (P < 0.05). In a study of the digestibility of lysine transgenic rice in the ileum of Wuzhishan miniature pigs, Hu I-Quan et al. (2010) found that the apparent digestibility and true digestibility of protein of lysine transgenic rice were not significantly different from those of the parental rice (P>0.05) and the digestive effect was good.

 

4.2 Application in chicken production Liu Weidong et al. (2010) found that the addition of 1.5% to 2% of rice peptide to the diet of Goodyear chickens significantly increased their productivity and body protein deposition (P < 0.05), and the serum levels of growth hormone (GH), triiodothyronine (T3), and insulin-like growth factor-Ⅰ increased significantly (P < 0.05), and the value of triiodothyronine (T3)/tetraiodothyronine (T4) increased significantly (P < 0.05). The serum levels of growth hormone (GH), triiodothyronine (T3) and insulin-like growth factor-I were significantly increased (P < 0.05), and the values of triiodothyronine (T3)/tetraiodothyronine (T4) were significantly increased (P < 0.05).

 

Liu Weidong et al. (2012b) showed that under heat stress conditions, the addition of 1.5% to 2% rice protein peptide significantly increased the egg production rate, feed intake, total protein, total lipid and thyroxine concentration in blood (P < 0.05), significantly reduced the feed-to-egg ratio, blood glucose, aldosterone and cortisol in laying hens (P < 0.05), and the mortality rate was reduced to a certain extent. In addition, the mortality rate of chickens was reduced to a certain extent. Liu Weidong et al. (2012a) found that the addition of 1.5%-2% of rice protein peptide to the diet significantly reduced the concentration of ammonia and hydrogen sulfide in the laying hen house (P < 0.05), and significantly increased the egg production rate and feed efficiency (P < 0.05).

 

5 Summary

In summary, rice protein has a reasonable amino acid composition, high bioefficiency, hypoallergenic and high nutritional value. China is a large rice-producing country with abundant rice resources, which provides a basis for the development and utilization of rice protein. As a plant protein resource with great development value, rice protein provides a feasible way to solve the problem of insufficient feed protein resources in China.

 

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