What Is the Nutrition Fact and Use of Black Rice?

As of 2003, specialty rice resources such as rice with colored seed coats (including purple rice, black rice, fragrant rice, and red rice) accounted for one tenth of the documented rice varieties in the variety resource bank [1]. Studies have shown [2] that the comprehensive nutritional value of specialty rice is superior to that of ordinary rice. Therefore, driven by the consumer concept of pursuing health and wellness, the development of specialty rice products is receiving increasing attention, and the value of black rice is also being increasingly valued. This is because black rice has very high nutritional value, and more and more people are now consuming it, and research on its cultivation, sales and further processing is also increasing.

Purple black rice [3] (mainly including black and purple seed coats) is also considered a special type of rice because the rice bran is rich in anthocyanins, mainly anthocyanins and nutrients, and therefore appears blackish brown, black, or purple black in color. Studies have shown that the anthocyanin pigments in black rice have a variety of medicinal values and effects, so black rice is also regarded as a special type of rice, and is known as “immortal rice”, “medicine rice” and “blood-tonifying rice”. [4] In China, black rice has a long history of cultivation. According to the “Qimin Yao Shu” (Essentials of the People's Livelihood), it has been cultivated for 1,500 years. [5] Black rice is also divided into indica, glutinous, and japonica varieties, which are further divided into indica-glutinous, indica-sticky, japonica-glutinous, and japonica-sticky types, accounting for more than 98% of the total variety resources. [6, 7] Black rice is a precious specialty rice resource with a complete range of colors, flavors, aromas and rich nutritional value. It is also known as one of the two rice that can be used for both food and medicine. This paper focuses on the nutritional value of black rice, its application in food, and the characteristics of black rice anthocyanins and black rice polysaccharides. It aims to provide a theoretical basis and reference for the study of black rice.

1 Nutritional value of black rice

The nutritional value of black rice is superior to that of ordinary white rice, both in terms of variety and content. The content of basic nutrients (protein, fat, carbohydrates, vitamins, etc.) in black rice is mostly superior to that of ordinary white rice. In addition to basic nutrients, black rice also contains special nutrients (essential amino acids, minerals such as calcium, iron, and zinc) in higher quantities than ordinary rice. Some of these special substances, such as anthocyanins, are bioactive substances not found in ordinary white rice.

1.1 Basic nutritional composition

A comparison of the basic nutritional composition of black rice and white rice is shown in Table 1. The basic nutritional composition of rice mainly includes starch, protein, fat and unsaturated fatty acids. From the perspective of basic nutritional composition, the starch content is not much different from that of white rice, while the average content of the other three is better than that of white rice. Among them, the minimum and are better than white rice, at 1.25 and 1.51 times, 2.96 and 1.56 times, and 3.45 and 1.80 times the levels of white rice, respectively. The fat content of black rice is lowest (2.37%), which is close to the highest fat content of white rice (2.50%), and the content of unsaturated fatty acids shows the same trend.

1.2 Special nutrients

The special nutrients mentioned here mainly refer to amino acids, vitamins, minerals, anthocyanins and other active substances. A comparison of the cultivated “black glutinous 178” with ordinary rice found that the protein, fat and crude fiber were 49.69%, 2.1 times and 4.9 times that of ordinary rice, respectively. The mineral elements P, K, Na, Mg, Fe and Mn are 2.3 times, 3.9 times, 3.2 times, 3.75 times, 1.4 times and 3.3 times those of ordinary rice, respectively. [9] Xiong Yanzhen et al. [10] made a thorough comparison of the special nutrients in black rice and found the following:

1.2.1 Minerals

The average mineral content of newly created special rice germplasm [11] is shown in Table 2. The mineral content (except magnesium) of colored rice (mainly red rice, black rice, etc.) is higher than that of ordinary white rice. Among the same type of colored rice, the mineral content is basically Mg>Ca>Se>Zn>Mn>Fe>Cu. Among them, Black rice is also a selenium-rich product, with 2.07 times the selenium content of ordinary white rice.

1.2.2 Essential amino acids

The comparison of the essential amino acid content of black rice with that of white rice and other grains [12] is shown in Table 3. The average essential amino acid content of black rice is generally higher than that of white rice. Whether it is compared with the maximum value or the minimum value, the essential amino acid content of black rice is at least more than twice that of white rice, except for threonine. in descending order of the multiple of the degree of higher than white rice, they are tryptophan (1.5 times), methionine (1.36 times), valine (1.30 times), lysine (1.28 times), phenylalanine (1.18 times), leucine (1.15 times), and isoleucine (1.1 25 times). Table 3 also shows that black rice is not only high in essential amino acids, but also contains a wide variety of them. Among them, methionine, valine, tryptophan and threonine are particularly notable. Therefore, black rice is a top choice for people who urgently need to supplement amino acids.

1.2.3 Bioactive ingredients

Bioactive substances are a class of substances with anti-inflammatory, anticancer and antioxidant functions. Dietary fiber, fructooligosaccharides, phytosterols, carotenoids and flavonoids have been identified as the main functional factors.

Black rice contains the main source of vitamin A, also known as carotene, which is not detected in white rice [13, 14]. As we all know, vitamin A is beneficial for the treatment of night blindness, dry eye disease, epithelial tissue keratosis, and tooth tissue deficiencies. It is mainly involved in the synthesis of rhodopsin in the body, maintains dark vision in the eyes, maintains the integrity of the structure and function of epithelial tissue, and promotes growth and development.

In addition to vitamin A, functional factors such as vitamin B1, vitamin B2 and vitamin C have also been detected in black rice, and the content of vitamin B1 and vitamin B2 is 1.91 times and 2.79 times that of white rice, respectively. Vitamin C (L-ascorbic acid) can be detected in black rice at a content of 23.93 mg/kg, but not in white rice. [15] Among them, vitamin B1 and vitamin B2 deficiencies can cause a variety of human discomforts, such as gastrointestinal dysfunction, beriberi, fatigue, weakness, angular cheilitis and other diseases, while vitamin C deficiency can easily cause scurvy. [16]

Black rice not only contains flavonoids, phenols, and dietary fiber, but also anthocyanins, which have not been detected in white rice. The anthocyanin content of black rice is as high as 0.65–14.98 mg/kg, the flavonoid content is 0.10%–1.70%, while white rice only contains 0.02%–0.32%, which is more than five times that of white rice; the phenol content of black rice is 2.31~4.84 g/1000g, which is also much higher than that of white rice; the dietary fiber content of black rice is 0.88%~5.00%. [17] Studies have shown that anthocyanins are the main source of the health benefits of black rice [18]. The colors exhibited by black rice, including red, dark red, blackish purple, and black, are mainly due to the presence of anthocyanins. Anthocyanins are a type of bioactive substance formed by the combination of an anthocyanin parent compound with one or more sugar groups via a glycosidic bond. The presence of anthocyanins is one of the characteristics of natural anthocyanins. [19] There are six main types of anthocyanins that are important in foods, which are pelargonidin, cyanin, delphinidin, peonidin, pentunidin and malvidin. Their colors vary with the substituents at the 3rd, 5th and 7th carbon positions of the B ring. [20]

2 Research progress on the characteristics of some functional components of black rice

2.1 Anthocyanins in black rice

Black rice contains special ingredients such as carotenoids, ascorbic acid, and anthocyanins. However, anthocyanin research is the most extensive, so this section focuses on summarizing the research progress of anthocyanins in black rice. In 2014, Hao Jie [21] conducted a study on the effect of black rice anthocyanins on the liver and kidneys of the body. The results showed that black rice anthocyanin extract has a protective effect on the liver and kidneys. Black rice anthocyanins have physiological functions such as antioxidant activity, blood lipid regulation and anti-inflammation. Among them, there have been many studies on the antioxidant activity of black rice. This section focuses on the research on the antioxidant properties of black rice anthocyanins. An overview of the research on the antioxidant properties of black rice anthocyanins in recent years is shown in Table 4. Studies have shown that different extraction methods and different operating conditions can affect the final antioxidant activity of the extracted black rice anthocyanins. Compared with the antioxidant activity of some substances, black rice exhibits strong antioxidant properties.

As can be seen from Table 5, the extraction of black rice pigments is affected by many factors, the more critical of which are the extraction time, temperature, liquid-to-material ratio, solvent, and pH value. Before 2010, the main extraction method was maceration, using ethanol, distilled water, hydrochloric acid or acetic acid as the solvent. Ethanol extraction technology is relatively mature and has a high yield. Other methods, such as the acid method, have a low yield and take a long time. Therefore, in order to increase the yield, auxiliary techniques such as mixing, microwaving, freezing, heating, mechanical action, etc., to break cell walls and cell membranes, thereby maximizing the release of anthocyanins in the vacuoles, and improving permeability to shorten the extraction time. Finally, the pigment yield is increased and the quality of the product is improved. These auxiliary methods are often used in combination during the application process, which can increase the pigment yield. However, issues such as the conditions for using auxiliary techniques and energy consumption require further research. [42] From 2010 onwards, with the development of science and technology, extraction technology has gradually matured and developed rapidly. Supercritical extraction, ultrasonic-assisted extraction, enzymatic-assisted extraction and other methods have been successively used in the extraction and use of black rice anthocyanins. Supercritical extraction has many advantages such as high efficiency, being green, safe and non-polluting, and is widely used in extraction operations. Supercritical extraction has the advantages of low processing temperatures, making it particularly suitable for heat-sensitive substances. However, due to its high cost, this technology is still a long way from being used on a large scale or in an industrial setting in the food industry.

2.2 Black rice polysaccharides

We all know that after black rice is cooked, it takes a long time to cook and the texture is not good because of its low stickiness. This is mainly caused by the characteristics of starch polysaccharides. The higher the amylose content, the lower the viscosity of the starch paste. Therefore, studying the characteristics of black rice polysaccharides is of great significance for improving the performance of black rice. The polysaccharides in black rice mainly refer to starch. Nowadays, the extraction and purification of anthocyanins in black rice and their in vivo antioxidant activities are relatively well understood, but there are few reports on the extraction of polysaccharides in black rice. [43] At present, polysaccharide extraction mainly uses chemical methods such as hot water extraction, alkali extraction, and acid extraction. [44] With the development of biotechnology, the sources of enzymes are becoming more and more extensive. [45] Wu Yilian et al. [46] used cellulase for enzymatic extraction, showing that under suitable conditions, black rice polysaccharides can be extracted. A single-factor test showed that the effect of the conditions on the extraction results was in the order of enzymatic hydrolysis time > pH > enzymatic hydrolysis temperature. This is a new technique for extracting black rice polysaccharides in recent years, but judging from the results, the yield is low. Therefore, further research is needed on the extraction of black rice polysaccharides, and a method with a higher yield needs to be explored in order to make full use of the characteristics of black rice and increase added value.

There has also been relatively little research on the properties of black rice polysaccharides. In recent years, Xiao Xin et al. (2017) [47], Zhang Jie (2019) [48] and Li Xuehong et al. (2015) [49] have mainly conducted research on the physicochemical properties of black rice starch after extraction from different perspectives. The three studies have certain commonalities, and the results all indicate that black rice starch has its own unique properties. From the research results of Li Xuehong et al. [50], it can be seen that the three types of black rice compared have the following properties: black japonica rice > black indica rice > black glutinous rice in terms of amylose content; black japonica rice < black indica rice < black glutinous rice in terms of viscosity.

At the same time, it was found that black glutinous rice has relatively high light transmittance and is less prone to aging. The density of cross-linking points and the density of cross-linking points and the number of effective cross-links are positively correlated with the elasticity of the starch gel [51]. Xiao Xin et al. [52] showed that under certain conditions, black rice starch has a better texture but poorer viscosity. Its gel hardness and chewiness are superior to those of japonica rice and glutinous rice starches. The starch particles are closely packed in an angular or diamond shape. The gelation and sedimentation stability of black rice starch is poorer than that of rice starch, and it exhibits characteristics of easy aging. Zhang Jie [53] showed that the amylose of black rice starch has the same particle structure under the microscope; its transparency is worse than that of the other two starches, and the solubility and swelling power of black rice starch are higher than that of japonica rice starch at higher temperatures.

Functional research on black rice starch is currently in its infancy. However, based on the results of research into its physical and chemical properties, it can be inferred that it has unique properties and that its functional properties are also different from those of other rice starches, although further research is required.

3 Comprehensive application of black rice

Black rice is rich in nutrients and contains vitamins A and C, anthocyanins, also known as carotenoids, ascorbic acid and anthocyanins, which are not found in white rice. Therefore, the comprehensive research and application of black rice and its active substances in food processing is of great significance.

3.1 Development and application in food processing

3.1.1 Traditional application: porridge

Although rice is used as a staple food in the home, black rice has a gel consistency of only 54 mm [54] and is relatively low in viscosity, which results in a relatively rough texture. To increase the sense of viscosity, it is necessary to add lard, baking soda, etc. during re-cooking. Not only that, but it also takes 3 to 4 hours to cook thoroughly. Therefore, it is rarely used as a staple food in home food alone. In order to address this weakness, many agricultural science and technology workers have carried out experiments to improve black rice varieties by increasing the glutinous components. At present, there are many new black rice varieties with better glutinousness. The average content of amylose can be increased by about 1.4%, and the gel consistency has increased to more than 100 mm, making it a nutrient-rich product suitable for consumption by young and old alike. The texture has been greatly improved.

3.1.2 Processed food

The application of black rice products in processing is shown in Table 6.

Although black glutinous rice improves the taste to a certain extent, it still has some defects in terms of its soluble and cooking qualities due to its dense skin texture. Therefore, processed products [58] are mostly used as auxiliary materials, such as black rice noodles, rice flour, rice wine, toast bread, rice vinegar, rice cakes, rice crackers and other products. In recent years, fermented foods have become a research hotspot because they contain beneficial bacteria such as probiotics, which are more easily digested and absorbed by the body. Representative black rice products include black rice fermented foods, black rice fermented beverages, black rice fermented milk, and black rice fermented alcoholic beverages. The development of black rice fermented foods mainly focuses on black rice buns, bread, pancakes, black rice cakes, black rice sauce, and black rice dietary fiber biscuits.

With the development of the food industry, there are also some new functional foods that are constantly being developed. In addition to solid and liquid drinks, the research of semi-solid gelatinous foods is also the direction of deep processing of black rice. For example, Lei Jing et al. [59] studied black rice fiber jelly, but in the research process of black rice jelly, the conventional traditional jelly process was also used, and additives such as carrageenan were added as a gelling aid. Currently, there is little research on the use of the gel properties of black rice polysaccharides themselves to directly produce semi-solid leisure black rice products. There is also the use of special black rice ingredients as functional health ingredients extracted and added to certain drinks, such as the development of anthocyanin-rich rice vinegar-based flavorable jelly [60]. Currently, there is relatively little research on the separate extraction of black rice anthocyanin ingredients for direct consumption in drinks, and further development and utilization is needed.

3.2 Development and application in the chemical industry

Since the black rice epidermis is rich in black rice pigments, etc., accounting for one-tenth of the weight of the rice grain, it is highly soluble in water and alcohol, and can also be mixed with ice cream and lard to form a stable system with even distribution. Therefore, these pigments can be extracted for use in the chemical industry to make dyes or colorants. Major studies include those of Xu Jing et al. [61], Jia Yanmei et al. [62], and Yu Xuezhi et al. [63]. The former used 50% ethanol to extract the pigment from black rice. The extraction conditions were as follows: the black rice was soaked in an ethanol solution at a temperature of 80°C for 30 minutes. The extracted black rice pigment was used for post-media dyeing, and the optimal dyeing process was 1% mordant concentration. The latter two both use distilled water extraction, and the black rice pigment is extracted and used to dye silk materials.

3.3 Development and application in medicine

Black rice is rich in bioactive ingredients such as flavonoids, anthocyanins and natural pigments. These bioactive substances can be refined through certain advanced methods to obtain ingredients with high purity to exert their physiological functions and exert the unique therapeutic effect of black rice.

In terms of medicinal value, anthocyanins in black rice are the first to be mentioned. Ling Wenhua et al. [64] used an alcohol solution extraction method to extract black rice anthocyanins, prepare them into capsules, and conduct adjuvant therapeutic interventions on 60 hyperlipidemia subjects. After a period of time, they were able to lower blood lipid levels and improve the inflammatory response of abnormal blood lipid factors.

Chen Lu [65] took the dietary supplement of black rice anthocyanins as an opportunity to develop tablet preparations, and studied the stability, process parameters, and process conditions of the preparation process, with the aim of developing black rice anthocyanin preparations that are nutrient-rich and have obvious therapeutic effects and a certain shelf life. Hou Fangli et al. [66] analyzed and demonstrated that black rice anthocyanins can regulate the body's lipid metabolism level when administered to test animals. Adding black rice to the feed of animals (mice, piglets, etc.) can lower blood lipid levels, and it has been shown that the mechanism of regulation is different from that of chemical drugs, with fewer side effects. Liu Wenyan [67] used black rice anthocyanins as raw material and for the first time used dry granulation to prepare black rice anthocyanin film-coated tablets for dietary therapy, which have a certain effect on improving some bodily functions. Li Wen et al. [68] used black rice anthocyanin to produce black rice anthocyanin compound soft capsules for dietary supplement use. Chomean et al. [69] used natural dye black glutinous rice extract to stain sperm cell nuclei instead of hematoxylin. The staining effect was consistent with hematoxylin, positively correlated, and safe with no side effects and affordable.

4 Prospects for black rice processing

Black rice contains not only the nutrients found in ordinary rice, but also special nutrients and bioactive substances. Its unique health benefits are gradually being recognised, and this has opened up broad market prospects and considerable profit potential for its comprehensive utilisation. However, the development and utilisation of black rice's functional ingredients, which have high added value, is mainly in the pharmaceutical field, and there is less in the food field.

In the field of black rice processing, most products are currently concentrated in the food sector, with black rice as the main ingredient or auxiliary ingredient being used in the food sector for product development and production with low added value. There is little research on the development of high value-added black rice products that use black rice as the main ingredient and make full use of its functional ingredients as auxiliary materials for further processing in the food industry. If leading enterprises can be cultivated and introduced to lead the industry with their technology [70] and fully develop new high value-added black rice foods, this will drive the development of the entire black rice industry chain and further expand the black rice market and increase the comprehensive utilization value of black rice. At the same time, although it is not difficult to find that there have been a lot of studies on the extraction and functional properties of anthocyanins, their application in the development of the food industry is still rare. Therefore, making full use of the functional properties of black rice anthocyanins as a further development and research of snack food additives is a new direction. The development and utilization of this new type of food should have broad application prospects.

In terms of research into the properties of black rice, most studies of its functional properties have focused on the functional properties of anthocyanins and flavonoids. There has been less research into the extraction and physical and chemical properties of black rice polysaccharides, and further research is needed to provide guidance for processing and utilization. The structure of the polysaccharides is precisely the source of its poor taste, and its unique structure and the ratio of branched chains and amylopectin are among the main factors affecting its taste. If we want to improve people's daily diet and increase nutrient intake, it is of great significance to conduct more in-depth research on black rice starch polysaccharides in order to meet people's pursuit of health and nutrition, improve its commercial value, and broaden its scope of use to meet the needs of consumers.

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