How to Extract Natural Red Coloring?

In recent years, the Sudan Red incident in the food industry has been exposed frequently, making people particularly wary of the safety of food coloring, and they even avoid the color red. Therefore, although various synthetic red dyes are bright in color, have strong coloring power, are highly stable, and are relatively stable in nature, they are gradually disappearing from widespread use as people's pursuit of “green food” increases. On the contrary, although Natural Red Coloring has poor color, it is highly safe, and some also have certain nutritional value or pharmacological effects, and is gradually being valued in the development of the modern food industry [1].

In fact, research on natural red coloring began relatively early abroad, but started relatively late in China, and there have been relatively few reports. To date, natural pigment materials that have been studied include capsanthin, lycopene, gardenia red pigment, rose red pigment, radish red pigment, scarlet red pigment, monascus red pigment[2], grape red pigment, and tea red pigment, among others. At the same time, the development of new varieties of natural red coloring and the improvement of the extraction process for Natural Red Coloring have become very urgent issues in the coloring industry.

1 Recent developments in Natural Red Coloring research

1.1 Capsanthin

At present, the research on capsanthin at home and abroad is relatively mature. Capsanthin is a carotenoid pigment that is fat-soluble (the structural formula is shown in Figure 1). It is a dark red viscous oily liquid with the aroma of chili peppers, soluble in most non-volatile oils, partially soluble in organic solvents such as ethanol, acetone, hexane, grease, and vegetable oil, insoluble in water and glycerin, stable to visible light, but easily discolored under ultraviolet light [3].

Paprika red is a high-quality natural pigment with bright color, light, heat, acid and alkali resistance, oxidation resistance, dyeing function and nutritional and health-care function. It has no toxic side effects and is currently the best and most widely sold red pigment internationally. It is mainly used in food, beverages, feed, health care products and cosmetics[3].

The main producing areas of paprika red are Spain, Central Europe, the United States, Asia (India) and Africa. It is usually made from paprika using physical methods of extraction and refinement. The main components of red pepper and the content of paprika red vary with the place of origin, and the corresponding paprika red extraction process also varies accordingly. The main methods of extracting paprika red pigment are the edible oil solvent method (oil-soluble method), the organic solvent (such as petroleum ether) method, the supercritical CO2 fluid extraction method, and the ultrasonic solvent extraction method.

The oil-solvent method involves immersing the red pepper peel or dried chili powder in edible oil (soybean oil, cottonseed oil, rapeseed oil, etc.) at room temperature to dissolve the capsanthin in the edible oil, and then extracting the capsanthin from the oil through a certain process. The disadvantages of this method are a low extraction rate, difficulty separating the oil from the pigment, and difficulty obtaining a product with a high color value.

Supercritical CO2 fluid extraction has a good separation effect, a high yield (68.3%, higher than the values reported in the literature for other extraction methods), and the advantage of a high chroma value, making it more suitable for the extraction of paprika red. Therefore, research is currently active both domestically and abroad. In addition, foreign researchers such as Cserhati et al. [4-5] have also reported the use of a high-fluid sleeve color plate-diode array method to separate and purify red paprika pigments, with a yield of 80.67% to 97.21%.

At present, paprika red is the world's best-selling natural pigment, and is the most popular product, in short supply. The United States needs 4,000 tons of natural pigments a year, of which paprika red is about 1,000 tons; Japan needs about 500 tons of paprika red a year; Canada, Australia, Singapore, and Western Europe also have a large demand [3]; therefore, it is worth mass production.

At the same time, there are currently not many products developed in China for the deep processing and finishing of chili peppers, and the added value of the products is not high, resulting in underutilization of resources and even a certain degree of waste. If the fat-soluble pigments can be converted into water-soluble pigments, and if chemical processing and emulsification technologies can be further developed, and the techniques for removing the pungency and the refining techniques for separating the pigment monomers can be improved at the same time, it will be beneficial to improving the technological content of deep-processed chili pepper products and enhancing the competitiveness of the products in domestic and foreign markets, producing significant economic and social benefits.

1.2 Lycopene

Lycopene is a unique carotenoid without a cyclic structure (its structural formula is shown in Figure 2). It is a dark red crystal with a melting point of 174°C, insoluble in water, slightly soluble in methanol and ethanol, soluble in ether, petroleum ether, hexane, acetone, and easily soluble in organic solvents such as chloroform, carbon disulfide, and benzene. It is easily oxidized and decomposed when exposed to light, oxygen, acids, alkalis and active agents, and the decomposition is accelerated by higher temperatures. Lycopene is a red powder or red liquid, and its thinner oily liquid is yellowish orange [6].

Lycopene has strong antioxidant activity and various physiological activities. Its singlet oxygen activity is twice that of beta-carotene. It also has the effect of inhibiting cell proliferation and diffusion, and has a certain preventive and inhibitory effect on various cancers. It can also effectively prevent the oxidation of low-density lipoprotein cholesterol, thereby reducing atherosclerosis and coronary heart disease [7].

Lycopene is also a food coloring with a wide range of colors. It has been approved as a yellow/red food coloring in Europe and Japan, and is therefore a health-promoting food coloring that is increasingly being used in the fields of nutritional fortification and food coloring.

Research on lycopene preparation methods and extraction processes is active both domestically and abroad. Currently, there are five main methods: extraction, high pressure, enzymatic reaction, supercritical extraction and chemical synthesis. The most common method is extraction, which uses the solubility of lycopene in different solvents to extract lycopene with an lipophilic organic solvent. Although research into the high-pressure method in the food industry is still in its infancy, high-pressure technology has unparalleled advantages in terms of sterilization and preservation and the crushing of microbial cells. Some studies have shown that a pressure of 200 MPa and a pressurization time of 15 min are suitable for lycopene extraction; increasing the number of pressurization cycles can greatly improve the extraction efficiency [8].

In Japan, a patent describes a method for extracting lycopene by using the tomato peel's own enzymatic reaction. Under slightly alkaline conditions (pH = 7.5 to 9), the pectinase and cellulase in the tomato peel are allowed to react, decomposing pectin and cellulose, causing the lycopene protein complex to dissolve from the cells. The resulting pigment is the water-dispersible lycopene [8].

However, the supercritical extraction method [9-10] is more suitable for the extraction of lycopene. Because the supercritical extraction method has many advantages, such as simple process, low energy consumption, cheap, non-toxic and easily recyclable extraction agent, and can be processed at low temperature, it is suitable for the extraction of heat-sensitive ingredients such as lycopene. Therefore, in recent years, there has been a trend to use the supercritical extraction method instead of the conventional extraction method.

Lycopene can also be obtained by chemical synthesis. The main methods can be divided into two types according to the basic chemical reactions involved: the Wittig synthesis and the aldehyde-sulfone synthesis. They can also be divided according to the number of carbon atoms bonded: 2C15 + C10, C20 + C20, 2C10 + C20, etc. At present, the main manufacturers of lycopene produced by synthetic processes are Roche and Bash [6].

In short, there are many ways to obtain lycopene. Given that lycopene also has nutritional value, it is necessary to continue to strengthen research in this area in order to reduce costs and improve efficiency.

1.3 Mulberry red pigment

Mulberry red pigment is extracted from mulberries (its structural formula is shown in Figure 3). It is a kind of anthocyanin pigment, which is a safe, non-toxic natural food coloring that is widely found in plants.

Mulberry red pigment is easily soluble in water, ethanol and methanol, but insoluble in acetone and petroleum ether. It is a water-soluble pigment and a polar pigment.

Mulberry red pigment is sensitive to acids, alkalis, light, oxidants, reducing agents, food additives, and metal ions. This is closely related to its molecular structure, but it has good heat resistance. For example, mulberry red pigment is sensitive to SO2, which can cause it to fade [11].

Studies on mulberry red pigment extract have also found that the extract contains 17 amino acids, 7 of which are essential amino acids, and a variety of trace elements, such as iron, zinc, copper, chromium, molybdenum, calcium, nickel, potassium, magnesium, and phosphorus, especially zinc. Mulberry red pigment is therefore rich in nutritional value [11]. China included mulberry red pigment in the national hygiene standard for the use of food additives (GB 2760-1996) in 1989.

Currently, the main methods for extracting mulberry red pigment are the maceration method and the resin method.

The main process of the extraction method is as follows: weigh fresh, ripe, purple-black mulberries, crush, add an extraction agent [(95% ethanol: 0.1% HCl = 1:1): material = 10:1], mix well, extract at 75°C for 2 h, cool, filter, vacuum-dry the filtrate, and the finished mulberry red pigment is obtained.

The extraction method, whether the extracting agent is ethanol or a combination of ethanol and hydrochloric acid, produces a pigment with a high content of impurities (sugars, proteins, etc.) after concentration. The color value is low (generally less than 4.0), and it is often in the form of an extract, which is difficult to make into a powder. The shelf life is short [11].

In the resin method, the upper layer of the supernatant after centrifugation is adsorbed by a macroporous resin, and then eluted with acidic ethanol. After concentration and drying, the pigment powder is obtained. Pigment powder can be easily prepared by purification with resin, and the color value is significantly improved, usually exceeding 100, which is 25 to 30 times higher than before purification. The pigment impurities after purification are also greatly reduced, and the properties are more stable. Therefore, the resin method is a good choice.

1.4 Red balsam

Red balsam is the flower of Ceris chinensis (Bge), a legume plant. Red balsam is a type of anthocyanin compound with good acid and alkali resistance. It is stable in acidic solutions and appears bright red. It is also thermally stable and has strong light resistance. It is very low in toxicity and is almost non-toxic.

The main extraction and purification methods for red balsam of the redbuds are solvent extraction, microwave-surfactant synergy, supercritical CO2 extraction, and membrane separation. For example, in the solvent extraction method, an appropriate amount of redbud sample is taken and 95% food-grade ethanol is added. It is appropriate to immerse the sample, and after 40 h, a dark red pigment extract can be obtained. The evaporated ethanol can be recycled and used as an extractant [12].

A microwave-surfactant synergistic method can also be used [13]. The application of microwave-assisted extraction technology, together with the aid of surfactants, has the effect of reducing the interfacial tension between the solid and liquid phases, increasing the leaching efficiency of the pigment and the extraction rate, and jointly extracting the red pigment from the redbud. This method reduces the contamination of the pigment product by organic solvents and has the advantages of being fast and having a high extraction rate.

Supercritical CO2 extraction and membrane separation methods, although having the advantages of low energy consumption, cheap, non-toxic extractants that are easy to recycle, have not yet been applied to industrial production and are yet to be further improved and developed.

Bauhinia variegata is distributed in most parts of the country, and its flowering season lasts for 7 to 8 months, from mid-October to May of the following year. According to the “Dictionary of Traditional Chinese Medicine,” Bauhinia variegata has the effect of clearing away heat and cooling the blood, dispelling wind and detoxifying, and can be used to treat rheumatism, muscle and bone pain, and nasal sores[12]. Therefore, strengthening its research may have multiple benefits.

1.5 Rhodiola Rosea Pigments

Rhodiola Rosea is a plant distributed in high-altitude areas such as Jilin, Xinjiang, and Tibet in China, as well as East Asia, Central Asia, Siberia, and North America. Preliminary studies have shown that Rhodiola pigment is relatively stable in the pH range of 2 to 6, is unstable to light, and has obvious functions such as resistance to hypoxia, cold, fatigue, and microwave radiation. It can also increase work rate, delay aging, and prevent age-related diseases. Therefore, it is of great significance in military medicine and sports medicine [14].

At present, the extraction of Rhodiola rosea red pigment is mainly carried out by the ethanol extraction method, which involves adding an extractant such as ethanol or citric acid to pretreated Rhodiola rosea, heating, stirring, soaking, filtering, and repeating the above steps to obtain the pigment [14]. Since the extraction solution needs to be constantly replaced during the soaking process, the solvent consumption is relatively high. If a Soxhlet apparatus is used, and 95% ethanol is used as the solvent, the rhodiola pigment can also be obtained by reflux, vacuum distillation and other steps [14]. Since this is a continuous solvent extraction method, the concentration of the extraction solution increases continuously, and the amount of extraction agent used is small. Therefore, the Soxhlet extraction method is conducive to improving efficiency, reducing costs, and is more suitable for continuous industrial production.

1.6 Rose red pigment

The rose is a common flower of the Rosaceae family, with large, multi-layered flowers and a deep color. Rose red pigment is a water-soluble pigment, but it is also alcohol-soluble. It has good stability under acidic conditions and is stable to ultraviolet light, but it is not resistant to reduction. In the past, rose pigment was extracted and separated using thin-layer chromatography (TLC) [15], and the pigment composition was analyzed using high-performance liquid chromatography (HPLC), with remarkable results.

However, the commonly used method is still the extraction method [16], and the extraction agent can be 65% ethanol (or 2% to 10% citric acid solution). Crush the rose petals and extract them in a constant temperature water bath at 60°C for 50 h. The ethanol solution is rose red. The rose petals were removed by suction filtration, and the solvent was evaporated off in a rotary evaporator to obtain a dark red extract. The dark red extract was dissolved in a small amount of 65% ethanol, and then extracted with petroleum ether and chloroform, respectively, to remove fat-soluble substances, and a rose red pigment solution was obtained.

In addition to being used to extract pigments, roses also have the characteristics of being highly fragrant, yielding a high oil yield, and being edible. Fresh flowers can be used to make rose wine, rose petals can be used as a spice for candy, pastries and preserved fruits, dried flowers can be used to make tea, rose oil is a valuable spice, and roses can also be used in medicine. Therefore, strengthening research on the comprehensive utilization of roses is of value in many ways.

1.7 Astringin

In contrast, there have been relatively few reports on the research, development and utilization of rubia cordifolia pigment at home and abroad. Preliminary research has found that rubia cordifolia pigment is an anthocyanin, an alcohol-soluble pigment with good thermal stability. It dissolves well in acidic aqueous solutions and organic solvents with high polarity, and has very low toxicity.

Salvia red pigment is not easily extracted directly in water, ethanol, methanol and other solvents, but is more easily extracted in acidic media [17]. Experiments have shown that, given the factors of non-toxicity, low cost, high extraction rate and ease of recovery, the best extraction solvent is 0.1 mol/LHCl /65% ethanol solution. After filtration and vacuum concentration, a paste-like dark red pigment is obtained.

Salvia splendens ker-gawl is a member of the Lamiaceae family of herbs that is native to the Americas. It is an annual plant that is widely cultivated in China. It grows quickly, is easy to manage, produces abundant flowers, has a bright color, and has a long flowering period. It is therefore a Natural Red Coloring resource with development and utilization value.

1.8 Red radish pigment

Red radish pigment (structural formula shown in Figure 4) is easily soluble in polar solvents such as water, methanol, and ethanol, but not in non-polar solvents such as acetone, hexane, and ethyl acetate. Compared with other red pigments, radish red pigment has a wider range of red colors, and it appears bright red at pH values of 1.0 to 6.0 [18], which gives it a wider range of applications.

Radish red has strong light resistance in acidic media, and its light stability is better than that of carmine. The stability of radish red decreases with increasing temperature, as the degradation of anthocyanins increases [18].

Radish red is mostly extracted from red-fleshed radish, which is rich in pigments. Therefore, it is a natural food coloring that is safe, non-toxic, abundant in resources, and has certain nutritional value and pharmacological effects.

Research on radish red pigment in China is relatively mature, and there are many extraction methods [19], such as the common extraction method using 60% ethanol as the extraction solution, and extracting with 0.5 mol/L HCl aqueous solution at 50 ~ 30°C for 1 h. After straining, decompression, and concentration, a paste-like purple-red gel can be obtained. Radish red pigment can also be extracted and purified using a macroporous adsorption resin. First, ion exchange resin is used to remove sugars and organic acids, so that the pigment in the radish is separated from non-pigment substances and the peculiar odor is removed, thereby improving the quality of the natural pigment. This is conducive to the promotion and application of natural pigments [19].

In short, as one of the natural pigments permitted for use by the National Additive Standards Committee, radish red pigment has great application potential in the fields of food, cosmetics and medicine. An important part of its production and processing is the separation and concentration of radish red pigment, which improves the stability and dyeing power of this natural pigment.

1.9 Other

There are many types of natural red coloring. In addition to the more mature red pigments mentioned above, there are also other natural red pigments, such as monascus red pigment, camellia red pigment[20], nectarine peel red pigment, grape red pigment[21], and pomelo red pigment[22].

For example, Wild et al. [23] used HPLC to isolate a new monascin from red yeast rice powder and used mass spectrometry to determine its molecular formula as C15H12O4.

Saito et al. [24-25] extracted six types of pelargonidin from the safflower flowers. Naito et al. [26] also extracted pelargonidin from sweet potatoes.

Meanwhile, the extraction methods for red pigments are also being constantly updated. For example, the CO2 supercritical extraction method is widely used, and has been used abroad for the micronization of red 60 pigment [27-28]; Gao Yong et al. [29] also successfully extracted and separated red lake carbon pigment using this method. In addition, there are membrane separation, enzyme reaction and other methods, which are continuously favored and valued by researchers due to their advantages of simple process, low energy consumption, and cheap extractant.

2 Conclusion

In the international market, the consumption of natural red coloring is growing at a rate of 10% per year, covering more than 60% of the market. At the same time, with the expansion of the functional food market and the interpenetration and development of new research technologies, I believe that the extraction methods of natural red coloring will become more comprehensive and perfect, and the efficiency will also be greatly improved, so as to better meet people's daily needs. Most natural red colorings are rich in trace elements and have high physiological activity and pharmacological effects. Therefore, the development of natural red coloring products has broad market prospects and economic value.

In addition, while we are keen to find and use new resources, we should also change our thinking and make the most of everything. Making full use and development of existing plants is of great significance to the food, pharmaceutical and cosmetics industries. For example, the Bauhinia flower was previously used only as a greening species, and its flowers were considered waste. In fact, it can be used instead of the rose of Sharon to extract red pigment, because the rose of Sharon is a cultivated crop, and its raw material is expensive and the output is limited. Using Bauhinia flowers instead of rose of Sharon to extract red pigment not only makes use of waste resources, but also conserves precious rose of Sharon resources.

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