What Is the Use of Natural Color?

Synthetic pigments are artificially synthesized pigments, with aniline dyes as the main raw material, which are mainly derived from coal tar produced by the dry distillation of coal. The first synthetic pigment was the organic pigment aniline violet [1]. It dominated the food coloring market due to its good stability, low price, bright color, and ease of color matching. However, synthetic pigments have no nutritional value for the human body, and the processing of synthetic pigments leaves behind lead, arsenic, phenols, aniline, chlorides, azo benzene, sulfates and other chemicals have carcinogenic, teratogenic, and ADHD-inducing risks[2,3]. Compared to synthetic pigments, natural colors come from nature, are healthy and harmless, and most have certain pharmacological functions[4].

Currently, new types of natural colors are being discovered all the time, while the types of synthetic colors that are permitted for use in various countries are constantly being reduced. The former Ministry of Health of China stipulated in the implementation guidelines for GB 2760-2014 “National Food Safety Standard Food Additive Use Standards” that there are a total of 67 types of colors that can be added to food in China, of which only 11 are synthetic colors [5]. In the United States, only 9 of the 35 synthetic colors that were allowed to be used in 1960 can still be used today, and Japan currently allows only 12 synthetic colors to be used [6]. This article mainly reviews recent research on the extraction methods and applications of Natural Color.

1. Sources and classification of Natural Color

Natural Color, also known as natural dyes, is a general term for colored substances derived from nature[7]. In a narrow sense, Natural Color refers to pigments extracted from animal and plant tissues, microorganisms or microbial metabolites. Natural Color resources are abundant and diverse, with more than 600 types of carotenoids discovered so far[8].

(1) They can be divided according to their source: animal pigments, such as lac pigments in the blood of lac insects; plant pigments, such as zeaxanthin and petunidin; microbial pigments, such as monascus red and monascus rubra[9].

(2) They can be divided according to their chemical structure: isoprenoid pigments, such as lycopene and zeaxanthin; pyrrole derivative pigments, such as chlorophyll and hemoglobin, etc.; ketones and quinones, such as xanthophyll, apigenin, cochineal pigment, lac pigment, madder pigment, Japanese madder pigment, etc.; phenols (anthocyanins and tannins) pigments, such as geranium pigment, orange peel pigment, etc.; indole pigments, such as sour date pigment, indigo, etc.

(3) Natural colors can be divided into the following color families: yellow, such as curcumin and gardenia pigment; brown, such as caramel pigment and malt pigment; green, such as chlorophyll; and black, such as black brown plum skin pigment, banana skin melanin, black sesame melanin, and black peanut hull pigment [10–12].

2 Extraction of Natural Color

As research into natural colors continues to deepen, the methods of extracting natural colors continue to multiply. Depending on the properties of the different natural colors, the commonly used extraction methods currently include ultrasonic-assisted extraction, solvent extraction, supercritical CO2 extraction, chromatography, etc. Compared with traditional extraction methods, enzyme reaction methods, pressing methods, boiling methods, etc., the current methods have the characteristics of simple operation, high extraction rate, low reagent consumption, short time consumption and easy separation. The following describes each of these commonly used methods.

2.1 Solvent extraction (CSE)

Solvent extraction is a method that uses the different solubilities of different substances in solvents such as water, ethanol, acetone, and alkenes to dissolve and separate the desired components. The principle of like dissolves like is used to separate the target components from plant tissues. This method has the characteristics of high extraction rate, cheap materials, and simple equipment, and is currently one of the commonly used methods. Jia Baozhu et al. [13] used acetone with a volume fraction of 75% as an extractant to extract proanthocyanidins from banana peels under the conditions of a liquid-to-liquid ratio of 1:12, a temperature of 50 °C, and an extraction time of 1.5 h. The extraction yield of proanthocyanidins was 0.459%, and the purity was 6.01%. Feng Jing et al. [14] used ethanol with a mass fraction of 61% as an extractant to extract grape skin pigments under the conditions of a pH of 1, a liquid-to-material ratio of 10:1, an extraction temperature of 70.34°C, and 3 extraction times, with an extraction concentration of 7.1635 mg/g. Zhang Yan et al. [15] used ethanol with a mass fraction of 90% as the extraction solution to extract the pulp pigment of cherry tomatoes under the conditions of a liquid-to-material ratio of 1:1, an extraction temperature of 40°C, and an extraction time of 1 h, with an absorbance of 1.186.

2.2 Ultrasonic-assisted extraction (UAE)

Ultrasonic-assisted extraction is a method that uses the cavitation and secondary effects of ultrasound to fully mix and dissolve the solute in the solvent, thereby extracting the desired pigment components. Ultrasonic-assisted extraction is a common method for extracting natural colors due to its high extraction efficiency and short extraction time [16]. Li Shiyin et al. [17] used this method to extract the pigment from the seed coat of the Chinese lantern plant at a material-to-liquid ratio of 1:30, a power of 80 W, a temperature of 90 °C, and an extraction time of 50 min, with an extraction rate of 23.17%. Ren Wenming et al. [18] extracted betalain using this method under the conditions of 500 W power, 1:5.02 liquid-to-material ratio, 30 ℃ temperature, and 25 min time, with an extraction rate of 85.42%. Guan Guanyu et al. [19] used 65% ethanol by mass to extract the pigment from mulberry pomace using this method under the conditions of 59 W power, a liquid-to-material ratio of 1:4, and an extraction time of 0.2 h, with an extraction rate of 93.17%.

2.3 Supercritical CO2 extraction

Supercritical CO2 extraction is one of the methods derived under the working principle of supercritical fluid extraction technology. The supercritical fluid used in supercritical CO2 extraction is CO2. In addition, ammonia, ethanol, carbon monoxide, water, etc. can also be used as supercritical fluids [20]. Because of its high extraction rate and short time, it has become a commonly used method for extracting natural colors. Weng Ting [21] extracted Antarctic krill astaxanthin using this method under the conditions of a pressure of 40 MPa, an entrainer of 1.00 mL/g, a temperature of 45 °C, static extraction for 0.5 h, and dynamic extraction for 3 h, with an extraction rate of 87.96%. Kong Lingzhong [22] used this method to extract capsanthin from particles with a particle size of about 60 mesh, with a CO2 flow rate of 10L/h, a pressure of 15MPa, an extraction separation temperature of 40°C and 50°C, respectively, and an extraction time of 3h, with an extraction rate of 4.68%.

The above are several commonly used methods with high extraction rates. In actual application, other more suitable methods can also be selected according to the physical and chemical properties of different pigments.

3. Current application status of Natural Color

Natural Color is diverse in composition. As a dye, it is soft and has a natural hue. As a food additive, it has certain physiological activity and pharmacological functions, and is widely used in various industries.

3.1. Application in the food sector

Natural Color is mainly used as a food coloring agent, and is often used in seasonings such as soy sauce and vinegar, as well as coloring candy, alcoholic beverages, ice cream, soy milk products, cooked meat products, jelly, etc., or for further processing of fruits and vegetables [23]. Candies need a beautiful sugar coating, and alcoholic beverages and drinks need a brilliant color to attract customers. These products are often presented in transparent containers in order to show off their color, so they require light- and oxidation-stable, water-soluble Natural Colors. The milk proteins in dairy products can bind with oil-soluble colors and are stable. In order to maintain the original color of the hemoglobin in canned meat products such as canned fish and canned beef, the previous measure was to soak the fish and beef in sauerkraut liquid containing nitrite and nitrate for a period of time. Nitrite is a carcinogen and harmful to health[24], so natural colors such as paprika red, beet red and alkanet red are widely used as alternatives in cooked meat products. When vegetables and fruits are processed into related by-products, the original color will be lost due to high temperatures, drying, dehydration and other processes. Plant-derived natural colors such as chlorophyll, paprika yellow and turmeric yellow are needed to maintain the original color[25].

3.2 Natural Color in the field of cosmetics

The excessive use of traditional cosmetics can cause adverse reactions such as skin inflammation and hair loss. When choosing various cosmetics, people pay more attention to whether the ingredients are safe and tend to buy natural and mild products, making Natural Color popular in the cosmetics industry. For example, the pigments of the pit of the dragon fruit, sorghum red pigment, tomato red pigment, and blueberry pigment are widely used in the manufacture of lipsticks; the madder pigment, capsicum red pigment, alizarin, safflower pigment, gardenia yellow pigment, safflower yellow pigment, safflower pigment, etc., are widely used in the production of hair dyes; gardenia yellow pigment, safflower yellow pigment, safflower pigment, etc., are widely used in the production of sunscreens and skin care products [26].

3.3 Antioxidant effect of Natural Color

One of the characteristics of Natural Color is that it has certain pharmacological and health-promoting functions. Proper intake can protect and improve human health and prevent the occurrence of certain diseases [27]. For example, lycopene, paprika red pigment, ginger yellow pigment, grape skin pigment, etc., can remove free radicals produced in the human body to reduce the production of peroxides, hinder the complete expression of cancer cell genes, induce apoptosis, improve lipids in the liver and serum, resist mutagens, and resist radiation[28].

Lycopene is the most powerful antioxidant pigment in nature. It can scavenge singlet oxygen 100 times more effectively than the commonly used antioxidant VE and more than twice as effectively as beta-carotene [29, 30]. Its mechanism of action is to prevent various cancers (breast cancer, skin cancer, digestive tract cancer, etc.) by preventing the oxidation of proteins and DNA, inhibit the formation of cholesterol (LDL) oxide to prevent coronary heart disease, block the oxidation of phagocytes, and promote the production of lymphocyte B and T cells to slow down aging. In addition, lycopene can also promote the secretion of interleukin 2 and interleukin 4, thereby enhancing immunity [31].

Paprika red is an internationally recognized red pigment. Not only is it classified as a Group A pigment by the United Nations Food and Agriculture Organization, but its use in various industries is also unrestricted [32]. The mechanism of action of this pigment is mainly threefold: 1) the β-carotene component of capsanthin can remove active oxygen in the body and prevent the chemical action of free radicals in the body; 2) it can prevent the synthesis of harmful lipoproteins to reduce blood clots and thus prevent the effects of atherosclerosis; 3) it can inhibit the effects of gamma rays. Studies have shown that the characteristic of capsanthin as a spice can protect cell DNA from gamma ray radiation[33].

The mechanism of action of curcumin is mainly twofold: 1) it can directly metabolize or indirectly inhibit mutagenic metabolites. Specifically, it inhibits the oxidation of lipid compounds in the air and by Fe/Cu, plays an antioxidant role, prevents the oxidation of hemoglobin by nitrite, and prevents damage to DNA molecules by oxidation [34]. 2) It inhibits further oxidation of cells, and the processing and modification of low-density lipoprotein (LDL) can indirectly lower blood lipids and prevent atherosclerosis. In addition, curcumin has an inhibitory effect on Escherichia coli and other bacteria, and can have anti-inflammatory, anticoagulant, and anti-infective effects[35].

4 Summary

Natural colors in China are widely used in food, medicine, dyes, daily necessities, etc., and have a bright future. However, most natural colors still have problems such as poor color strength, uneven dyeing, sensitivity to light, heat, metals, and temperature, and the color they display changes with changes in pH, resulting in poor pigment stability. With the continuous research of scientists and the strong support of relevant state departments, the problem of poor pigment stability has been improved. With the development of science and technology, the extraction methods of natural colors have been continuously innovated, and there are already a variety of new methods. However, the application, extraction technology and separation technology of some natural colors are not yet mature, and researchers need to continue to explore and research in depth in order to steadily promote the development and application of natural colors.

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