What Is the Use of Natural Colour?

Synthetic pigments are mostly tar dyes, which not only have no nutritional value, but some are also harmful to the human body. Therefore, edible natural colors extracted from plants have recently received widespread attention from scholars at home and abroad, and there is a trend of replacing synthetic pigments. The types, extraction methods and development and application of natural colors are summarized below.

1 Types

Natural colors come from a wide range of sources, have complex compositions and are highly diverse. They can be divided into four main categories according to the extraction method: liquid or solid pigments extracted from animals and plants by juicing or solvent extraction; powdered pigments obtained by drying and grinding colored animals and plants; pigments that are fermented by microorganisms, the metabolites of which are separated into liquids or further processed into solid powders; and pigments made from natural products by enzymatic action. In addition to the turmeric, beetroot red, sodium copper chlorophyllin, paprika red, red yeast rice pigment, and carotene that were already in use, research has recently led to the development of corn yellow, sorghum pigment, radish red, rose bengal, gardenia yellow, tea pigment, indigo, sorghum red, black rice pigment, and safflower pigment. The development of China's natural color industry has begun to take shape. The total national output of natural colors has reached more than 100,000 tons, of which the most representative ones are caramel color (i.e., sugar color), gardenia yellow, gardenia blue, turmeric, amaranth red, paprika red, tea pigment, etc. [1].

2 Extraction and separation

2. 1 Extraction methods

Solvent extraction: The most commonly used method is to follow the principle of like dissolves like, based on the polarity of the extracted ingredients in the raw material and the different physical and chemical properties of coexisting impurities, so that the mass transfer process of the active ingredients transferring from the solid surface or inside the tissue of the raw material to the solvent. Solvent extraction methods include maceration, percolation, decoction and reflux extraction. Water can be used as a solvent for Natural Colour extraction using the maceration and decoction methods, and organic solvents can be used for extraction using the reflux method. The solvents used for extracting some Natural Colours are shown in Table 1.

Supercritical fluid extraction: This uses a fluid between a gas and a liquid for extraction. The fluid must have excellent solvent properties, low viscosity, high density, and good fluidity, mass transfer, heat transfer and solubility properties. The most commonly used solvent at present, CO2, is non-toxic, non-flammable, chemically inert, inexpensive, highly pure and environmentally friendly. Under high pressure, the solute is dissolved in the fluid, and then the pressure of the fluid solution is lowered or the temperature is increased, causing the solute dissolved in the supercritical fluid to precipitate due to a decrease in density and solubility. Compared with traditional processes, it has the advantages of low operating temperature, simple process, high efficiency and no pollution. Rozzi[2] studied the extraction of lycopene from tomato by-products at 32-86 °C and 13.7800-48.2686 kPa, and the maximum extraction rate of 38. 8% was obtained at 86 ℃ and 34. 478 6 kpa.

Ultrasonic extraction: Ultrasonic waves are elastic waves that can generate and transmit powerful energy. They can penetrate deeper into plant tissue cells than electromagnetic waves and remain there for a longer period of time. Ultrasonic waves can cause liquid to be broken into many small cavities that close instantaneously, generating an instantaneous pressure of up to 3,000 Mpa, i.e., cavitation, which causes plant cells to rupture. In addition, ultrasound also has multiple effects such as mechanical vibration, emulsification and dispersion, and crushing, which can transfer, diffuse and extract the effective ingredients in plants. Therefore, using ultrasound to extract pigments is easy and fast, does not require heating, and has high extraction efficiency, speed and effectiveness, without damaging the structure. Li Yunyang et al. [3] used ultrasonic technology to extract brown pigment from chestnut shells and compared it with the conventional method. The results showed that ultrasonic extraction saves time and energy and has a high extraction rate. The optimal process parameters for ultrasonic extraction are a mass ratio of 1:10, an aqueous ethanol solution of 30% as the solvent, and 2 extraction times at 70 °C (1 h each). Wang Zhenyu et al. [4] found that at an ultrasonic frequency of 30 KHz, the highest extraction rate was obtained using dilute H2 So4 as the extracting agent at a mass fraction of 2% for 40 min at 50 °C.

In addition, good results were also obtained using ultrasound to extract mulberry red pigment.

Microwave extraction: Microwave technology has the advantages of rapid temperature rise, easy control, even heating, energy saving, etc. It can strengthen the extraction process, shorten the production cycle, reduce energy consumption, reduce waste, improve yield and extract purity, and has low operating costs and is environmentally friendly, with good development prospects. At present, reports on the use of microwave technology for the extraction of pigments involve alkaloids, flavonoids, tannins and other substances. Li Yu et al. [5] studied the microwave extraction of chamomile yellow pigments, using anhydrous ethanol as the extracting agent (the mass ratio of chamomile to anhydrous ethanol was 1:70), microwave power 800 W, extraction time 450 s, and extraction was carried out three times. Compared with the solvent extraction method, the extraction time was reduced from 12 h to 450 s, and the extraction rate increased from 88.6% to 91.1%. Cai Jinxing et al. [6] used a microwave-ultrasonic method to extract strawberry pigments and study their physical and chemical properties. The results showed that the combined treatment of microwave and ultrasound can break the bonds of pigments, break up tissue cells, and improve the extraction rate of strawberry pigments.

Enzyme method: Plant pigments are often enclosed in the cell walls, which in most plants are made of cellulose. Using cellulase can break the β-D-glucoside bond, destroying the plant cell wall and facilitating the extraction of ingredients. Based on this principle, using cellulase to digest before extracting plant ingredients can increase the extraction rate of active ingredients. Regardless of whether enzymes are used or not, the composition of the extract is the same, which shows that enzymatic hydrolysis does not destroy the plant components.

Air bursting method: This method uses the fact that the air in plant tissues is compressed, and when the pressure is suddenly released, the strong pressure released breaks the plant cell walls and tears the plant tissues, loosening the plant structure and facilitating the penetration of solvents into the interior of the plant, as well as greatly increasing the surface area of contact to extract the active ingredients. This method is suitable for the extraction of fibrous tissues such as roots, stems, bark and leaves, but there is not much research on it yet.

2. 2 Separation methods

Solution separation: The most common method is to use the different solubilities of the components of a mixture in a solution to achieve separation. For example, if the distribution coefficients of pigments in different polar solvents are different, select 3 to 4 solvents of different polarities (such as petroleum ether, chloroform, ethyl acetate and n-butanol) and perform stepwise extraction of the extraction solution from low polarity to high polarity to obtain extraction solutions with different polarities. Next, pigments can be separated at different pH values. For example, gradient pH extraction is a classic method for separating free anthraquinone derivatives and is also the most commonly used method for separating flavonoids. In addition, certain solvents or precipitants can be added to the sample solution to precipitate the separated substances in solid form through chemical reactions or by changing the pH or temperature of the solution, such as tannin pigments. For example, the precipitation of tannins with metal salts can be used for separation. Flavonoid pigments can also be separated using the precipitation method with lead salts. Adding a saturated aqueous solution of neutral lead acetate to an ethanol or methanol extract of traditional Chinese medicine causes flavonoids with o-dihydroxy or hydroxy groups to precipitate out.

Membrane separation: This refers to the use of natural or synthetic polymeric membranes to separate, classify, purify and concentrate mixtures using external energy or a chemical potential difference as the driving force. In pigment separation, the difference in molecular size between the pigment and impurities is used. The use of a fibrous ultrafiltration membrane and a reverse osmosis membrane can retain various insoluble macromolecules (such as polysaccharides and proteins). This method is simple and highly effective and can be used to separate cocoa pigments and red yeast pigments. It is also possible to first remove more than 90% of macromolecular substances such as pectin through an ultrafiltration membrane, and then use a reverse osmosis membrane to concentrate it to more than 20% solids. Operating at room temperature can cause the membrane to 100% retain the pigment. Anthocyanin separation is carried out by extracting with water containing sulfurous acid, and then selectively removing sugars or other low molecular substances using an ultrafiltration membrane, which can increase the yield by 2 times [7].

Column chromatography: refers to the use of different adsorbents or stationary phases to separate and purify pigments by column chromatography. For example, ion exchange resin column chromatography can be used to purify grape skin pigments and remove impurities such as sugars and organic acids; polyamide column chromatography is suitable for the separation of flavonoids, quinones, and phenolic pigments, such as safflower yellow pigments and red pigments; silica gel column chromatography is suitable for the separation of small molecule fat-soluble pigments; activated carbon column chromatography is used to separate water-soluble components, such as benzopyran pigments (anthocyanins, alizarin, and genipin, etc.). Macroporous adsorption resin has a strong adsorption effect on pigments and has a good adsorption and purification effect on a variety of Natural Colour. The adsorption and screening effect of the proposed separation substance using macroporous adsorption resin can achieve the purpose of separation. Most Natural Colors prepared using traditional processes have strong hygroscopicity. However, after being treated with a macroporous adsorption resin column chromatography, hygroscopic components such as sugars, inorganic salts, and mucus in the aqueous or alcoholic extract can be effectively removed, enhancing the stability of the product. Shen Yonggen et al. [8] found that X-5 resin has strong adsorption capacity at a pH of 4 and a flow rate of 2.0 mL/min. At room temperature and a desorption flow rate of 1.5 mL/min, the elution effect is best when 60% ethanol is used as the desorbent. Purification of purple sweet potato pigment with it can obtain a high purity pigment. Rukye Musa [9] used AB-8 macroporous adsorption resin column chromatography to enrich and purify the purple sweet potato extract, obtaining high-quality, high-purity anthocyanin-based Natural Colour.

Thin-layer chromatography (TLC): The principle is basically similar to that of column chromatography, with the main difference being that TLC requires a finer particle size of the adsorbent, generally greater than 250 mesh, and a uniform particle size. The TLC method can be used for the analysis of individual or a class of pigments, including anthocyanins, flavonoids, carotenoids, etc. Some people used TLC to analyze the pigments in red-heart radish [10], studying the effects of pigment quantity, eluent, elution time and other factors on pigment separation. Fu Zhengsheng et al. [11] chose a diluent with silica gel G as the stationary phase (the ratio of pigment quantity in the pigment viscous liquid to water was 1:1), the mixture [anhydrous ethanol-water (5:1)] or [anhydrous ethanol-water-petroleum ether (6:1:1)] as the developing agent, the development time was 80-100 min, and the development distance was 6-10 cm. When these conditions were used, the pigments were completely separated and the spots were clear and concentrated.

High-performance liquid chromatography (HPLC): HPLC is used for the separation and analysis of unknown samples. There are mainly four types: adsorption chromatography, distribution chromatography, ion chromatography and molecular exclusion chromatography. HPLC analysis can be used to identify unknown components in a sample, especially when used in conjunction with mass spectrometry or nuclear magnetic resonance. It can also be used to determine the exact content of a component in a sample based on the height or area of a chromatographic peak in a comparative relationship with the amount of the component being measured under defined conditions. It can also be used to quickly determine the structure of trace amounts of unknown compounds. Liu Xiaoling et al. [12] used liquid chromatography-mass spectrometry (HPLC-MS) to separate the different components of the pigments in dragon fruit and identify the structures of the separated components. The results showed that the pigments in the flesh and peel of dragon fruit were both betaine pigments, and four betaine pigments were isolated from the flesh.

High-speed counter-current chromatography (HSCCC): a new type of liquid-liquid separation chromatography technique that has rapidly developed in recent years. It relies on the directionality of the tetrafluoroethylene serpentine tube and the centrifugal force generated by a specific high-speed planetary rotation to stably retain the carrier-free stationary phase in the serpentine tube and pass the mobile phase through the stationary phase in one direction at a low speed, achieving the purpose of continuously extracting and separating substances. The advantages are that there is no need to use a carrier to eliminate irreversible adsorption, sample denaturation and contamination, and abnormal tailing of chromatographic peaks. The sample can be quantitatively recovered, and it is suitable for separating non-polar and polar components. This method has been used to separate anthocyanins from red wine and carotenoids from gardenia.

Centrifugal liquid chromatography: a major improvement on conventional column chromatography. A dish-shaped disc is used instead of a column, with the adsorbent spread on the disc. The sample is then added and eluted, and the components are separated in sequence by the action of centrifugal force and collected in segments by the detector. The operation is fully automated. This method has the advantages of short separation cycles and simple operation. The components can be collected according to the color band. In addition to the silica gel and alumina used in ordinary thin-layer chromatography, ion exchange resins and dextran gels can also be used as adsorbents.

3 Application

Grape skin pigment: The main component is malvinidin, which is usually extracted from grape skins used in wine production with sulfurous acid. The pH can affect the change in hue, and the more acidic the solution, the more vivid the red color. It is stable in an acidic environment, but not very stable to light and heat. When used in beverages, it can enhance the color and prevent quality degradation to a certain extent.

Hibiscus pigment: Hibiscus is a woodbine native to Africa that is an annual upright herb widely distributed in tropical and subtropical regions. The calyx of the hibiscus is purplish red, and its pigment is a type of anthocyanin. The main component is delphinidin, followed by cyanidin. Its aqueous extract has a characteristic sour taste, and the de-pectin extract can also be used as a natural acidulant. The pigment is similar in nature to grape skin pigment. When used to colour drinks, it is prone to browning if stored for a long time.

Curcumin: A relatively stable pigment with a bright color and exceptional luster. It is heat- and light-resistant and is mainly used as a yellow food coloring. It has the functions of clearing away heat, stimulating the appetite and strengthening the spleen, promoting digestion, and coloring. It is mostly used in the production of pastries, candies, canned goods, soft drinks, pickles, etc., and has particularly high medicinal value. It has been widely used in the pharmaceutical industry. Curcuminoids are the main active ingredients in the Chinese herb turmeric, which contains about 3% to 6% of them, including curcumin, demethoxycurcumin and bisdemethoxycurcumin. The similar structures of these three phenolic pigments (interconversion between the benzene ring diketone structure and enone, phenolic hydroxyl group on the benzene ring) give them similar pharmacological effects in many ways, such as anti-inflammatory, antioxidant and free radical scavenging, and regulation of blood lipids. In particular, their antimutagenic and anticancer effects have become a research hotspot, but the slight structural differences mean that the three curcuminoids have very different abilities in terms of anticancer and antioxidant effects. For example, demethoxycurcumin is best at inhibiting tumor cell proliferation caused by TPA, followed by curcumin; and demethoxycurcumin is best at preventing the formation of lipid peroxides in cells, followed by demethoxycurcumin. Therefore, extracting a purer mixture of the three curcuminoids from turmeric and further separating them can provide a good guide and practical value for actual drug production.

Lycopene: found in ripe tomatoes, as well as in watermelons, grapes and some other fruits and vegetables. 1 kg of fresh, ripe tomatoes contains 0.02 g of lycopene. Studies have shown that lycopene accounts for about 50% of the carotenoids in human serum and is most easily absorbed, metabolized and utilized by the body. It is also found in high concentrations in the testicles, adrenal glands and prostate. Since mammals cannot synthesize carotenoids (including lycopene) in the body, lycopene in the human body mainly comes from fruits and vegetables, especially tomatoes and tomato products. Epidemiological studies have shown that lycopene can reduce the risk of lung cancer, stomach cancer, prostate cancer, pancreatic cancer, colon cancer, esophageal cancer, oral cancer and uterine cancer. The 1997 report of the American Cancer Research Conference and the American Cancer Society Annual Meeting ranked tomatoes as the top anti-cancer food. At present, the world's development and production of lycopene is mainly extracted from the plant tomato, chemical synthesis and other methods, Israel, Japan, Russia and other countries, as well as multinational companies such as Roche and BASF are in the leading position.

Saffron yellow pigment: It is relatively stable to light and heat and is not affected by pH. Previous studies have shown that the active ingredients of safflower that promote blood circulation are mainly concentrated in the water-soluble safflower yellow pigment. Saffron yellow pigment is a chalconoid compound with a variety of pharmacological effects, such as coronary artery dilation, anti-oxidation, myocardial protection, blood pressure reduction, immunosuppression and cerebral protection. The content of safflower yellow pigment is one of the main indicators for evaluating the efficacy of safflower. Studies have shown that safflower yellow pigment is the pharmacological component of the Chinese herbal medicine safflower, which has no toxic side effects. It can inhibit platelet aggregation and release induced by platelet-activating factor, and can competitively inhibit the binding of platelet-activating factor to platelet receptors. It can be directly used in medicine, as well as in health products, food, cosmetics and textile coloring.

Paprika Oleoresin: a natural color contained in ripe chili peppers. Its main ingredients are β-carotene and capsanthin and capsorubin, which belong to the xanthophyll family. It is a hot spot in the research of natural colors and has a broad domestic and international market and high application value. The powdered pigment made by extracting and roasting paprika is easily soluble in water, heat-resistant, salt-resistant, acid-resistant, metal-resistant and microbe-resistant. It has strong coloring power and is good in dispersion and hiding power. It is a high-quality natural color that can be widely used in the production of food, medicine and cosmetics. Compared with general natural pigments, it is not only sold at a higher price, but also has a lower production cost.

Zein and zein alcohol: can be widely used in the pharmaceutical, food and packaging industries. Zein is a carotenoid pigment extracted from the by-product of corn starch production (corn zein), and it is a natural colouring agent that can be used to colour food and cosmetics. Zein alcohol is a kind of edible protein that has good film-forming, adhesive and moisture-proof properties. It can be used to make moisture-proof outer coatings for pharmaceutical products and fresh-keeping coatings for food products. Zein is an edible protein with excellent film-forming, adhesive and moisture-proof properties. It is used in the pharmaceutical industry for moisture-proof outer coatings and in the food industry for fresh-keeping coatings.

Gardenia yellow: It has attracted much attention in recent years due to its advantages of being safe and non-toxic, promoting gallbladder function and detoxification, and providing a natural and vibrant colour. The Chinese people have long used gardenia as a pigment for food and object dyeing. Gardenia yellow pigment has excellent dyeing ability for starches, proteins, etc., and is widely used in various foods such as pastries, pasta, beverages, and candy. It is also used in the fields of medicine and cosmetics. It is highly safe, has good stability, is non-toxic, and has no side effects. Therefore, the demand for it at home and abroad is increasing year by year.

Amaranth red pigment: Amaranth stems and leaves are purple or green in color, rich in nutrients, and the seeds can be used as a topping. The purple stems and leaves contain bright pigments that can be used as food dyes or to color medicinal liquids. They are non-toxic, harmless, and stable. Natural amaranth red pigment is made from red amaranth (mature period is June to August), obtained by physical extraction and refinement. Its main components are amaranthine and betaine. It is a dark purple-red dry powder that is highly soluble in water and soluble in low-grade ethanol. Its solution is a clear purple-red color in the pH range of 2 to 7, with a soft and natural hue. Its physical and chemical properties are similar to those of the internationally-used beet red pigment, and it is suitable for coloring frozen foods and beverages that are stored for a short period of time.

Walnut pigment: The walnut is a plant in the Juglans genus of the Juglandaceae family. It is a special cash crop that combines the functions of medicinal herb, dye, and oil. The green walnut skin not only has antifungal and antitumor medicinal effects, but can also be used as a natural color and dye, and is widely used in foods, cosmetics, etc. [13-14].

4 Prospects

At present, China has developed and utilized relatively few varieties of natural food coloring. Most are limited by production conditions and the seasons, with unstable raw material sources. Moreover, the raw materials have complex compositions, low pigment content, and high production costs, which are not conducive to widespread promotion. However, technological progress has made their widespread use possible. The first commercially used natural color was simply a plant extract. It is expected that in the future, people will be able to produce high-quality products with uniform standards and a longer shelf life, further increasing the demand for Natural Colour. More and more food and beverage processing companies will replace synthetic colours with Natural Colour, thus promoting more customers to pursue a natural and healthy lifestyle. Therefore, accelerating the development and utilization of Natural Colour has a positive and far-reaching significance for protecting people's health and improving the economic value of agricultural and sideline products.

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