Study on Plant Source Natural Color

Colors can be divided into synthetic colors and natural colors. Natural colors are colors that are extracted from natural organisms and then purified and refined artificially. Natural colors can be divided into plant pigments (also known as plant-derived natural colors), animal pigments, microbial pigments and mineral pigments according to their source[1]. Animal and microbial pigments are mainly derived from insects and microorganisms. The most famous animal pigment is carmine (a red natural dye). Carmine is a pigment obtained by drying female cochineal insects that parasitise cactus-like plants. Its chemical composition is carminic acid[2]. Most mineral pigments are harmful to the human body and therefore cannot be used for colouring in the food industry. Plant pigments are mainly derived from plant tissues such as flowers, fruits, leaves, stems and seeds. Natural colors are derived mainly from plant tissues, such as flowers, fruits, leaves, stems and seeds. Most of the so-called natural colors come from plant materials[3], and the plant species involved span many families and genera. According to incomplete statistics, there are currently more than 80 known natural colors[4], and more than 30 plants have been used in the development and research of natural colors[5]. Because plant-derived natural colors are safe and non-toxic, they are often used to improve the appearance and color of foods, medicines, cosmetics, etc.

Synthetic colorants are organic pigments obtained by artificial chemical synthesis. They are generally made from chemical products such as benzene, toluene and naphthalene as the main raw materials, and are formed through a series of reactions. Most synthetic colorants are organic synthetic pigments, and common ones include carmine, indigo and sunset yellow. Synthetic colorants have many advantages. Their colors are generally bright, they are stable, and they are not easily affected by environmental factors such as light, heat and oxygen. Synthetic pigments have strong coloring power and can be used as colorants or dyes, which are easy to dye food or textiles. In addition, the production process and process of synthetic pigments are easy to control, and their industrial production costs are low [6]. However, synthetic pigments themselves have no nutritional value, and most pigments are toxic to human health and even have the risk of causing teratogenic and carcinogenic effects. With the improvement of people's living standards, and in particular the various food safety problems caused by synthetic pigments, people's demand for the safety of food additives is increasing. Natural pigments, with their natural and healthy attributes, are non-toxic and will inevitably replace synthetic pigments, and are widely used in food, pharmaceuticals and light industry.

1 History and characteristics of the use of plant-derived natural colors

1.1 History of the use of plant-derived natural colors

Natural colors were the first pigments used by humans. As early as the 10th century BC, the people of ancient Britain made rose-colored candy from the juice of madder plants. This is the earliest record of the use of natural colors by ancient people. In ancient Egypt, merchants used natural plant extracts and wine to improve the color of candy. In China, there is also a long history of using natural colors, which are widely used in fabric dyeing, food coloring, and the production of rouge and makeup. For example, the rouge that was popular among ancient Chinese women since the Shang Dynasty was made from the pigment in the petals of safflowers. For example, the madder and gardenia used to prepare dyes were already cultivated on a large scale in the Eastern Zhou Dynasty (221 BC) [8]. In today's society, the types and scope of use of Natural Color have been continuously expanded, and it has broad application prospects.

1.2 Characteristics of plant-derived Natural Color

Natural colors from plant sources are produced as a result of the natural growth and metabolism of plant tissues, and have some advantages over synthetic pigments, such as: ① most plant-derived natural colors are non-toxic and have no side effects; some plant-derived natural colors with high safety can be widely used as pharmaceutical or food additives; ② plant-derived natural colors reflect the colors of the plants themselves, so the shades are very natural. As food additives or colorants, they can make the shades closer to the colors of natural objects, making it more acceptable; ③Many edible plant-derived natural colors contain essential nutrients that the body cannot synthesize on its own. These plant-derived natural colors not only improve the color of food, but also supplement the body's essential nutrients and even have preventive and therapeutic effects on certain diseases. For example, β-carotene can be converted into vitamin A in the human body, and vitamin A has the effect of treating dry eye disease and preventing night blindness.

Although plant-derived natural colors have many advantages, they still have some disadvantages: ① They are difficult to purify. Plant-derived natural colors are compounds that exist in plants and often coexist with other complex substances in plants, which makes the extraction process complicated. The pigment extracts obtained often contain other substances and are relatively low in purity. Moreover, due to the current main problems in the production process of plant-derived natural colors, such as immature processes and less advanced equipment, the extraction rate of plant-derived natural colors is low and the price is expensive. ②The hue of plant-derived natural colors is unstable and often changes due to external environmental factors such as light, temperature, oxygen, pH value, and metal ions, making it less stable [10]. Moreover, plant-derived natural colors are easily oxidized, which shortens their service life. They need to be frequently supplemented with antioxidants or pigment stabilizers, which makes their use cumbersome. ③There are many types of plant-derived natural colors, and their properties are complex. In particular, their own physical and chemical properties limit their scope of application and make them highly specialized.

2 Classification of plant-derived natural colors

In addition to being classified according to their source, plant-derived natural colors can be divided into fat-soluble pigments and water-soluble pigments according to their solubility properties; and according to their chemical structure, they can be divided into the following major pigment types: porphyrins, pyrrole derivatives, quinones and xanthones, polyene pigments, and polyphenol derivatives[9]. According to their functional ingredients, they can be divided into anthocyanins, carotenoids, flavonoids, pyrroles and other major pigments [10].

2.1 Anthocyanins

Anthocyanins, also known as anthocyanidins, are a type of water-soluble pigment that generally exist in the form of anthocyanins in the flowers, leaves, fruits, stems and other parts of plants. According to incomplete statistics, 27 families and 72 genera of plants contain different amounts of anthocyanins[11]. Anthocyanins display different colors under different pH conditions, appearing red under acidic conditions, purple under neutral conditions, and blue under alkaline conditions. The different colors of red, purple, and blue displayed by plants are also the result of the coloration of anthocyanins under different pH conditions in the cell vacuole. Anthocyanins have high biological activity and are hydroxyl donors that can be used as free radical scavengers. Studies have shown that anthocyanins have pharmacological effects such as anti-oxidation and anti-aging, anti-inflammatory and anti-cancer, immunity enhancement, cardiovascular protection and disease prevention [12]. Many plants in nature are rich in anthocyanins. For example, black goji berries have the highest anthocyanin content of any plant discovered so far. Purple sweet potatoes are an ideal raw material for anthocyanin extraction because of their high anthocyanin content and high yield.

2.2 Carotenoids

Carotenoids, also known as polyene pigments, are a class of fat-soluble terpenoid polymers, mainly divided into carotenes and carotenoids. Carotenoids are widely found in plant parts that appear yellow, orange-red or red. The carotenoids in chloroplasts are mainly carotene (orange-yellow) and lutein (yellow), which play an important role in photosynthesis. Vitamin A (retinol) is an important substance for maintaining normal visual function and maintaining healthy skin. Some carotenoids can be converted into vitamin A. These are called provitamin A. The most common of these is beta-carotene, which is converted into vitamin A when the body needs it. Not all carotenoids can be converted into vitamin A. Lycopene (found mainly in tomatoes, watermelons and guavas), for example, cannot be converted into vitamin A. Some plant-derived carotenoids that have been tested for safety can be added directly to foods such as pastries, dairy products, cold drinks, and candy as colorants.

2.3 Flavonoids

Flavonoids can generally be divided into flavones and flavonols, dihydroflavonoids, and dihydroflavonols. They are widely found in many plant tissues, mostly pale yellow or even colorless, and a few are bright orange. Among them, the most widely distributed in the plant world are flavonoids and flavonols, of which more than 400 have been discovered so far [13-14]. Flavonoids have important physiological functions and play an important role in protecting human health by anti-oxidation, anti-mutation and delaying aging. Moreover, when used in synergy with anthocyanins, flavonoids can reduce the oxidation of anthocyanins and have a certain color-enhancing effect. The most common type of flavonoid is curcumin, a yellow pigment found mainly in the turmeric and saffron tubers. It is widely used in food and medicine due to its strong antioxidant, anti-inflammatory, anticancer, coloring and toxin-inhibiting properties.

2.4 Pyrrole

Pyrrole pigments mainly include chlorophyll and its copper and sodium salts and zinc and sodium salts. They are widely found in the chloroplasts of green plants, especially in the green parts of the leaves and fruits of higher plants, where they combine with proteins to form chloroplasts. These pigments have many pharmacological effects in human medicine, such as anti-viral, anti-ulcer, antibacterial, and liver protection and detoxification.

2.5 Other pigments

Other pigments mainly include anthraquinone pigments and red yeast rice pigments. Anthraquinone pigments mainly include carmine and lac dye, which are found in the underground stems and roots of plants and red beets. They have medical effects such as antibacterial and detoxification [15].

3 Extraction and purification methods of plant-derived natural colors

There are many methods for extracting and purifying plant-derived natural colors. Common extraction methods include traditional solvent extraction, supercritical fluid extraction, microwave-assisted extraction, and pressurized solvent extraction. Common purification methods include column chromatography, membrane separation, and liquid chromatography. The following briefly describes several methods for extracting and purifying plant-derived natural colors.

3.1 Traditional solvent extraction

The traditional solvent extraction method is mainly used to extract both alcohol-soluble and water-soluble pigments. The method involves drying and crushing the raw materials, and then selecting a solvent to extract the pigments from the raw materials based on the solubility and polarity of the pigments and coexisting impurities. The crude pigment extract is then filtered, concentrated under reduced pressure, dried in a vacuum, and refined to obtain the finished product. The traditional solvent extraction method mainly includes the maceration method, the decoction method and the reflux extraction method. The traditional solvent extraction method has relatively simple equipment and process requirements, but the time taken for extraction and filtration is long, solvent consumption is high, the yield and purity of the product is low, and there is a risk of odors or solvent residues, which affects the quality of the product. Therefore, extracts obtained using the traditional solvent extraction method require further purification.

3.2 Supercritical fluid extraction

The supercritical state refers to the state when the gas-liquid interface disappears above a certain temperature or pressure. The fluid in this state is called a supercritical fluid. Supercritical fluid extraction (SFE) is a new separation and extraction technology developed in recent years, which uses supercritical fluids as extraction agents to extract target substances. The principle of supercritical fluid extraction is as follows: the solute is first dissolved in the supercritical fluid under high pressure, and then the pressure of the system is lowered or the temperature of the system is increased, causing the solute in the fluid to precipitate due to a decrease in density and solubility. Supercritical fluids mainly include carbon dioxide, ammonia, ethanol, nitric oxide, toluene, benzene, water, etc.

The supercritical temperature of carbon dioxide (31 °C) is close to room temperature, it is non-toxic and non-polluting, and it does not corrode equipment. Therefore, carbon dioxide is the most common supercritical fluid [16]. The optimal extraction process conditions for different types of plant-derived Natural Color supercritical fluids will vary, but the extraction process is generally in the range of 10 to 50 MPa, 31 to 80 °C, and 3 to 20 h [17]. Compared with traditional solvent extraction methods, supercritical fluid extraction has many advantages, such as low extraction temperature, high extraction rate, fast speed, no extraction reagent residue, no pollution, and many other advantages. However, supercritical fluid extraction technology is limited in actual promotion and application due to its high equipment investment and operating costs and imperfect technology.

3.3 Microwave-assisted extraction

Microwave-assisted extraction, also known as microwave-assisted extraction (MAE for short), is a separation and extraction technique that combines microwave heating, which can be used to selectively heat, with solvent extraction technology. The principle of microwave-assisted extraction is as follows: the target component is selectively heated in a microwave field, generating a large amount of heat in a short period of time, which causes the hydrogen bonds between molecules in the cell membrane to break, thereby destroying the cell membrane structure. This accelerates the diffusion of the Natural Color within the cell to the solvent, which has a lower dielectric constant and relatively weak microwave absorption capacity, thereby achieving the goal of rapid extraction of Natural Color. Microwave-assisted extraction (MAE) has many advantages for extracting plant-derived natural colors. Firstly, it can accelerate the dissolution of plant-derived natural colors in the extraction solvent, improve the extraction efficiency, and reduce the extraction time. Secondly, it can simultaneously extract multiple components of the sample, with a small amount of solvent and good repeatability of results. Therefore, microwave-assisted extraction shows good development prospects and great application potential in the development and utilization of plant-derived natural colors.

3.4 Column chromatography

The most common purification method for plant-derived natural colors is column chromatography. Column chromatography refers to the method of separating and purifying natural colors by filtering a mixed solution containing natural colors through a column containing different adsorbents or stationary phases. The main methods of column chromatography are macroporous resin column chromatography, gel chromatography, silica gel column chromatography, ion exchange resin method, activated carbon column chromatography and polyamide method. Among them, the most commonly used purification methods are macroporous resin column chromatography and gel chromatography.

Macroporous resin column chromatography is based on the fact that macroporous resin has a good adsorption and screening effect on Natural Color, thereby separating and purifying natural pigments. This method can also effectively remove impurities such as inorganic salts, sugars, and mucus from Natural Color. The operation of macroporous resin column chromatography is relatively simple, mainly including the processes of loading, elution, and rinsing. The purification of vegetable-derived natural colors using macroporous resin column chromatography has many advantages, including low solvent consumption, high adsorption capacity, fast adsorption speed, easy desorption, and reusability.

Another common method for purifying vegetable-derived natural colors is gel chromatography. The commonly used gels for this method are polyacrylamide gel, agarose gel, and dextran gel. The principle of gel chromatography is that when a natural pigment extract passes through gel particles with a porous, highly cross-linked structure, macromolecular substances can easily move downwards with the eluent through the gaps between the gel particles, while small molecules enter the interior of the gel particles through the pore size. Since entering the interior of the gel particles causes the small molecules to travel a long way and move slowly, the purpose of separating Natural Color is achieved using molecular size. Compared with other purification methods, gel chromatography is easy to operate, requires simple equipment, does not require regeneration after each chromatography, and fully preserves the biological activity of the separated substances. Therefore, gel chromatography is widely used in the purification process of plant-derived natural colors.

4 Problems and prospects

Most natural colors are generally safe and non-toxic for the human body, and those that have been consumed for a long time are relatively safe. However, some natural colors are still toxic (e.g., the highly toxic garcinia). Therefore, the safety of natural colors should not be ignored. Therefore, food safety has become the main issue in the development and utilization of natural colors. In particular, with the further development of technology, some plant-derived natural colors that were previously considered safe and approved for use were found to have certain mutagenic effects in subsequent toxicological tests, such as rose bengal. It can be seen that there is a problem of lagging toxicological evaluation in the research of plant-derived natural colors. Therefore, it is necessary to focus on the development of toxicological research on plant-derived natural colors and strengthen the further toxicological evaluation of natural colors.

China is a vast country with many plant resources rich in natural colors. This provides a rich source of pigment materials for the development and utilization of plant-derived natural colors. Therefore, it is necessary to make full use of this plant resource advantage, further explore characteristic plant resources that can be used for the extraction and utilization of natural colors, and continuously improve equipment and production processes to increase the yield of natural colors and reduce production costs. Moreover, the waste and by-products of some crops can be fully utilized as raw materials for extracting natural colors, such as the use of orange peel to extract hesperidin and the use of sorghum hulls to extract sorghum red pigment, etc., taking a green and environmentally friendly path of turning waste into treasure and comprehensively utilizing resources.

Although it is currently unrealistic to completely replace synthetic pigments with plant-derived natural colors, with the continuous improvement of people's living standards and health awareness, the demand for natural colors is constantly increasing. Coupled with the continuous maturity of natural color purification technology and the continuous development of toxicological research, it is believed that in the near future, plant-derived natural colors can overcome the many shortcomings of synthetic pigments and be widely used as colorants and additives in the food, pharmaceutical, and cosmetics industries.

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