What Is the Category of Natural Pigment?

In many tests, it has been found that many varieties of chemically synthesized pigments have serious chronic toxicity and carcinogenicity, and they will inevitably be gradually replaced by natural pigments [1]. Natural pigments, as food additives, have the characteristics of high safety and natural bright colors. In addition, some natural pigments also have pharmacological effects such as treatment and prevention, as well as health functions. The physiological functions and preparation techniques of several important natural pigments are introduced below.

1 Types and physiological functions of natural pigments

1.1 Carotenoid pigments

These pigments can be divided into two categories according to their chemical properties and solubility: carotenoids and xanthophylls.

1.1.1 Carotenoids

Carotenoids are conjugated polyenes, which are characterized by their ability to effectively prevent damage to the inner lipid membrane caused by free radicals. Beta-carotene is found in many yellow-green vegetables (carrots, pumpkins, sweet potatoes, etc.), fruits, seaweed, and wolfberries. It is reported that wolfberries have a high carotene content (19.61 mg/100g) [2]. Beta-carotene is a precursor to vitamin A. In the body, after beta-carotene is absorbed, it is broken down in the small intestine or liver and converted into vitamin A. Some studies have found that one molecule of beta-carotene can be converted into two molecules of vitamin A by the action of enzymes in the body,  and is the most abundant in food, so it is considered the main source of vitamin A for the human body [3]. Some studies have shown [4] that vitamin A can improve the body's immune function, protect against ultraviolet radiation, prevent vitamin A deficiency, prevent and treat cardiovascular diseases such as stroke and myocardial infarction, and has an anti-cancer effect.

1.1.2 Lutein

It is an oxygen-containing derivative of conjugated polyenes and is an effective oxygen radical trap. Lutein is mainly found in corn, chili peppers, citrus fruits and wolfberries. It is a scavenger of singlet oxygen and free radicals, and can quickly react with oxygen and free radicals produced by the oxidation of linoleic acid to prevent the chain transfer of lipid peroxidation. Its antioxidant effect is comparable to that of BHT, and it will become a new generation of nutritional antioxidants. Western medical researchers discovered a long time ago[19] that natural lutein can prevent a series of diseases caused by aging in the human body, such as preventing the decline in vision and even blindness caused by macular degeneration of the retina of the elderly, and also reducing the incidence of cancer, cardiovascular disease, and eye disease.

1.2 Flavonoids

Flavonoids are a type of water-soluble phenolic substance with a C6-C3-C6 structure. They can capture lipid peroxide radicals and superoxides in the biological membrane, thereby cutting off the lipid peroxidation chain reaction in the body that causes aging and disease. They also have the effect of chelating metal ions and blocking oxidase. Wang Wei [12] and others found that flavonoids can also be used as a vascular protective agent to prevent atherosclerosis and embolism. Among flavonoids, sorghum red pigment, cocoa pigment, and onion pigment have strong antioxidant activity. Sun Ling [13] and others extracted melanin from black rice, black beans, and black sesame seeds and found that it has strong oxygen radical scavenging capacity.

1.3 Anthocyanin pigments

Anthocyanin pigments are generally found in the flowers, leaves, and fruits of plants and are composed of a glycoside group and sugar. They are generally water-soluble, but their color can change depending on the pH value. Anthocyanins are also sensitive to light, temperature and oxygen. Anthocyanins can strongly absorb ultraviolet light and act as a UV barrier in the body. They also have a preventive effect on coronary heart disease and myocardial defects[14]. In addition, they can treat circulatory disorders and angina pectoris, and delay the growth of cancer cells. Jiang Pingping [15] and others extracted purple sweet potato anthocyanin from purple sweet potatoes. The research found that it can remove oxygen free radicals, resist lipid peroxidation, and resist the hemolysis of red blood cells caused by H2O2. Some scholars extracted perilla pigment from the dual-purpose perilla in China and found that it has the effects of detoxification, dispersing cold, promoting circulation of Qi, and soothing the stomach.

1.4 Chlorophyll (porphyrin pigments)

Chlorophyll is found in algae and the leaves and fruits of higher plants, and is unstable to light. Generally, the magnesium in the center of the porphyrin ring can be removed by dilute acid separation to form demagnesium chlorophyll, and then copper can be used to replace magnesium to obtain the more stable derivative chlorophyllin copper sodium. Studies have found that it has the effect of nourishing blood, promoting blood production, activating cells, fighting infection and inflammation, and inhibiting the growth of cancer cells.

2 Preparation method

2.1 Solvent extraction method

The process of this method is to first extract with an organic solvent, and then filter, reduce the pressure to concentrate, vacuum dry and refine to finally obtain the product. Depending on the nature of the pigment and the raw material, the choice of extraction agent also varies, and it is generally divided into inorganic and organic. Common inorganic extraction agents include water and acid-base solutions; organic solvents include ethanol, acetone, alkanes, benzene, oils and fats, carbon dioxide, etc. The solvent extraction method is simple, requires less equipment investment, is easier to operate, does not pollute the environment, and is convenient for production; however, the extraction time is long, labor intensity is high, energy consumption is high during raw material pretreatment, the solubility of the pigment is poor, and the color changes greatly. The amount of solvent required for the extraction process is large, and recovery is difficult, resulting in a higher production cost for the product. Some literature reports [16] that ethanol is an ideal organic solvent for the extraction of natural food colors. Cai Jian [17] and others reported that for red peppers and tulips with low moisture content, the use of 95% ethanol is more effective; for radishes with high moisture content, anhydrous ethanol can be used for extraction.

2.2 Supercritical fluid extraction

Supercritical fluid extraction is an emerging extraction and separation technology in the food industry. It uses the characteristics of liquids in the supercritical region, which have both the amphoteric properties of gas and liquid (i.e., they have the high permeability and low viscosity of a gas, as well as the high density and good solubility of a liquid), to achieve the dissolution and separation of solutes. The main feature of this technology is that it combines the dual functions of traditional solvent extraction and distillation, and it is especially effective at separating heat-sensitive and non-volatile substances. Supercritical CO2 extraction technology can also drive the development of related industries and improve chemical solvent methods in China, thereby promoting the development of the industry. It is an excellent method for extracting, preparing and analyzing samples from natural substances, and therefore is more in line with the trend of people returning to nature.

2.3 Freezing-melting method

This method is gentle, with an operating temperature not exceeding room temperature, and therefore less damaging to heat-sensitive natural food colors. It is a method often used in biochemical research to break the cell walls of microorganisms [17]. Compared with conventional solvent extraction, this method avoids the mass transfer process through the cell wall, so the extraction time is greatly shortened. After the plant cell wall is broken, the intracellular soluble substances will be dissolved. In order to obtain a purer product, ethanol is generally used as the extracting agent. Yang Li [20] and others used gardenia, rubia cordifolia, and maple leaves as raw materials to extract three pigments. The results showed that this process has broad applicability for extracting water-alcohol soluble plant pigments, and can therefore be extended to the industrial production of other similar pigments.

2.4 Enzyme extraction method

The enzyme extraction method is suitable for raw materials that are surrounded by cell walls and are not easily extracted, such as safflower yellow pigment [17]. Safflower yellow pigment is found in the tubular petals of safflowers, and the chemical composition of plant material in this area is mainly cellulose-like substances, which form a barrier to the diffusion of safflower yellow pigment from plant material to the extraction medium. The action of cellulase on the tubular flowers of safflower causes the degradation of cellulose, hemicellulose and other substances in the cell walls and intercellular matrix, resulting in local changes such as loosening, expansion and collapse of the structure of the cell walls and intercellular matrix, thereby increasing the mass transfer area for the diffusion of the active ingredient, i.e. safflower yellow pigment, to the extraction medium and reducing the mass transfer resistance, thus facilitating the extraction of safflower yellow pigment.

2.5 Microwave extraction method

Microwaves are electromagnetic waves with a frequency of 300–300,000 MHz, and are generally referred to as ultra-high frequency waves. The mechanism of microwave extraction [17] is that the microwaves freely pass through the transparent extraction medium during microwave irradiation, reaching the internal vascular bundles and glandular cell systems of the biological material. Because of the absorption of microwave energy, the temperature inside the material will suddenly increase. Generally, the temperature of the vascular bundles and glandular systems of natural materials will increase faster, and then maintain that temperature until the internal pressure exceeds the ability of the cell wall to expand, and the cells begin to break. At this time, the active ingredients in the cells will flow freely out of the broken cell walls, and then be captured by the extraction medium at a lower temperature and dissolved into the extraction medium. Finally, the residue is filtered out to obtain the extract. On the other hand, the electromagnetic field generated by microwaves can accelerate the diffusion rate of the extracted material from the inside to the extraction solvent. The advantages of microwave extraction are high and accurate extraction rates, fast speed, low operating costs, reduced pre-processing costs for raw materials, and it is also environmentally friendly. Microwave radiation has excellent penetrability, and it can be applied to any natural biological material, and is more effective for the extraction of heat-sensitive ingredients. In addition, this method can be combined with supercritical fluid extraction to solve the problem of solvent residue during microwave extraction, which is difficult to achieve with other existing extraction methods.

3 Outlook

China has a vast territory, abundant and diverse plant resources, and natural pigment raw materials in both the north and the south, all of which can be developed and utilized. This is especially true of some agricultural products, such as corn, sorghum, chili peppers, radishes, etc., which can be found everywhere. It is also worth mentioning that wolfberries, which are cultivated in large plantations in northwest China, contain a wide variety of pigments in high concentrations, providing a rich source for the development of natural food colors and opening up new avenues for the deep processing of agricultural and sideline products. With the improvement of people's living standards and the continuous development of the food industry, as well as the increasing emphasis of China's food industry on high-tech, natural food colors have broad development prospects. For this reason, we should seize the opportunity, based on the domestic market, actively explore the international market, and vigorously develop natural, nutritious, and multifunctional natural food colors, such as carotene, flavonoids, lycopene, chlorophyll, red yeast rice pigment, etc. At the same time, we should also adopt advanced technology, continuously improve the level of equipment and product output, so as to enhance the market competitiveness of the products.

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