What Are the Sources of Natural Colours?

Pigments are divided into two categories: natural colors and synthetic colors. Natural colors have a long history of use. In ancient China, there were already records of Natural Colors being used as dyes for textiles, rice, wine, cosmetics, etc. Since the British chemist Parkin synthesized the first artificial color, aniline violet, in 1856, synthetic colors have played a very important role in people's lives due to their bright colors, strong coloring power, stability and low cost. With the continuous development of toxicology and analytical chemistry, humans have gradually realized that most synthetic pigments are more toxic and teratogenic and carcinogenic to the human body. Countries around the world have strictly restricted the use of synthetic pigments whose safety has not been clarified, and many synthetic pigments have been successively removed from the list of approved uses in various countries. Therefore, the use of non-toxic and harmless natural substances to extract food, cosmetic and medical pigments has become a new development trend. Natural Colours come from natural substances and are now mainly extracted from plant tissues, but also include some pigments from animals and microorganisms. Natural Colours are safe and healthy, and many also have certain nutritional value and pharmacological functions.

1 Extraction of Natural Colours

Natural colours are produced and obtained in three main ways: direct extraction, synthesis and biotechnology[1]. At present, the vast majority of Natural Colours are produced using direct extraction. Pigment extraction methods include solvent extraction, extraction, enzymatic methods, pressing, crushing, tissue culture, microbial fermentation and synthetic methods. The following describes only a few commonly used and promising Natural Colours extraction methods.

1.1 Solvent extraction

Solvent extraction is currently a commonly used method for extracting pigments from plants and animals. Solvent extraction methods include maceration, percolation, decoction and reflux extraction. Natural pigments extracted with water as the solvent can be extracted by maceration or decoction. The former is suitable for raw materials whose active ingredients are soluble in water, stable to moisture and heat, and not easily volatile. Organic solvents can be used for extraction, and reflux extraction can be used. The red pigment of sorghum is extracted by soaking in a 0.1% aqueous solution of hydrochloric acid for 2 hours, removing impurities and discoloration, then extracting with a 7% aqueous solution of ethanol at 40°C, followed by filtration, concentration and drying [2]. The yellow pigment in turmeric is extracted using organic solvents such as methanol, ethanol, acetone, ethyl acetate, etc. [3]. The shrimp shells were soaked in hydrochloric acid for 24 hours and then filtered. The residue was soaked in 95% ethanol, and the extract was concentrated to obtain a crude astaxanthin extract by distillation [4]. In comparison, the organic solvent extraction method is relatively inexpensive, has simple equipment, is easy to operate, and has a high extraction rate. However, the quality of some products extracted using this method is poor, the purity is low, there is an odor or solvent residue, and this affects the scope of application of the product.

1.2 Supercritical fluid extraction method

At present, the most commonly used solvent in supercritical fluid extraction (SFE) technology is CO2. Supercritical carbon dioxide fluid extraction is a new type of material separation and purification technology that has developed in the past 20 years. It combines the characteristics of low viscosity, high diffusion of gases and high density and solubility of fluids [5]. Because the supercritical CO2 extraction process is simple, energy consumption is low, the extractant is cheap, and the extracted product has the advantages of high purity, low solvent residue, and no toxic side effects, it is increasingly being valued. Rozzi.NL studied the extraction of lycopene from tomato by-products at temperatures of 32–86°C and pressures of 13.7800–48.2686 kPa. The results showed that the maximum extraction rate of 38.8% was obtained at 86°C and 34.4786 kPa [6].

Supercritical CO2 fluid extraction technology is a new type of green separation technology, but because of the problems of imperfect technology, complex and expensive equipment, and high operating costs, the development and application of this extraction method in this field has been limited.

1.3 Microwave extraction method

Microwave extraction is a method in which the sample and organic solvent are heated with microwaves in a sealed container to extract the substance to be measured from the sample matrix. It can extract multiple sample components in a short time, with a small amount of solvent and good reproducibility of results [7]. At present, there are continuous reports of the use of microwave technology for the extraction of pigments, involving substances such as alkaloids, flavonoids, and tannins. Wang Weihua et al. have studied the optimal process conditions for the extraction of lycopene using microwaves, which achieves an extraction rate of 95% and greatly reduces the extraction time. Compared with supercritical CO2 extraction, it is low in cost, requires less investment and has a high extraction rate [8]. Although the microwave extraction of natural colors technology has achieved some important results in experimental work, its application scope is limited due to its characteristics.

1.4 Enzyme method

In the process of extracting plant pigments, the pigments are often enclosed in the cell walls, and the cell walls of most plants are composed of cellulose. Using cellulase can break the β-D-glucoside bond, causing the plant cell wall to break down, which is beneficial for the extraction of ingredients. Based on this principle, enzymolysis with cellulase is carried out before extracting plant ingredients, so that the plant cell wall is broken down before extraction, which can improve 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 enzymolysis does not destroy the composition of the plant pigments.

In summary, traditional extraction methods such as crushing, pressing, organic solvent extraction, reflux, etc., have the disadvantages of long extraction times, high labor intensity, high energy consumption in raw material pretreatment, heat-sensitive components that are easily damaged, poor purity of the produced pigment products, off-flavors and solvent residues, etc., which directly affect the development and application of Natural Colours.

For the synthetic method, due to the complexity of biosynthesis, many natural color substances are difficult to synthesize chemically under artificial control. The synthetic method can only produce a very small number of substances with the chemical composition and molecular structure of natural pigments, such as carotene.

In recent years, with the development of biotechnology, the use of biotechnology to produce Natural Colours has opened up a wide range of possibilities. The production of various natural pigments such as blue pigments and red yeast pigments using microbial fermentation methods has become a reality [10].

2 Classification and application of Natural Colours

Natural Colours can be divided into three main categories according to their source: plant pigments, animal pigments and microbial pigments; they can also be divided into water-soluble pigments and fat-soluble pigments according to their solubility properties;   Natural Colours can be divided into the following categories according to their functional ingredients: carotenoids; flavonoids; anthocyanins; chlorophylls; and other pigments such as turmeric, red yeast rice, beetroot red, alizarin red, lac dye, and cyanobacteria. Here, we will introduce the physiological functions and uses of just a few representative Natural Colours.

2.1 Carotenoids

Carotenoids are natural pigments synthesized by plants and microorganisms. They are representatives of the polyene pigment class and are the collective name for carotene and xanthophylls. They are widely distributed in the living world and more than 600 types have been discovered so far. Carotenoids can be divided into two categories according to their composition and solubility: carotenoids and xanthophylls. Carotenoids are internationally recognized as physiologically active functional antioxidants. They are effective singlet oxygen quenchers, can eliminate hydroxyl radicals, combine with lipids in cells and cell membranes, and effectively inhibit lipid oxidation. A high intake of carotenoids can reduce age-related prostatosis and age-related macular degeneration of the retina [11]. In recent years, it has been reported that carotenoids also have many new functional values in terms of anti-cancer and anti-aging effects. Therefore, carotenoid pigments are a type of functional food additive with very broad development prospects.

2.2 Flavonoid pigments

Flavonoid pigments are a type of natural pigment widely distributed in plant tissues. Most are light yellow or even colorless, and a few are bright orange-yellow. Flavonoids have long been the focus of attention as antioxidants and free radical scavengers. Numerous studies have shown that flavonoids have a wide range of biological activities, such as antioxidant, anti-mutagenic, anti-aging, anti-tumor, antibacterial, etc. [12]. The most important of these is the antioxidant activity of flavonoids, which is mainly manifested in the reduction of free radical production and scavenging of free radicals. Among flavonoid pigments, sorghum red pigments, cocoa pigments and onion pigments have strong antioxidant activity. Wang Li et al. isolated and purified four flavonoid substances from pigments extracted from the leaves of the Ufan tree. The ability of these four substances to scavenge active oxygen radicals was also studied, and it was found that all of the extracts of the black rice tree had a strong ability to scavenge free radicals, with the strongest scavenging ability of quercetin and the IC50 of the 6# extract reaching about 519 μg/mL [13].

2.3 Anthocyanin pigments

Anthocyanins, also known as anthocyanins, are found in the vacuoles of cells in the fruits, flowers, stems and leaves of plants. They are a type of water-soluble pigment in plants. Due to the differences in molecular structure or pH of various anthocyanins, they appear in different colors such as red, purple and blue. Anthocyanins are not only abundant in resources and have a gorgeous color, but also constitute the majority of the pigment kingdom. They also have high physiological activity. They are hydroxyl donors and free radical scavengers [14] and are effective in ophthalmology and the treatment of various blood circulation disorders.

2.4 Chlorophyll pigments

This type of pigment mainly includes chlorophyll and its copper and sodium salts and zinc and sodium salts. Chlorophyll is widely found in the leaves, fruits and algae of higher plants. Chlorophyll has the effects of nourishing the blood, promoting blood production, activating cells, and fighting infection and inflammation. In recent years, it has also been found to inhibit the growth of cancer cells, making it a health-promoting food colouring agent. Sodium zinc chlorophyllin is used in medicine to treat chronic osteomyelitis, chronic ulcers, skin wounds, leukopenia and other conditions. It is also a good colouring agent and nutritional enhancer in the food industry. In addition to being used as a green pigment and deodorant, sodium copper chlorophyllin also has many physiological functions in medicine. For example, sodium copper chlorophyllin or its derivatives can promote cell metabolism in the body, heal gastrointestinal ulcers and restore liver function. It can be used to treat infectious hepatitis, hemorrhoids, leukemia, uterine disorders, and gastric and duodenal ulcers. It can also enhance hematopoietic function and promote recovery from radiation damage to the body.

2.5 Other pigments

mainly refer to red yeast rice pigment, turmeric pigment and purple grass pigment.

In addition to being used in food to enhance flavor and to have antibacterial, bacteriostatic and shelf-life-extending effects, red yeast rice pigment also has antibacterial effects. In addition, as a traditional Chinese medicine, red yeast rice pigment also has the functions of lowering blood lipids, blood pressure and blood sugar, inhibiting tumor cells, and treating dysentery and stomach diseases.

Turmeric yellow pigment is widely used in food and medicine because of its strong coloring power and its anti-inflammatory, free radical scavenging, toxin-inhibiting and anticancer properties. It is a rare medicinal herb that can be used for both food and medicine, and is also a spice. Its most important physiological function is antioxidant and related physiological functions.

As a traditional Chinese medicine, comfrey has been used in clinical practice for a long time. It has the effects of cooling the blood, promoting blood circulation, detoxifying, promoting eruption, and anti-inflammatory, antibacterial and antiviral effects. The natural pigment extracted from it, comfrey pigment, is a promising biological pigment. Its bright red color is used as a cosmetic additive, which can significantly improve the antibacterial, anti-inflammatory, emollient, and freckle-removing effects of cosmetics. It can also be used as a food additive in fruit wine, beverages, snacks, and other foods to increase their functions of clearing away heat and disinfection, anti-inflammatory and antibacterial, disease prevention and anti-cancer.

3 Problems and research directions in the development of natural colors

Compared with synthetic pigments, natural colors also have some shortcomings that need to be improved: (1) most natural colors are sensitive to light, heat, oxygen, metals, etc., and have poor stability; (2) most natural colors have poor dyeing power and are not easily dyed evenly; (3) natural colors are very sensitive to changes in pH, the hue will change greatly accordingly; (4) there are many types of natural pigments with complex properties. For a single type of natural color, it is relatively specific in application and has a narrow scope of application. Another problem is that many undiscovered pigments may be located in undeveloped land and oceans, and it is difficult to commercially develop them. In view of these difficulties, current research on natural colors is mainly focused on the following three areas: synthesis processes, production techniques and alternative pigment sources. In addition to researching and developing new edible natural colors, it is also necessary to strengthen research on the stability of natural colors and the stabilization techniques for pigments during use.

There are a wide variety of pigments produced by microorganisms in nature, and they have the advantage of not being limited by resources, environment or space. Therefore, they have great potential for development and utilization. In particular, Natural Colours products can be obtained by culturing a large number of microorganisms, which greatly reduces production costs, while protecting the environment and ecological balance, solving the contradiction of resource shortages, and having the advantage of sustainable development and utilization. The pigments produced have the characteristics of Natural Colours and can be used in food, pharmaceutical, and cosmetic industries. Microbial pigments will also be a major direction for the development of Natural Colours.

References

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