Study on Natural Pigment

Pigments are inextricably linked to human life. As food additives, they give food an attractive color, and as dyes, they can dye textiles and other materials a beautiful color. Pigments can be divided into natural and synthetic pigments according to their sources. Natural pigments are natural coloring substances obtained by extracting and purifying plant, animal and microbial resources. Synthetic pigments refer to artificially synthesized pigments. It has been found that some synthetic pigments are harmful to human health and carcinogenic. In particular, the carcinogenic effect of some azo compound synthetic pigments is particularly obvious, such as 4-dimethylazobenzene, which can induce liver cancer.

Although natural pigments have disadvantages such as instability and decomposition under light and high temperatures, which greatly limits their application, they are extracted from plants and animals, are safe and non-toxic, and have good physiological activity. Anthocyanin pigments and carotenoids 2-1 both have the effect of scavenging free radicals and preventing oxidation. Anthocyanins also have the effect of ascorbic acid and improve the spontaneous activity and learning and memory of mice on a high-fat diet. 5, not only do pigments have a certain physiological activity when used alone, but the combination of pigments can also have effects that no single pigment can have. The combination of anthocyanins and lutein can protect the retinal tissue of diabetics.

Therefore, in recent years, the research, development and application of natural pigments has become a hot topic. Japan and India have made research on natural dyes part of the UNDP (United Nations Development Programme) plan for technical cooperation between developing countries. Two international symposiums have been held, and European and American countries have mainly focused on the development of new natural pigments, the properties, stability and extraction of natural pigments. China has also been advocating the preferential use of natural pigments in foods.

After more than 20 years of development, the country has standardized and improved the management of natural pigments as food additives in terms of development, production, and use regulations. In 2004, the total production and sales volume of natural pigments was 211.013 million tons. The world natural pigment market is growing rapidly at twice the rate of synthetic pigments, and the replacement of synthetic pigments by natural pigments has become a major trend in the development of the pigment industry. Therefore, this article discusses natural pigments from several aspects, including their classification, coloration principles, stability, extraction, and applications, in the hope of providing a basis for better establishing the research direction of natural pigments in the future.

1. Classification of natural pigments

Natural pigments are natural coloring substances obtained by extracting and purifying plant, animal, and microbial resources. Natural pigments can be classified in various ways. For example, they can be classified according to their source into three main categories: plant pigments, animal pigments, microbial pigments, which can be divided into three categories according to their sources; according to their chemical structures, they can be divided into pyrrole pigments, carotenoid pigments, anthocyanin pigments, flavonoid pigments, quinone pigments, etc.; according to their solubility, they can be divided into water-soluble pigments and fat-soluble pigments, etc.

However, Zhang Shengwan, Liu Shuling7 and others first proposed a “binary system” for natural pigments based on a study of their structure and behavior, i.e., one type is a class of fat pigments with a long-chain conjugated structure (see Figure 1). Examples include gardenia yellow, tomato red, and corn yellow. The other type is an aromatic pigment with a conjugated structure (see Figure 2). such as grape skin red pigment, sorghum skin red pigment, safflower yellow pigment, and rose red pigment, etc., and pointed out that the reason why fat pigments have color is because of their long conjugated structures, and the more conjugated structures there are, the darker the color will be, which can also lead to a red shift of the absorption peak. Aromatic pigments mainly have absorption in the visible light range due to their stable conjugated aromatic structures and multiple phenolic hydroxyl groups. At the same time, aliphatic pigments mainly fade due to photochemical oxidation and hydrolytic rearrangement, while aromatic natural pigments mainly change color due to structural rearrangement. This classification method classifies pigments structurally and studies the mechanism of natural pigments having color and fading by studying the structure. This achieves a combination of structure and behavior.

2. Stability of natural pigments

2.1 Factors affecting the stability of natural pigments

Natural pigments have the advantages of high safety and nutritional value compared to synthetic pigments, but they have the disadvantage of poor stability. Zhu Beipei, Jin Yingshi and others investigated the effects of factors such as temperature, light, pH, reducing agents and oxidants on bilberry pigments. The results showed that bilberry natural pigments are heat-resistant, light-resistant and oxidatively stable, but are relatively stable to common reducing agents, stable under acidic conditions, and discolored under alkaline conditions. However, Chen Cunshe, Dong Yinmao and others, through the study of three pigments: radish red pigment, tulip red pigment and tulip yellow pigment, concluded that among these three pigments, tulip pigment has good light resistance and poor heat resistance, while radish red pigment has poor light resistance. Reducing agents have a certain effect on the stability of pigments. Tulip red pigment has good resistance to oxidation and reduction, while radish red pigment and tulip red pigment have the opposite properties. The use of the three natural pigments is not affected by sugars.

Shi Haixiang, Zhong Shanmin 0 and others studied the natural pigments of the pomelo and concluded that different metal ions have different effects on the pigments, and the effects of the metal ion concentration also differ. From the above research data, it can be found that current research on the stability of natural pigments focuses on the factors affecting the stability of individual pigments, and the factors affecting and the results of the effects also differ depending on the pigment. Qiao Hua, Zhang Shengwan and others studied the relationship between the molecular structure of 16 different types of natural pigments and their stability, and found that if natural pigments are divided into two categories: aliphatic natural pigments and aromatic natural pigments, there are qualitative differences in the three aspects of their performance behavior, mechanism of action, and main factors affecting stability, there are qualitative differences in the three aspects.

Aliphatic natural pigments mainly fade due to photochemical oxidation and hydrolytic rearrangement, while aromatic natural pigments mainly change color due to structural rearrangement and the reaction with metal ions to form complexes. Light, oxidation, and increased medium polarity are the main factors affecting the fading of aliphatic natural pigments. Aliphatic natural pigments have poor light and oxidation resistance. As the medium polarity increases, the stability of aliphatic natural pigments decreases significantly. The presence of metal ions and changes in pH are the main causes of discoloration of aromatic pigments, while light and oxidation have little effect. This research provides a basis for the preservation of natural pigments. The precautions for the preservation of aliphatic and aromatic pigments are different. First, we should understand the structure of the pigment to be preserved, and then preserve it according to the precautions for the preservation of aliphatic and aromatic pigments.

2.2 Methods to improve the stability of natural pigments

2.2.1 Add food additives

Food additives such as malic acid, succinic acid, ferulic acid, rutin, naringin, and paraben have a certain effect on improving pigment stability. Among them, the effects of succinic acid, ferulic acid, paraben, and naringin are stronger, and their combined use has a significant effect. Giulia Martellia²¹ and others have confirmed that high concentrations of sugar have good resistance to degradation of phycocyanin at high temperatures, and this property is independent of the type of sugar but is related to the concentration of sugar.

2.2.2 Forming complexes or inclusion compounds

Beta-cyclodextrin can form inclusion compounds with the aliphatic pigment gardenia yellow, which protects the gardenia yellow pigment. EDTA can form complexes with metal ions to eliminate the effect of metal ions on the pigment. 3. Citric acid can form stable compounds with some metal ions, such as iron ions, copper ions, and manganese ions, indirectly plays an antioxidant role, thereby improving the stability of the pigment. Persimmon anthocyanin pigment can combine with flavonoids to form complexes, deepen the color of the pigment, and enhance stability+.

2.2.3 Adding auxiliary pigments

The addition of methionine, tryptophan, valine, tyrosine and alanine can all increase the absorbance of persimmon pigments, and the stability of the pigments is also enhanced. The addition of hydroxy acid co-pigments can shift the maximum absorption wavelength of the pigments by 2–5 nm, and as the content of hydroxy acid co-pigments increases, the absorbance of persimmon pigments also increases.

2.2.4 Other methods

Chen Xuehong 5 et al. significantly improved the heat and light resistance of the pigment by acylation of the pigment with ferulic acid and salicylic acid. This reaction belongs to the acylation reaction of anthocyanin pigments, and acylated anthocyanins exhibit strong stability to changes in pH, heat treatment, light, etc. Il⁶-171. In addition to acylation, the processing technology can also be adjusted. According to the factors affecting the stability of the pigment, especially the pH, the stability of the pigment can be ensured by controlling the pH of the environment in which the pigment is located. Some people also use porous starch as an adsorbent and microencapsulate curcumin with gelatin.

The results show that the stability of microencapsulated curcumin to light, heat, pH, etc. has been significantly improved. 1 8 1.

3. Research on natural pigment dyeing

Natural pigments are used in the food industry and increasingly in the textile industry because they are biodegradable, mostly non-toxic and have no side effects and do not pollute the environment.

3.1 Dyeing protein fibres

At present, most protein fiber dyeing methods include three types: wool dyeing, silk dyeing, and hair dyeing. Take wool as an example. The macromolecules of wool fibers are mainly composed of polypeptide chains formed by α-amino acids linked by peptide bonds, which form salt bonds, disulfide cross-links, and hydrogen bonds. These spatial lateral bonds combine with each other by molecular attraction, salt bonds, disulfide bonds, and hydrogen bonds, etc., to form a relatively stable spatial helical structure. called α-keratin. Under certain conditions, when subjected to tension, the macromolecular chain stretches and transforms into β-keratin. After the tension is removed, under certain conditions, it returns to its original bent state – α-keratin, and sometimes even undergoes overshrinking.

Yu Boling, Li Qingrong 9 and others found through research on dyeing experiments with 10 natural pigments such as gardenia, turmeric, natural brown, cocoa, tomato, red yeast, sorghum red, paprika red, tea and coffee that they are all ideal pigments for dyeing wool. The direct dyeing of soaping fastness can reach more than level 3. Turmeric and natural brown can be used with aluminum, iron and and natural brown use metals such as aluminum, iron and copper as mordant agents. The washing fastness can be increased to level 4, but the 10 pigments are relatively poor at dyeing silk compared to wool. It can be seen that although silk and wool are both protein fibres, the colouring mechanism of natural pigments on the two is different, resulting in the difference in the colour fastness to soap after the two are dyed. However, there is currently no literature reporting on the different principles of natural pigments for dyeing these two protein fibres, and research in this area will be of great significance for the study of the colouring principles of natural pigments.

Jia Yanmei [et al. explored the effects of temperature, pH, dyeing time, mordant, etc. on the effect of black rice pigment dyeing wool, and found that black rice pigment is more stable under acidic conditions, so it should be dyed under acidic conditions. Different metal ions will cause black rice pigment to produce different colors. Prolonging the dyeing time will deepen the pigment color within a certain range. Raising the temperature is beneficial for dyeing, but too high a temperature will also cause the pigment to decompose.  Li Qian 12 concluded through experimentation that the optimal dyeing conditions for dyeing silk with turmeric pigment are pH = 4, dyeing temperature 70 degrees, dyeing time 60 minutes, electrolyte Na₂SO₄ dosage 2g/L, lanthanum-rich rare earth chloride dosage 2g/L, and the post-mordant method. Zhang Huan et al. 2 also verified that natural loess (containing a large number of metal ions such as Ca²⁴, Fe² and Mg² and no heavy metal ions such as lead, cobalt, chromium and nickel) is a good natural mordant for dyeing wool.

Wang Ru et al. [21 found through experiments that hematoxylin and catechin can color hair, and that adding different metal ions can produce different colors. They also studied the adsorption kinetics and thermodynamics of hematoxylin on hair to explore the mechanism of the hair dyeing process. They confirmed that the adsorption of hematoxylin on hair is spontaneous, exothermic, and entropy-increasing, and that low temperatures are conducive to the adsorption of hematoxylin on hair. and when the hair is stained with hematoxylin, if the dye is acidic, hematoxylin mainly reacts with the amino groups of the hair, and if it is alkaline, it reacts with the carboxyl groups. Under alkaline conditions, the structure of the interaction between the hair and hematoxylin is more stable. Although black rice pigment, ginger yellow pigment, umber pigment, and catechin pigment are different pigments, research results show that the dyeing of all five pigments is related to factors that affect their stability. Therefore, research to improve the stability of natural pigments is of great significance.

3.2 Dyeing of cellulose fibers

Cellulose fibers, taking linen fibers as an example, are found in the bast of the flax plant. In the cross-section of the flax stem, 30 to 50 individual fibers are glued together with a mucilage to form a fiber bundle. The fibers overlap each other to form a network structure in the bast. From the molecular structure of flax, it is possible to carry out at least the following two types of chemical reactions: one type is related to the glycosidic bond linking the glucose residue to the molecular structure of the flax, which is mainly the interaction of the hydrolytic agent with the glycosidic bond, which under certain conditions causes the glycosidic bond to break, reducing the molecular weight; the other type is related to the three free hydroxyl groups on the glucose residue in the molecular structure of the flax. Some people use cationic modification technology to chemically bond reactive ion modifiers to the hydroxyl groups on the fiber, which causes the fiber surface to become positively charged.

The modified linen fabric is then dyed with purple sweet potato red pigment. The dyed fabric has good rubbing fastness, but the washing fastness is slightly worse2. Yu Fei et al. 2 used cationic groups such as quaternary ammonium salts, reactive tertiary ammonium salts, and reactive groups such as epoxy groups to react with the hydroxyl groups on the cotton fiber macromolecules under alkaline conditions, and used food coloring to dye the modified cotton fabric.  The results show that the dyeing depth is high, the reagent is safe, the process is simple, and the high dyeing rate also makes it easy to purify the dyeing residual liquid. It is an efficient, energy-saving and environmentally-friendly dyeing process. It has also been found that its dyeing mechanism is similar to that of acid dyes on wool – the fiber is connected to the modifier by an ether bond, and the modifier is connected to the dye by an ionic bond, which gives a high fastness.

3.3 Dyeing synthetic fibers

Since many natural dyes have many hydroxyl groups and are highly hydrophilic, while synthetic fibers such as nylon and polyester are highly hydrophobic, there have been few reports of natural dyes being used on synthetic fibers. Zeng Ke, Wang Xiangrong125 and others have studied the dyeing of polyamide with chestnut shell pigments, and have shown that chestnut shell pigments dyeing polyamide fabrics have good rubbing, washing and light fastness, with grades generally around 4. There is also a certain change in the shade of the dyed fabric after coal dyeing.

4. Extraction of natural pigments

Natural pigments are now mainly obtained from plant and animal materials, mainly by conventional methods such as soaking, mashing and filtering, leaching with organic solvents, ultrasonic extraction, microwave extraction, enzymatic methods, supercritical CO₂ extraction, and only in recent years, the high-speed countercurrent chromatography method. Emulsion method 126]. High-speed countercurrent chromatography 127 is based on the distribution of a sample between two complementary miscible solvents. The individual components of the solute are separated according to their different distribution coefficients during the process of passing through the two solvent phases. Compared with other column chromatography, high-speed countercurrent chromatography does not have the problems of adsorption loss caused by solid carriers, sample denaturation, contamination and tailing distortion of the chromatographic peak shape. Therefore, in recent years, high-speed countercurrent chromatography has been widely used in the separation and preparation of natural products.

Emulsion method [28] By emulsion polymerization of acrylic acid, butyl acrylate, styrene, n-butanol, distilled water, sodium hydroxide, and potassium persulfate, a kind of polymeric surfactant was synthesized, and then a series of fat-soluble natural pigments were extracted. This method not only avoids the disadvantages of organic solvent extraction, such as high toxicity, heavy pollution, and high price, but also ensures that the extraction rate is not lower than that of organic solvent extraction. Although the technology for extracting pigments from plants and animals is well developed, the growth and reproduction of plant and animal materials are affected by factors such as season, climate, and place of origin, resulting in insufficient raw materials and high prices.

In addition, compared with foreign pigment production, pigment production in China generally has higher costs and lower extraction efficiency. In the development and utilization of pigment resources, many units only focus on development and do not pay attention to protection, resulting in resource depletion. Therefore, the production of natural pigments from microbial resources has shown its advantages. Microbial production of natural pigments can overcome the shortcomings of using plants and animals as raw materials for natural pigments, while also achieving industrialized production to meet market demand. The current situation of microbial production of natural pigments is summarized as follows:

Guo Fenghua29 isolated a strain from the soil that can produce yellow and blue pigments. The yellow pigment is more stable than the blue pigment, and preliminary research has concluded that the two pigments are non-toxic. Li Yiwei 30 discovered a strain that produces blue pigment. Through the analysis of the morphology, culture characteristics and physiological and biochemical properties of the strain, it was determined that the strain belongs to the genus Streptomyces and was named Streptomyces zhuhaiensis. The blue pigment produced is water-soluble and stable to temperature, sunlight, most metal ions, ascorbic acid and acidity regulators. It has no toxic side effects.

Wang Xiaodong studied the natural pigments in the mycelia of the strains RCEF4585, isolated from Cordyceps sinensis, RCEF4337, isolated from Cordyceps bisporus, and RCEF4022, isolated from Cordyceps coralina. It was found that the crude methanol extracts of strains RCEF4585 and RCEF4337 can produce a hydroxyanthraquinone pigment. The main pigment compound in the crude ethyl acetate extract of RCEF4337 is C₃OH₁₈O₁₃. It is tentatively judged that this compound is a new compound. The mycelium extract of strain RCEF4022 is an anthraquinone substance. Lou Zhihua [321 analyzed the liquid of Phellinus linteus and preliminarily identified the pigment produced by the liquid as an anthraquinone pigment through techniques such as chemical reagent color development.

5. Conclusion

Although there is currently a wide range of research on natural pigments, given the wide variety of natural pigments, most of the current research is limited to the study of a single pigment. There is less research on kinetics, and it is limited to qualitative research. The chemical structure and reaction process are not linked together, and the problem cannot be solved fundamentally. There is also a lack of comprehensive research on a class of pigments. Therefore, these aspects should be the focus of future research.

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