Study on Natural Pigments and Natural Dyes Development

Natural pigments and natural dyes both come from basically the same sources, most of which are extracted from plants. Both use extracted pigments as the element used, and are natural, non-toxic, harmless, environmentally friendly, renewable, and have no pollution. Entering the 21st century, natural pigments and natural dyes have become the dominant players in the international market, growing at a rate of more than 10% per year. With the increasing call for a return to nature, it is imperative to develop safe, economical and effective natural pigments and natural dyes.

1 Overview of natural pigments

Food coloring, also known as a colorant, is a very important food additive in food. It not only gives food a colorful appearance, stimulates the senses and increases appetite, but also gives people a sense of beauty and is the basis for assessing the quality of food. Food coloring can be divided into natural coloring and synthetic coloring according to its source.

1.1 Background

As the amount of synthetic coloring used increases and as society develops and people's living standards improve, more and more people are asking whether the use of synthetic coloring in food can pose a risk to human health. At the same time, a large number of research reports have pointed out that almost all synthetic coloring does not provide nutrients to the body, and some synthetic coloring can even be harmful to human health. As a result, people's attention has gradually been drawn to the issue of safety. As a result, many regulations have been introduced in countries around the world, such as the sharp reduction in the number of approved synthetic colors in the United States from 700 to 7. However, food adulteration continued for many years. Together with the media, which in recent years has been critical of food coloring and health, they have contributed to consumers' concern about food coloring additives. Natural coloring began to be widely used commercially 25 years ago and has since made great progress. Consumers' interest in natural products from a health perspective has driven the demand for natural coloring in the food industry [1].

1.2 History

The history of adding colorants to food can be traced back to ancient Egypt, where local confectioners used natural extracts and wine to improve the color of their sweets around 1500 BC. By the mid-nineteenth century, a spice called saffron was added to certain foods for decorative purposes. With the Industrial Revolution, the food industry developed rapidly. Mineral and metal compounds were also used as colorants to disguise poor-quality and adulterated foods. More terrible examples include: Pb3O4 and HgS were used as colorants for cheese and candy, and blackened tea could be recolored with arsenate and then sold on the market. In 1860, the use of colorants in desserts caused two deaths. Some toxic chemicals were also used as colorants in candy and pickles. In 1856, Sir W illian Henry P erkin invented the first synthetic color (mauvine). At the end of the 19th century and the beginning of the 20th century, unapproved color additives were widely used in various popular foods in the European and American markets, including ketchup, mustard, jelly and alcohol. More than 80 synthetic colors appeared on the market, some of which were used in the textile industry, not the food industry. Many food colors have never been tested for toxicological and other negative effects. Since 1900, a large number of synthetic colors have been produced using aniline as a raw material, a toxic petroleum product. Initially, these colors were called coal tar colors, as the raw material for synthesis came from coal containing asphalt. Early pigments derived from vegetable, animal and mineral sources were still used in food at the beginning of the 20th century, but were gradually phased out by manufacturers driven by economic interests. Chemically synthesized pigments are simple to produce, inexpensive, have very good coloring properties and can be used in small quantities without imparting an undesirable flavor to the food.

1.3 Current situation and outlook

Pigments are found in abundance in fruits, vegetables, seeds and roots. We consume large quantities of various pigments in our diet every day, especially anthocyanins, carotenoids (nature produces more than 1 million tons each year, of which the structure of 600 species has been confirmed) and chlorophyll. The natural pigments we ingest in our food every day are only a very small part of the total. The physical and chemical properties of natural pigments vary greatly. Many are sensitive to oxidation, pH changes and light, and their solubility varies greatly. Currently, 13 natural colors are approved for use in Europe, while the US has approved 26. Natural colors were initially thought to be far less stable than synthetic colors, more difficult to use and more expensive.

It is estimated that 70% of the world's plants have not been fully researched, and only 0.5% have been thoroughly researched. From this perspective, our research on natural food colors has only just begun. However, in reality, it is not easy to develop new colors. Before any new color can be approved for use by the FDA and in Europe, it must undergo a safety assessment, which takes a lot of time and money. Many undiscovered colors may be located in undeveloped land and oceans, and it is difficult to commercially develop them. In view of these difficulties, current research into natural colors is focused on the development of existing colors, mainly in the following three areas: synthesis processes, production techniques and alternative sources of color. These approaches have proven very successful and have greatly facilitated the use of natural colors in the food and beverage industry. Food manufacturers are optimistic that most natural colors offered by color suppliers meet requirements in terms of color, stability and use properties.

2 Overview of natural dyes

2.1 Definition

Natural dyes are dyes obtained from plants, animals or mineral resources that have not been synthesized artificially and have undergone little or no chemical processing.

2.2 Classification

(1) Natural dyes can be divided into plant dyes, animal dyes and mineral dyes according to their sources, with plant dyes being the main ones. Plant dyes include madder, woad, sumac, indigo, safflower, pomegranate, yellow gardenia, tea, coneflower, yarrow, persimmon, comfrey and sumac. Due to the different molecular structures of natural dyes, the dyeing methods also vary greatly. For protein fibers and cellulose fibers, the dyeing methods mainly include mordant-free dyeing, dyeing first and then mordanting, and mordanting first and then dyeing. For synthetic fibers, they are mainly divided into atmospheric pressure dyeing and high temperature and high pressure dyeing. The best dyeing process should be determined according to the nature of the dye. In particular, it should be noted that when the same natural dye is dyed with a different mordant, the dyed fiber often appears a different hue [2].

(2) Natural dyes have a variety of chemical structures, which can be divided into seven categories according to chemical composition: carotenoid, anthraquinone, naphthoquinone, flavonoid, curcuminoid, indigo and chlorophyll. Carotenoid dyes are named after the carrot, and have a long conjugated double bond (a double bond) in their structure. They are orange in color, and the main dyes are annatto and saffron. Red dyes all contain an anthraquinone structure, and most of them are extracted from plants, insects or animals. They are known for their good light fastness. Flavonoids are colorless organic compounds. Most natural yellow dyes are hydroxyl or methoxy substituents of flavonoids or flavonols.

(3) Classification by color refers to the classification of natural dyes according to the color of the dyed fibers or fabrics: there are red, yellow, blue, brown and other natural dyes. Among the natural dyes that have been counted, yellow and red varieties are the most common, while blue, green and black are the least common. Red dyes are not as abundant in nature as yellow dyes. Most red dyes are hidden in the roots of plants, the bark of trees or the dark grey bodies of insects. Despite the limited sources of red dyes, they are present in large quantities in the color bases of a plant and are therefore easy to extract. Carmine, the most beautiful red natural dye, is found in the body of the carmine bug that parasitises on cacti [3], but the more important and commonly used natural red dyes are madder and lac. Yellow is the most vivid and the most abundant of all the hues found in nature. There are far more plants that produce yellow dyes than those that produce dyes of other colours. Turmeric dye is a derivative of turmeric and is one of the most famous and vibrant yellow dyes of natural origin. Indigo has been one of the main and most commonly used blue dyes since ancient times. It is extracted from the leaves of a plant that grows in Asia, Africa, the Philippines and the United States. Catechu brown dye is extracted from the wood of a tree called catechu, which grows mainly in India.

2.3 History of natural dyes

Legend has it that as long ago as the time of the Yellow Emperor more than 4,500 years ago, people were already able to use plant juices to dye. Strong evidence for this is that in 1959 archaeologists discovered dyed silk threads in the tomb of the Yin Dynasty at Wangyukou, Anyang, Henan Province. This shows that people were already dyeing at least 3,000 years ago during the Yin Dynasty. At that time, people used natural dyes, mainly plant juices, and some also used insect blood to dye things. For thousands of years, the Chinese people have used plant dyes extensively. They have accumulated a lot of experience, such as turmeric extract from ginger juice, cochineal extract from cochineal insects, and madder extract from madder.

Since W. P erkin invented synthetic pigments in 1856, natural dyes have gradually lost their importance as textile dyes because they are time-consuming and laborious to use and cannot be pre-prepared as ready-to-use products. Nowadays, most of the dyes used on the market are synthetic dyes. Some synthetic dyes pose a threat to human health. Among synthetic dyes, some azo dyes can break down into toxic aromatic amines, pentachlorophenol, and some dyes are likely to cause skin allergies. If inappropriate dyes are used in the early production process of clothing, it may pose a carcinogenic risk to the wearer. EU countries have begun to legislate on the banning of azo dyes, and exported textiles are subject to strict environmental certification inspections. Germany banned certain azo dyes in 1996, having found that they are carcinogenic, teratogenic and allergenic. Therefore, people have turned their attention to “environmentally friendly” dyes. The reintroduction of natural environmentally friendly dyes has opened up business opportunities for the textile industry to use these dyes to open up new markets for naturally dyed products.

Natural plant dyes are not only non-toxic and harmless, they are truly environmentally friendly products. They also have a wide range of health care benefits, catering to the modern consumer's psychology of returning to nature and maintaining good health. Plant dyes are made from raw materials such as Chinese herbal medicine, flowers, tea and fruit, which are unique to China. They are refined through many processes such as crushing, soaking and cooking, and are pure natural plant dyes. They do not contain any chemicals, are non-toxic and harmless, and textiles and clothing dyed with them will not cause any harm to human health.

2.4 Advantages of natural dyes

The characteristics of natural dyeing are that in addition to the natural hue, some plants such as madder and indigo, etc., have insect-repellent and antibacterial effects when used to dye fabrics with natural dyes extracted from them. Coupled with the natural fragrance of natural dye-dyed fabrics, they are very popular with consumers. Although natural dyes cannot completely replace synthetic dyes, they have a place in the market and are increasingly valued by people, with broad prospects for development. The market for natural textile products is also a huge market that has yet to be fully developed. Making full use of these natural resources requires the concerted efforts of many parties to develop corresponding technologies, processes and products.

The main advantages of natural dyes are that they are non-toxic and harmless, non-allergenic and non-carcinogenic to the skin, and have good biodegradability and environmental compatibility. Their colors are soft, natural and distinctive, and they have broad development prospects in the fields of high-end silk products, health underwear, home textile products, decorative products and other fields. In the past 10 years, the Korean government has spent nearly 1 million US dollars on research and development of natural dyeing projects to promote the development of new ecological dyeing and finishing technologies. At present, great progress has been made in the research of natural dye extraction technology, level dyeing and improving light fastness, and some natural dye extraction processes have been industrialized. The development of natural dyes is not only beneficial to the protection of natural resources and the ecological environment, but also has broad development prospects for the development of some high value-added textiles [4-6].

2.5 Prospects

China's research and application of natural dyes is similar to the international level, unlike other industrial fields where there is a large gap. The core value of the application of natural dyes is its safety and biocompatibility. Although it is currently unrealistic to commercialize and completely replace synthetic dyes, it is necessary to inject new technology into the acquisition and dyeing of natural dyes, adopt modern equipment, and accelerate the pace of industrialization. It is believed that natural dyes will make the world more colorful [7].

3 The connection and difference between natural pigments and natural dyes

Natural pigments and natural dyes both come from basically the same source, mostly plants. Both are extracted pigments used as elements, and are natural. They are non-toxic, harmless and environmentally friendly. They can be regenerated and are pollution-free. The difference lies in the different objects of application and requirements, which results in a significant difference between the two. Natural pigments are mainly used in the food industry, where high safety requirements are needed. Some plant pigments cannot be used in the food industry or cosmetics industry, such as water white gourd, chestnut shell, and bindweed. Natural dyes are mainly used for dyeing textiles, and require washability, rubbing fastness, and washing and soap fastness. Some natural pigments do not meet these requirements. For example, carotene and anthocyanin, which are widely found in colored vegetables and fruits, are natural pigments, but they do not adhere well to textiles. Once washed, the pigments are gone, so they do not meet the requirements of natural dyes [8]. Natural pigments can be directly incorporated into foods to achieve coloration. However, many natural dyes are not easily dyed directly and require the help of mordants to achieve the desired effect.

4 Problems in the application process and possible solutions

Although natural pigments (dyes) have broad prospects for development, it is currently unrealistic to commercialize them and completely replace synthetic pigments (dyes) because there are many problems with their application.

4.1 Problems with the supply of raw materials

Natural dyes are mostly derived from animals and plants, which makes it difficult to produce them in a standardized way. Take plant dyes as an example. Even for the same plant, the place of origin, climatic conditions and time of collection will affect the composition and color of the pigment. This will inevitably lead to poor reproducibility of the dyeing. Natural dyes are difficult to produce in large quantities. Since the pigment content of plants is relatively small, a large number of plants must be picked or cut down to obtain enough dye. This will cause damage to the ecological environment, which defeats the original intention of using natural dyes for dyeing, which is to be eco-friendly and environmentally friendly. Many natural plant dyes are also Chinese herbal medicine resources with high medicinal and economic value. It is also not economical to use them for dye extraction in large quantities.

One way to solve the problem of raw material supply is to use bioengineering methods to cultivate plants. At present, using bioengineering methods, various plants such as comfrey and madder have been cultivated artificially. Because the method of biological culture can greatly accelerate cell growth, the production of natural dyes can be independent of plants in nature, and the output can be greatly increased. Secondly, the development of natural dye analogs is also of great value. This dye has the same structure as a certain natural dye and is an isomer. At the same time, it is produced by chemical synthesis, which has high purity, stable performance, abundant raw materials, does not compete with food for land, and can be produced on a large scale. As long as it does not contain harmful impurities such as heavy metals, its safety is not a problem, and the cost is much lower than that of naturally extracted dyes. Therefore, it is worth studying the varieties of natural dyes with excellent performance, determining their structures, and then researching their synthesis methods, gradually achieving large-scale industrial production, in order to replace the cultivation and extraction of natural dyes and synthetic dyes with questionable safety.

4.2 Extraction and preparation issues

Traditional methods for preparing natural pigments (dyes) include water extraction, distillation, and column chromatography. The disadvantages of these traditional methods are that the obtained pigment (dye) particles are large and the color fastness is not good. In order to improve the efficiency of pigment extraction, researchers used ethanol instead of water as a solvent. After the plant was crushed, it was placed in an airtight container, 95% ethanol by volume was poured in, and the mixture was allowed to macerate for 24 hours. After 24 hours, the solution was poured out, and the same ethanol was used to macerate for another 6 hours, repeating the process twice. Finally, all the solutions were mixed, filtered, and the resulting solution was used as a dye liquor. This method is very suitable for poorly soluble dyes. The dyeing effect can be improved by treating the dyeing solution with ultrasound and microwaves [9-11]. There is evidence that ultrasound and microwave experiments on the dyeing of cotton fabrics with natural dyes in the presence of mordants have shown that ultrasound and microwave oscillation accelerates the penetration of the dye and improves the depth of dyeing. In addition, the application of membrane separation technology combining ultrafiltration and nanofiltration to the purification of natural pigments (dyes) can also improve the dyeing effect.

4.3 Limitations of changing application performance

Natural pigments (dyes) have very little affinity for textile fibers, which leads to poor dyeing fastness, and it is difficult to meet the requirements even with the use of various mordants, especially light fastness and soap fastness [12-14]. Traditional natural dyeing methods also have problems such as low color yield and long dyeing times. In addition, most natural dyes require the use of mordants during dyeing, and most traditional mordants contain heavy metal ions, many of which are on the list of banned substances for ecological textiles. For this reason, Chinese dyeing workers have used rare earth-citric acid complexes as mordants for natural dyes to dye ramie fibres. Rare earth ions can complex dye ions as central ions and as ligands. In addition, it also has a similar electrolyte effect and promotes dyeing. After treatment, the surface of the cellulose fiber has pores of different sizes, so that the rare earth ions can enter the non-crystalline region of the fiber and even the edge of the crystalline region, forming a multi-component complex with the dye molecules and cellulose molecules, thereby improving the dyeing fastness. The formation of this multi-component complex system allows the dye to resist color changes caused by fluctuations in the pH of the solution [15-20].

5 Conclusion

Natural pigments (dyes) are loved by people all over the world for their natural hues, insect-repellent and antibacterial properties, and natural aromas. They have a bright future. However, it is still unrealistic to commercialize them and completely replace synthetic dyes. New technology must be used to extract and dye natural pigments (dyes), modern equipment must be used, and the speed of industrialization must be accelerated. It is believed that natural pigments (dyes) will make the world more colorful.

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