The Character and Types of Natural Food Coloring

Natural food coloring is a product obtained by extracting and refining natural raw materials (mainly plant materials). It is a natural food additive used for coloring food. Food coloring can be divided into natural pigments and synthetic pigments according to their source and nature. In China, 65 pigments have been approved for use in the food industry, including 48 plant pigments. At present, natural pigments account for 90% of the food coloring produced in China each year; of these, 20% are natural pigments extracted from plants and through microbial fermentation, with the remainder being caramel pigments. Overseas, especially in developed countries such as Europe and the United States, many new natural pigments have been developed for use in the food processing industry[1]. Natural food coloring has become the mainstream of future development. In recent years, with the development of society and the economy and the improvement of dietary standards, the food and beverage industry has increasingly high requirements for food coloring. Therefore, the stability and processing properties of food coloring are becoming increasingly important. New coloring agents are also being developed through the development and use of new technologies and processes to meet the needs of the food processing industry.

1 Characteristics of natural food coloring

Most natural colors come from edible plant and animal tissues. Compared with synthetic colors, natural colors have the characteristics of low side effects and high safety. Some natural colors also have health and nutritional benefits. Because natural colors can retain the color of the natural product and some can add a pleasant aroma to the food, they are more popular with consumers.

Plant pigments can add color and are similar to natural colors, which is a natural beauty. However, plant pigments are present in small amounts in plants and are difficult to isolate and purify. In most cases, the production cost of natural pigments is much higher than that of synthetic pigments, and it is greatly affected by the type of plant, its place of origin and the growing season. There are many types of plant pigments and their properties are complex. Some pigments may also produce peculiar or unpleasant odors in the presence of co-existing substances. Compared with synthetic pigments, most plant pigments have relatively weak coloring strength, are not easy to dye evenly, and are not as bright and vivid as synthetic pigments. They are also more sensitive to light, heat, oxygen, metal ions and changes in pH value, and have poorer stability. In use, some plant pigments need to be added with pigment stabilizers and antioxidants to improve the shelf life of the product. For a plant pigment, due to its own physical and chemical properties, it has strong specificity and a narrow range of applications. Therefore, the breadth of application of pigments is of great concern when researching and developing natural pigments.

2. Ways to produce natural food coloring

The main methods of producing natural food coloring are direct extraction, synthesis and production using biotechnology. Currently, the direct extraction method is mainly used to produce natural pigments. The synthetic method is currently limited to individual pigments such as carotene, which have a stable chemical composition and a relatively simple molecular structure. Most natural pigments are difficult to chemically synthesize due to their complex molecular structures and unstable physicochemical properties. Therefore, people are working hard to find new natural pigment resources in minerals, microorganisms, and especially in animals and plants.

In recent years, with the development of biotechnology, a whole new field has opened up for the production of natural pigments. At present, the use of transgenic plant cell culture and plant tissue culture technology to produce food pigments has shown great potential [2, 3]. Natural pigments are generally secondary metabolites of plants and microorganisms, and their content in plants and microorganisms is normally very low. The use of genetic engineering to create transgenic plants and transgenic high-efficiency bacteria that artificially alter the metabolic pathways of plants and microorganisms in order to increase secondary metabolites such as pigments is a new way to increase the production of natural pigments using biotechnology. Many natural pigments, such as carotenoids and red yeast pigments, can now be mass-produced through the fermentation of genetically modified microorganisms.

Natural food coloring is extracted mainly by solvent extraction, a process that includes steps such as pre-treatment, crushing, leaching, filtering, concentrating and drying of the raw materials. The solvent used depends on the physical and chemical properties of the pigment and the source of the pigment. Commonly used solvents in natural pigment extraction include organic solvents such as acetone, ethanol, alkanes and benzene, as well as acids, alkali solutions, fats and oils, carbon dioxide and water. In recent years, a new supercritical fluid extraction technology has been developed and applied to pigment extraction with remarkable results.

3 Types of natural food coloring

Natural food coloring is divided into plant pigments, animal pigments, microbial pigments and mineral pigments according to their source of raw materials; according to their chemical structure, they are divided into anthocyanins, carotenoids, chlorophylls and hemoglobins; according to the different shades of the pigments, they can be divided into reddish-purple series, yellow-orange series and blue-green series. This article focuses on the chemical composition and main properties of pigments.

3.1 Plant anthocyanins

Anthocyanins, also known as anthocyanidins, exist in the form of glycosides, i.e. sugar esters, in plant cells. The different anthocyanins contained in different tissues and organs of plants, such as stems, leaves and flowers, give them different colors. The disadvantage of plant anthocyanins is that their color is sensitive to changes in the acid-base environment and to changes in light and heat conditions. The advantage is that they are water-soluble and relatively stable to changes in light and heat conditions under slightly acidic conditions. For example, adding a certain amount of citric acid to the anthocyanin solution can enhance its stability. Nature is rich in plant resources, providing ample raw materials for the extraction of anthocyanins. In particular, the extraction of plant pigments is mainly a comprehensive utilization of agricultural and sideline products, which are inexpensive and low in cost, such as corn pigments, red rice pigments, black soybean hulls pigments, carotene, grape skin pigments, rose pigments, citrus pigments, etc. [4, 5]. Pigment extracts from fruits not only have a fruity aroma, but also contain certain nutrients. They are commonly used as flavor additives in beverages and coloring agents in candy and fruit wine.

3.2 Plant chlorophyll

Chlorophyll is an important component of plant chloroplasts. It is widely found in green plants and algae, and gives higher plants and green algae their green color. In clinical practice, chlorophyll has the effect of activating cells, promoting blood production and replenishing blood, and fighting infection. In recent years, it has also been found to inhibit the growth of cancer cells. Chlorophyll is a porphyrin compound that is divided into chlorophyll a and chlorophyll b. It is insoluble in water but soluble in organic solvents such as alcohol, acetone and petroleum ether.

Chlorophyll is stable under alkaline conditions, but under acidic conditions its Mg2+ is easily replaced by hydrogen ions, thus losing its green color [6, 7]. Therefore, to keep fresh green vegetables green, they can be frozen or freeze-dried, or blanched in 60-75 °C hot water, which can delay or reduce the oxidation and discoloration caused by high temperatures. Since chlorophyll is easily broken down by light, it is made into copper or sodium chlorophyllin in most cases to maintain its stability. Among food additives, the blue-green pigment series belongs to this type of product; other similar products include algin blue and gardenia blue. Copper chlorophyllin sodium salt is soluble in water, ethanol and fat, but it is unstable to light, heat and acids and is also expensive. However, it is difficult to replace it with other natural pigments because of its bright colour.

3.3 Plant carotenoids

Carotenoids are a class of coloured substances widely found in plants. There are currently more than 600 types of carotenoids. Different carotenoids have different molecular structures. Generally speaking, carotenoids are isoprene polymers, which chemically consist of a carbon skeleton with eight isoprene chains and contain conjugated double bonds. The carotenoids found in plant chloroplasts contain two pigments, namely carotene and lutein. The former is orange-yellow and the latter is yellow. There are three main isomers of carotene: α, β and γ. Carotenoids are soluble in fat but not in water. Physicochemically, they are relatively stable and only react weakly to acids, bases and high temperatures. However, they are easily broken down by light, oxidants and enzymes. Carotenoids are found in abundance in green vegetables, yellow fruits and vegetables and palm oil.

Among carotenoids, those that can be converted to retinol are called provitamin A, and the one with the highest biological potency is β-carotene. Lycopene, which is found in tomatoes, watermelons and guavas, is also a carotenoid but cannot be converted to vitamin A. Due to their strong antioxidant activity, carotenoids are often used as food antioxidants in the food industry, as they are 100 times more effective than VE. Carotenoids can effectively prevent the oxidation of low-density lipoprotein, protect the skin from ultraviolet damage, inhibit the growth and spread of cancer cells, and prevent the occurrence and deterioration of prostate cancer. A high intake of carotenoids can also reduce retinal macular degeneration and age-related prostate diseases[8~10] .

In addition to carotene and lutein, carotenoids also include capsanthin and gardenia yellow. Capsanthin is a fat-soluble pigment extracted from chili peppers that is reddish-orange in color. The pigments contained in chili peppers, in addition to capsanthin, also include capsaric acid, which is collectively referred to as capsanthin in a broad sense. Capsanthin has the advantages of being heat-, acid- and light-resistant, and is suitable for coloring foods such as pastries, margarine, canned goods, beverages and dairy products. Since chili peppers are themselves spices with a strong spicy flavor, the chili peppers need to be de-spiced during the extraction of capsanthin. Gardenia yellow pigment is a pigment extracted from the fruit of the gardenia jasminoides plant in the rubiaceae family. It is easily soluble in water, insoluble in oil, has little effect on hue depending on pH, is thermally stable, slightly less light stable, and quite stable to metal ions. It is hardly affected by aluminium, calcium, lead, copper, tin, etc., but turns black when exposed to iron. Gardenia yellow pigment is suitable for use in pastries, cold drinks, sweets, dairy products, etc. According to incomplete statistics, there are nearly 20 factories in Japan producing gardenia yellow pigment, and most of the raw materials are imported from China. China is rich in gardenia resources, and it grows in large quantities in provinces south of the Yangtze River, so it is worth developing.

3.4 Flavonoids

Flavonoids are widely found in plants and have many derivatives. So far, thousands of flavonoids have been discovered. The types of flavonoids contained in plants of different orders, families, genera and species are different, and the types of flavonoids contained in different tissues and organs of plants, such as flowers and roots, are also different. Generally speaking, cereals, vegetables and fruits have a high content of flavonoid pigments. Flavonoids are water-soluble phenolic compounds. In their molecular structure, a three-carbon link (C6 -C3 -C6 ) connects two benzene rings. Flavonoids have physiological activities such as antioxidant activity and free radical scavenging, and are effective in preventing cardiovascular disease and in fighting bacteria, viruses and allergies. Isoflavones, which are also contained in soybeans, have a positive effect on the treatment and prevention of diseases associated with low hormone levels, such as elevated blood lipids, osteoporosis and menopausal syndrome [11-13]. Some flavonoids can enhance the anti-hormone capacity of people with high estrogen levels, and are effective in preventing and treating various diseases such as lung, colon, skin, endometrial, breast, prostate, other cardiovascular diseases and leukemia, and can inhibit the growth of cancer cells.

3.5 Red yeast rice

Monascus Red is the trade name of the red yeast pigment, a natural food coloring agent secreted by Monascus mycelium. It is a secondary metabolite of Monascus. Fermenting Monascus with glutinous and non-glutinous rice can produce a large amount of red yeast pigment. Unprocessed red yeast rice can be directly used for coloring food. Monascus pigment is a complex substance, the red color of which is produced by a mixture of six components. Among these, monascorubrin and monascorubrin are reddish purple, monascorubrin and monascorubrin are reddish orange, and monascorubrin and monascorubrin are yellow. Compared with other natural pigments, red yeast rice pigment has strong coloring power, especially stronger coloring power on protein. It is also light and heat resistant, and has many other advantages such as resistance to acids and alkalis, redox agents and changes in metal ions. At present, red yeast rice pigment is widely used in coloring meat, aquatic products, fermented foods, soy products and wine[14~16] .

3.6 Caramel pigment

Caramel coloring is a water-soluble pigment derived from the dehydration and condensation of sugars. Depending on the production process, the product is divided into two categories: ammonium salt and non-ammonium salt. Ammonium salt caramel has the advantages of a better color, stability to acid and alkali, heat resistance, light resistance, simple processing methods, and a high yield recovery rate. Caramel color is widely used in coloring a variety of beverages, snacks, and even products such as soy sauce [17, 18].

3.7 Turmeric pigment

This is a pigment extracted from the underground rhizomes of plants in the ginger family, Curcuma. Its main component is curcumin. It is insoluble in cold water, soluble in ethanol and propylene glycol, and easily soluble in glacial acetic acid and lye. Its advantages are high color value, strong coloring power, thermal stability, and good light resistance of its alcoholic solution. It is suitable for use in candy, pastries, condiments, etc. [19, 20]. China has abundant turmeric resources, which are mainly produced in Sichuan, Fujian and other places and should be vigorously developed. In addition, the yellow-orange pigment series also includes paprika yellow pigment, safflower yellow pigment, corn yellow pigment, nuclear yellow pigment, annatto orange pigment, rehmannia yellow pigment, citrus pigment, etc.

3.8 Other

In addition to the types of pigments described above, there are currently pigments in the research stage in China, and some pigments are already produced in small quantities, such as betalain extracted from the tubers of red beet of the family Chenopodiaceae; shiso pigment extracted from the leaves of shiso of the family Lamiaceae; red cabbage pigment extracted from the leaves of purple cabbage; sorghum pigment extracted from the hull of sorghum of the family Gramineae; red rice pigment extracted from red rice; cocoa pigment made from the fermented and roasted ripe seeds of the cocoa tree; red pumpkin pigment extracted from red pumpkins; and amaranth red pigment extracted from red amaranth eaten by the amaranth family, etc. [21~23].

4 Prospects for the development of natural food coloring

As people's quality of life improves and their awareness of health care grows, the safety and nutritional value of food coloring are increasingly important. Compared with synthetic coloring, natural coloring has many advantages, such as fewer side effects and higher safety. Some natural coloring also has health and nutritional benefits, and natural plant pigments are naturally colored. In China, both biological and non-biological resources are abundant, and plant resources are found all over the country. Many of these plants are not only traditional Chinese medicinal herbs, but also high-quality raw materials for extracting various pigments such as red, yellow, and purple.

In addition, the deep processing of agricultural and sideline products also produces a wide variety of pigments. Therefore, the reasonable development and utilization of China's natural resources and agricultural and sideline products will provide the food industry with natural pigments that are not only highly safe but also have certain nutritional value. In recent years, with the technological progress and the increasingly perfect process of refining and using natural pigments, natural pigments have accounted for an increasingly large proportion of the pigments used in the light industry, especially in the food processing industry. It is an inevitable trend for natural food additives used in the food industry to gradually replace synthetic food additives with natural pigments[24~26] . Therefore, as a food coloring additive, natural pigments have very broad development prospects.

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