Study on Plant Food Coloring

Food Coloring can improve the color of food and is an important part of food additives. Food Coloring is divided into two types: synthetic pigments and natural pigments. With the development of technology and people's increasing concern about health, it has been discovered that many varieties of synthetic pigments have serious chronic toxicity and carcinogenicity. Studies at home and abroad have found that natural pigments are not only highly safe, have a soft hue, and reproduce the colors of nature, but also some have a certain degree of physiological activity, and are considered functional natural food coloring, which improves consumer trust in food safety.

At present, countries around the world are vigorously developing and using natural food coloring, and synthetic coloring will gradually be replaced by natural coloring. Natural pigments are widely found in many kinds of organisms. According to their sources, they can be divided into mineral pigments, plant food coloring, animal pigments, microbial pigments, etc.[1]. Most mineral pigments are harmful to the human body and cannot be used for coloring food. Animal and microbial pigments are rarely used. Plant food coloring is the most widely used type, and is used very widely for coloring food and health food such as beverages, candy, pastries, and alcohol[2].

1 Chlorophyll

Chlorophyll is widely found in the leaves, fruits and algae of higher plants. It is an important component of plant chloroplasts and gives most leafy vegetables and unripe fruits their green color. Natural chlorophyll is a fat-soluble pigment that turns a natural green color in solution and changes color in an acidic environment. The porphyrin ring of chlorophyll is not stable with magnesium ions, so magnesium can be replaced with copper or iron. Copper chlorophyll is stable to light and heat and can be used at any pH value, while iron chlorophyll is slightly brownish and has a deodorizing effect.

Chlorophyll can not only bind to DNA, mRNA and proteins, regulate the expression of proteins related to differentiation, proliferation and apoptosis in cells, change the expression and activity of enzymes in cells, and reduce the transport capacity of cell membranes for some carcinogens, but also has antioxidant and free radical scavenging effects, thereby having anti-mutagenic and anti-tumor effects [3]. In addition, the chlorophyll and its derivative porphyrin ring structure is easily bound to complexes with a polycyclic structure, such as planar aromatic hydrocarbon carcinogens, by non-covalent bonds to form an inactive complex and lose its aggressiveness, thereby reducing the carcinogenic effect [4]. A recent study by researchers at the Lawrence Livermore National Laboratory in the United States has shown that substances such as chlorophyll in green vegetables can effectively reduce the toxicity of the carcinogen aflatoxin and reduce the body's absorption of aflatoxin.

At present, natural chlorophyll is mainly used in baked goods such as vegetable buns, dumplings and cakes. During the processing of fresh vegetables into dehydrated vegetables, the original bright colors fade due to high temperatures and drying. For example, chlorophyll is needed for the vegetable topping in instant noodles, which requires the color of chlorophyll. Of course, chlorophyll is also used in cold drinks, such as ice cream and beverages such as apple juice and kiwi juice.

2  Red pigment

2.1    Lycopene

Lycopene is a carotenoid found in high concentrations in fruits such as tomatoes and watermelons. It is a fat-soluble pigment that is red in color. It is insoluble in strongly polar solvents such as water, but soluble in weak polar or non-polar organic solvents such as ether, acetone, and carbon disulfide. It has been approved as a yellow/red food coloring solvent in Europe and Japan.

Lycopene is a strong antioxidant that quenches singlet oxygen and scavenges free radicals, preventing oxidative damage to lipoproteins and DNA and thus preventing the occurrence of cancer. It also inhibits the formation of oxidized LDL cholesterol products and prevents the occurrence of coronary heart disease. It is a functional natural pigment with great development potential[5]. Scientific research has shown that lycopene accounts for about 50% of the carotenoids in human serum, and is most easily absorbed, metabolized and utilized by the human body. It protects phagocytes from their own oxidative damage, promotes the proliferation of T and B lymphocytes, stimulates the function of effector T cells, has the effect of delaying aging and reducing the incidence of disease. Lycopene also promotes the secretion of interleukin 2 (IL-2) and interleukin 4 (IL-4), enhances humoral immunity, and improves the immunity of the elderly [6].

At present, in addition to the economically viable extraction methods and the changes in the processing that require further research and discussion, research on tomato breeding to increase the lycopene content in tomatoes is being carried out from two aspects: conventional and genetic breeding. It has been reported that the lycopene gene has been successfully transferred to E. coli and expressed.

2.2 Beet red pigment

Beet red pigment is the general term for the coloring compounds in red beets. It is composed of two types of compounds: red betaxanthin and yellow betalain. Beet red pigment is widely found in various plants such as the Chenopodiaceae, Myrtaceae, and Cactaceae. The most familiar of the Chenopodiaceae is red beet; the leaves of the Gesneriaceae genus Gesnerium, the petals of Portulaca, and the fruits, peel and flesh of cacti in the Cactaceae family are all rich in betalain.

Betalain is easily soluble in water and aqueous solvents, and is a water-soluble pigment. It is difficult to dissolve in acetic acid and propylene glycol, and is insoluble in organic solvents such as absolute ethanol, glycerin, acetone, chloroform, grease, and ether. The color of its solution changes with the pH value. When the pH is between 3.0 and 7.0, the color is red and relatively stable; it is most stable when the pH is between 4.0 and 5.0. When the pH is less than 4.0 or greater than 7.0, the color changes from red to purple. When the pH is greater than 10.0, the betacyanin in beet red pigment is converted to betaxanthin, and the color of the solution quickly turns yellow. This shows that betacyanin is relatively stable under acidic and neutral conditions. Since the pH of most foods is between 3.0 and 7.0, and the color of betacyanin does not change in this pH range, the color of foods containing betacyanin is generally not affected by pH.

In 2004, Tesoriere et al. found that after eating prickly pear fruit containing betaine, lipid damage caused by oxidative stress can be significantly reduced, and the body's antioxidant level can be improved. Cultivating isolated red blood cells in a betaine solution for a period of time can also significantly delay hemolysis caused by the oxidant isopropylbenzene hydroperoxide. Lü Xiaoling et al. studied the antioxidant capacity of the main components of beetroot red pigment and finally determined that the main antioxidant part of beetroot red pigment is the red part, i.e., betalain [8]. In addition, beetroot red pigment contains betaine, which has a certain therapeutic effect on liver diseases (such as chronic hepatitis, cirrhosis, toxic hepatitis, metabolic liver disease, and liver dysfunction caused by biliary tract diseases) [9].

Currently, betalain is widely used in coloring various beverages, fruit flavoring powders, fruit juice drinks, soft drinks, candies, pastries, ice cream sandwiches, canned foods, concentrated fruit juices, ice cream, jelly, and sausage products. It not only adds a beautiful appearance to foods, but also improves their nutritional value.

2.3 Capsanthin

Paprika pigments are a type of carotenoid, of which the more polar red components are mainly paprika red and paprika reddish-orange. Pure paprika red is a spicy-smelling dark red needle-like crystal that is non-spicy, soluble in most non-volatile oils, almost insoluble in water, partially soluble in ethanol, and insoluble in glycerin. It is highly soluble in alkaline solutions, resistant to acids and bases, and oxidation. Paprika red is a bright red color with strong coloring power that is not affected by pH. When used at pH 3 to 12, the hue does not change.

Capsanthin in capsicum red is currently a popular antioxidant. Many test results have shown that various antioxidants have a certain degree of anti-cancer effect. In addition, the beta-carotene in capsicum red can prevent the formation of a particularly harmful type of low-density lipoprotein, which is effective in preventing and treating atherosclerosis and its malignant development. Therefore, many spicy ethnic groups such as those in Southeast Asia and India have a lower risk of cancer than people in Western countries. Paprika red is derived from paprika and is a type of spice. Indian researchers compared the effectiveness of various spices in preventing radiation and found that paprika red had the most significant protective effect [10].

Paprika red is a food red coloring that can be used according to the national standard GB-2706-86. It can be used in the processing of oily foods, sauces, processed aquatic products, vegetable products, jellies, ice cream, cream, margarine, cheese, salads, sauces, rice products, etc. In these applications, paprika red pigment not only has no toxic side effects on the human body, but also increases the carotenoid-like compounds in the body, which has certain nutritional value. Zhang Fusheng and others have shown that paprika red pigment, when used in imitation foods, has good stability, light resistance, heat resistance, acid and alkali resistance, etc., which can effectively solve the problem of fading in the processing of imitation foods, effectively extending the shelf life of the food. It is an ideal edible natural pigment in imitation foods.

2.4 Sorghum red pigment

Sorghum red pigment is found in sorghum husks, sorghum seed coats and stalks. Sorghum red pigment is mainly composed of anthocyanin compounds and has a flavonoid structure. Sorghum red is a brick-red amorphous powder, paste or lump with a slightly characteristic odor. It dissolves in water, ethanol, methanol and salt solutions, but is insoluble in non-polar solutions such as oil, ether, n-ethane, trichloromethane and ethyl acetate. The pH of sorghum red is 4–12, and its hue changes with changes in pH. It becomes lighter in acid and darker in alkali. When the pH of the solution is less than 4, the pigment will precipitate from the solution, so it is not suitable for coloring acidic liquid foods with a pH less than 4. When the pH is greater than 12, the hue changes from red to brown. It has good transparency and a bright color when the pH is 7-8 [11].

Sorghum red pigment is a typical flavonoid compound with strong antioxidant properties. According to research by Zhu Yaohua et al., sorghum red pigment has a good scavenging effect on the 1,1-diphenyl-2-picrylhydrazyl free radical (DPPH-), can effectively scavenge superoxide radicals (O2-) and hydroxyl radicals (-OH), and has a certain inhibitory effect on lipid peroxidation induced by Fe2+, as well as a significant inhibitory effect on the β-carotene/linoleic acid autoxidation system. It can be seen that sorghum red pigment has good antioxidant capacity [12]. In addition, the main component of sorghum red is flavonoid galactoside, which has the effects of anti-inflammatory and antipyretic, lowering blood sugar and blood pressure.

Sorghum red pigment has good coloring properties for proteins, and the color is close to the natural color of meat, giving a realistic appearance. It can achieve an ideal coloring effect in products such as ham and sausage. Compared with other coloring pigments for sausage, sorghum red pigment has better light resistance and can extend the shelf life of the product. In addition, an aqueous solution of sorghum red pigment is also a coloring agent for fruit candies, pastries, jellies, and vegetable proteins.

3 Yellow pigments

3.1 Gardenia yellow

Gardenia yellow is a natural water-soluble yellow pigment extracted from the gardenia fruit in traditional Chinese medicine. It is a mixed pigment, the main components of which are carotenoids such as crocin and crotonic acid. Gardenia yellow is easily soluble in polar solvents such as water and ethanol, but hardly soluble in non-polar solvents such as benzene and gasoline. The pigment has strong thermal stability and is stable below 80°C. The pH has a significant effect on the pigment's maximum absorption wavelength and stability. Above pH 5.02, the maximum absorption wavelength shifts significantly. The initial absorption peak is higher under acidic conditions than under alkaline conditions, but the residual rate is much higher under alkaline conditions than under acidic conditions. Sucrose, starch, sodium chloride, common preservatives, and low temperatures are conducive to the stability of the pigment; the pigment is not very lightfast, has average redox resistance, and is not very tolerant of Na2SO3; Cu2+, Zn2+, Sn2+, Ca2+, and Al3+ have little effect on the pigment, while Fe3+ has a destructive effect on the pigment[13].

Gardenia yellow pigment has an inhibitory effect on Staphylococcus aureus, Neisseria meningitidis, Neisseria gonorrhoeae, and Streptococcus catarrhalis, and has antibacterial and antiviral effects. It also has an anti-inflammatory effect on soft tissue damage, relieving inflammation and pain. Gardenia yellow pigment is extracted from gardenia, which is one of the first dual-purpose food and medicine resources promulgated by the Ministry of Health. It has been used in traditional Chinese medicine for a long time and is the first choice for the treatment of jaundice hepatitis. Recent studies have found that gardenia yellow pigment also has a protective effect on the liver [14].

Gardenia yellow is widely used in the food processing and beverage manufacturing industries. Due to its bright color and heat resistance, gardenia yellow is currently used as a food coloring agent in the United States, the United Kingdom, Canada and other countries. It is also widely used in Japan in foods such as egg rolls, biscuits, flour products, candy, preserved fruits and jelly. China is the world's leading exporter of gardenia yellow, with the majority of the product exported to Japan. Gardenia is currently used as a yellow food coloring, and can be used in the preparation of beverages, wine, pastries, popsicles, ice cream, candied fruit, puffed food, jelly, candies, etc.

3.2 Corn yellow

Zein is a carotenoid that is a mixture of β-carotene, zeaxanthin, cryptoxanthin and lutein. It is extracted from corn gluten meal, a by-product of wet-milled corn starch. Zein is a fat-soluble pigment that is insoluble in cold water but slightly soluble in hot water. It is easily oxidized by light and is unstable in the presence of ions such as Al3+ and Fe3+. However, it is stable in the presence of heat, acids, alkalis, reducing agents, food additives and ions such as Fe2+, Zn2+, Na+ and K+ [15].

Zein has strong antioxidant properties, which can prevent the oxidation of lipids and vitamins in food. In addition, its main components, zeaxanthin and lutein, can effectively remove free radicals in the body, reduce the adhesion molecules on the surface of aortic endothelial cells, and play an important role in preventing the development of atherosclerosis. Zein belongs to the group of carotenoids. Epidemiological studies have shown that consuming foods rich in carotenoids can improve health and reduce the risk of cancer, cardiovascular disease, eye disease, cataracts, etc.[16].

Wang Wei et al. [17] added zeaxanthin to margarine, hard candy, and soft candy. The results showed that the margarine turned a natural yellow color, the hard candy a stable yellow color, and the soft candy a natural, realistic color after coloring, which can replace the use of synthetic pigments. In addition, zeaxanthin is widely found in fruits, vegetables and flowers. The New Orleans Food Research Institute in the United States is currently collaborating with Kemin Corporation to jointly launch an eye health drink containing zeaxanthin nutrients.

3.3 Turmeric yellow pigment

Turmeric yellow pigment, also known as curcumin, is a natural pigment extracted from the Chinese herb turmeric. The main components of curcumin include curcumin, demethoxycurcumin and bisdemethoxycurcumin, which are extremely rare colored diketones in nature.

The crystals of curcumin are orange-yellow powder with a special aromatic odor. It is lipophilic, easily soluble in methanol, ethanol, alkali and glacial acetic acid, slightly soluble in water, benzene and ether, but unstable in aqueous solution. It appears yellow in acidic and neutral solutions and red in alkaline solutions with a pH > 9. Metal ions such as A1 3+, Fe3+ and Cu2+ as well as strong light and high temperatures affect the stability of turmeric yellow, but sucrose, starch and ions such as Na+, C1 - and Zn2+ have little effect on the pigment [18].

Turmeric yellow pigment has an antimutagenic effect by altering the metabolism of mutagens and indirectly inhibiting the metabolism of mutagens; it has an antitumor effect by scavenging free radicals, inhibiting cancer cell expression and the activation of carcinogens; it has an antioxidant effect by inhibiting the oxidation of lipids in the air and by Fe2+ and Cu2+, and inhibiting the oxidation of hemoglobin by nitrous acid to prevent oxidative damage to DNA; it can inhibit the oxidation of low-density lipoprotein (LDL) in cells, thereby lowering blood lipids and preventing atherosclerosis [19]. Liang Jinli et al. showed that curcumin has a certain preservative effect on cooked mutton, bread, tofu and grape juice [20].

Curcumin has a bright color, strong coloring power, good dispersion and heat stability, and is an ideal coloring agent for puddings, cream products, meat products, soy products, pickles, etc. Turmeric extract has a certain preservative effect, so it can be used as both a coloring agent and a preservative in fruit and vegetable drinks.

4 Black Coloring

Black coloring is a large class of substances with similar properties. It is a heterogeneous polyphenol polymer widely found in animals, plants and microorganisms. The main component of most plant melanins is anthocyanin. Anthocyanins are water-soluble pigments that are easily soluble in polar solvents such as water, ethanol and acetone, but not in non-polar solvents such as n-hexane, glycerol and peanut oil. They are stable in acidic environments but not in neutral or alkaline environments[21]. Melanin is usually brown or black in colour, but other colours can also be observed.

Black pigment is an important active substance in the human body and is closely related to growth and development and health. Its pharmacological effects mainly include the following: prolonging the activity of adrenaline, thereby maintaining the normal osmotic pressure of blood vessels, softening blood vessels and shortening the time of bleeding; antibacterial and anticancer effects, inhibiting bacteria at low concentrations and killing bacteria and inhibiting the growth of cancer cells at high concentrations; antispasmodic effects. Melanin can also effectively scavenge hydroxyl radicals (-OH), reactive oxygen species, and scavenged diphenylpicrylhydrazyl radicals (DPPH-) [22]. In addition, melanin is currently the only known natural endogenous biopolymer that protects organisms from radiation damage.

Currently, black coloring is not only added to ordinary foods such as black baked goods, black rice noodles, black bread, black soybean milk, etc. on the market, but also used in health-preserving functional foods to produce oral liquids, nutritional health tablets, multi-dimensional capsules, etc. Based on the theory of Chinese medicine that promotes the homology of medicine and food, researchers are working hard to develop and research new black products that combine function, nutrition, and health.

5 Prospects

Plant-based natural pigments are generally secondary metabolites of plants, and their content is very low and their stability is poor. In particular, the stability of natural pigments during processing is related to the quality of the food. Therefore, it is important to choose the right natural pigments, develop new varieties with high stability, explore new sources of natural pigments, improve the production process of existing natural pigments, expand the scope of application of natural pigments, and reduce the production cost of natural pigments. This will give natural pigments a broader development prospect and has become a very urgent issue for the additives industry and researchers.

In addition, natural pigments come from natural products and have problems that synthetic pigments do not have. The composition of natural pigments is complex, and their components have not been completely separated, refined, or identified. How to figure out the structure, properties, functionality, and safety of the main components of natural pigments is also a major issue facing natural pigments today.

References

[1] Sava V M, Yang S M, Hong M Y, et al. Isolation and characterization of melanic pigments [J]. Food Science, 1994(2):15.

[2] Deng X Y, Wang S J, Li F, et al. Resources and applications of natural pigments [J]. Chinese Condiments, 2006(10):49-53.

[3] Fahey J W, Stephenson K K, Dinkova-Kostova A T, et al. Chbrophyll chbrophyll-in and related tetrapyrroles are significant inducers of mammalian phase2cytoprotective genes[J]. Carcinog-enesis , 2005 , 26(7):  1247-1255.

[4] Smith W A , Freeman J  W , Gupta  R  C. Effect of chemop  reventive agen ts  on  DNA  adduction  induced  by  the  potentm  ammary  carcinogen diben zo[a. l] . pyrene in the human breast cells MCF-7 [J]. MutatR es, 2001, 480-481:97-108.

[5] Cheng J, Zeng QX. Research progress on the properties and physiological functions of lycopene [J]. Food and Fermentation Industry, 2002, 26(2):75-79.

[6] Watzl B, Bub A, Briviba K, et al. Supplementation of a low carotenoid diet with tomato or carrot juice modulates immune functions in healthy men [J]. Ann Nutr Metab, 2003, 47(6):255-261.

[8] Lv Xiaoling, Wang Yuping, Zhou Ping, et al. Antioxidant capacity of the main components of beet red pigment [J]. Food Research and Development, 2009(6):39-43.

[9] Wang Changquan, Liu Tao, Wang Baoshan. Research progress of phytobetaine [J]. Botanical Bulletin, 2006, 23(3): 302-311.

[10] Peng Shulian, Ding Fanglin. Several functional ingredients in chili peppers and their applications [J]. Chili Pepper Magazine, 2008(1): 62-65.

[11] Lv Yuzhang, Zhang Hongyu, Li Tao. Development and utilization status of sorghum red pigment [J]. Agricultural Products Processing, 2010(6):39-40.

[12] Zhu Yaohua, Yang Jianxiong, Dai Bin. In vitro antioxidant research of sorghum red pigment [J]. Journal of Shaanxi Normal University, 2009(3):66-69.

[13] Ai Zhilu, Zhang Xiaoyu, Qiao Mingwu. Research on the application characteristics of natural food coloring gardenia yellow [J]. Food Industry Science and Technology, 2003, 24(12): 69-73.

[14] Zhang Dequan, Lv Feijie, Tai Jianxiang, et al. Effect of gardenia yellow pigment on carbon tetrachloride-induced liver damage in mice [J]. Journal of Nutrition, 2002, 24(3):269-272.

[15] Hurst W J. Methods of Analysis for Functional Foods and Nutareeutieals [M]. Boca Raton Lon-don New York Washington D C: CRCPressLLC, 2002.

[16] Navesmmv, Morenofs. Beta-carotene and cancer chemo-prevention: from epidemiological association to cellular mechanisms of action [J]. Nutrition Research, 1998, 18(10):1807-1824.

[17] Wang Wei, Wang Chunli, Yan Bingzong. Research on natural edible corn yellow pigment [J]. Food and Fermentation Industry, 1994(2):35-40.

[18] Wang Xianchun. Research on the stability of turmeric yellow pigment [J]. Food and Fermentation Industry, 1994(1): 63-66.

[19] Li Xia, Lin Huting. Research progress on the anti-mutagenic and anti-cancer effects of curcumin [J]. Foreign Medical and Health Credit Book, 1996, 23(5):278-281.

[20] Liang Jinli, Meng Yuzhu, Lei Changgui. Study on the preservative properties of curcumin [J]. Cereals, Oils and Foodstuffs Science and Technology, 2007, 15(6):67-71.

[21] Chen Xuefeng, Su Guifeng, Zhou Qingli. Research status of melanin microbial resources in China [J]. Food Industry Science and Technology, 2008(6): 318-319.

[22] Cong Jianmin. Research on the nutritional composition analysis of black beans [J]. Food Industry Science and Technology, 2008(4): 262-264.