Study on Natural Blue Coloring

Blue is one of the three primary colors and is used in industries such as food and dyeing. There is no shortage of blue pigments in nature, but most of the blue pigments currently used in food processing are synthetic. Natural blue coloring that fully meets the needs of the food industry is rare.

Colorants are divided into two main categories: natural and synthetic. Natural colorants can be obtained from natural resources such as animals, plants and microorganisms. Colorants extracted from plants are usually derived from bark, roots, rhizomes, leaves, flowers and fruits. Common natural colorants include anthocyanins, betalains, chlorophyll, curcumin, carmine, lycopene and carotenoids. Synthetic pigments are obtained through chemical synthesis. They have almost replaced natural pigments due to their advantages of high yield, good durability, low price, strong coloring power and diverse colors. However, studies have shown that synthetic pigments contain substances such as arsenic, lead, mercury and chloride, all of which are toxic to varying degrees [1]. In addition, due to frequent incidents such as “Sudan Red”, people have gradually become aware of the health and environmental problems caused by synthetic pigments. Compared with synthetic pigments, natural pigments, which are mostly derived from plants, are stable in nature, have low toxicity, no carcinogenic risk, and at the same time have good biocompatibility and biodegradability. Therefore, natural pigments have once again returned to the public eye and have become the main force of food coloring in the new era.

Natural Blue Coloring

Natural blue coloring has extremely high application value and is now widely used in many fields such as medicine, cosmetics, printing and dyeing, and food. However, in fact, among the common natural pigments, the natural sources of blue pigments are very limited, which causes consumers to label related blue foods as “artificial blue” when they see them, thinking that they may have potential health risks [2]. In addition, color often has a corresponding visual and psychological effect on consumers. In the past, researchers compared the consumption of yellow and blue potatoes to verify the psychological impact and consumption choices of blue foods on consumers. The results showed that only 28% of consumers chose the blue potatoes, simply because they were curious about the new product. However, this phenomenon has largely disappeared in recent years, and blue ice cream, blue cakes, and blue candies that are popular with children can be found everywhere in the market. Classic blue has also been named the 2020 trend color by Pantone, the world's most authoritative color research institution.

Natural Blue Food Coloring

Common Natural Blue Colorings include cyclohexene derivative blue, algal blue, indigo blue, oyster green and anthocyanin [3]. In China's current food additive use standard GB2760-2011, the only Natural Blue Colorings allowed for use are gardenia blue, algal blue and indigo blue.

Cyclohexene ether terpene derivative blue pigment Cyclohexene ether terpene derivative blue pigment has a long safety record. Compared with algal blue pigment and anthocyanin, cyclohexene ether terpene derivative blue pigment is more stable to heat, light and pH changes. The precursors of this type of blue pigment are found in Rubiaceae plants, such as China's gardenia and the American plant Genipa. Gardenia is an evergreen plant in the Rubiaceae family that is widely cultivated in China. Its small, orange, oval fruit is 1.25–2.5 cm long [4]. Gardenin is one of the main cyclic ene-ether glycosides found in gardenia fruit and can be used as a natural yellow coloring agent. After being extracted from plant material, gardenia blue is hydrolyzed by β-glucosidase to release genipin and glucose. When genipin reacts with primary amines such as amino acids, it forms gardenia blue. When genipin reacts with different amino acids in a condensation reaction, it forms different gardenia blues. Gardenia blue is mainly used for coloring hard candy, drinks, fruit juice, etc., and the dosage is 0–1.5 g/kg. In addition, because gardenia blue has good solubility and strong coloring power, and common metal ions, acids, and alkalis have little effect on its hue, it is also widely used in medicine, cosmetics, and biomaterials.

Japanese researchers have conducted in-depth research on the production mechanism and applicability of gardenia blue pigment, solved the problem of the dark shade of gardenia blue pigment, and developed gardenia blue pigment with excellent stability and bright color. In the 1980s, research on gardenia blue pigment began in China. Researchers used a two-step method that separated microbial fermentation and enzymatic fermentation reactions to prepare gardenia blue pigment with a bright color.

Some researchers have also used macroporous adsorption resins and ion exchange resins to separate and purify gardenia blue pigment, producing gardenia blue pigment with high purity and excellent color value. Great progress has been made in the study of gardenia blue pigment.

Based on previous research by other scholars, the author's team used a new strain of Leifsonia bacteria (Leifsonia sp. ZF2019) in microbial fermentation to innovatively prepare gardenia blue pigment from gardenia yellow waste liquid with bright color and good stability. In addition, a highly active β-glucosidase was also screened from this strain, which can be used to catalyze the targeted production of gardenia blue pigment with excellent performance and stability. However, there has been relatively little research on the biological activity of gardenia blue pigment. Only a few studies have indicated that it has good antioxidant properties, so further research and exploration is needed.

Genipap is a native American plant that also belongs to the Rubiaceae family [5]. The fruit is oval-shaped, 5–8 cm long, and edible. Due to its juicy flesh and sweet-sour taste, ripe genipap fruits are mainly used for the production of juices, jams and liquors. When genipin is extracted from genipap fruits, the maturity of the fruit needs to be monitored, as genipin is only present in large quantities in unripe fruits. When an unripe juniper berry is cut and exposed to air, its flesh gradually turns blue. This is because the pinene reacts with an amino acid naturally present in the flesh of the juniper berry to produce a blue pigment when exposed to oxygen. Compared to unripe berries, ripe berries have 90% less pinene.

Phycocyanin

Phycocyanin is a blue coloring agent derived from eukaryotic algae such as red algae, cryptophytes and cyanobacteria. It is mainly derived from the dinoflagellate Phycocyanobacterium phycocyanum, commonly known as spirulina. The commercial name for its dried biomass is spirulina. Phycocyanin is considered to be a more versatile food coloring agent, and is mainly used in jelly and soft candy foods. However, because phycocyanin is essentially a protein, its stability is limited by many factors, so it is mainly used in sugary foods with a neutral or slightly acidic pH. In general, phycocyanin is mainly suitable for food environments with high sugar, high protein, low water content, and alcohol content below 20%.

Phycocyanin has received a lot of attention and research in recent years, and has high nutritional and medicinal value [6]. Its various biological activities, such as immune regulation, anti-cancer, anti-diabetes, anti-oxidation, anti-inflammatory, anti-viral, anti-fungal, anti-fibrosis, lipid-lowering, metal chelation, and protective effects on the reproductive system and various organs, have been widely reported. In addition, in vitro experiments on phycocyanin have shown that it has a similar effect to erythropoietin and can stimulate erythrocyte colony formation. In addition, many studies have further supported the view that increasing spirulina biomass and purifying phycocyanin will be beneficial to human health. Phycocyanin also has unique fluorescent properties. Chinese scholars have found that highly purified phycocyanin can be used as a tracer for biomedical research and diagnosis, as well as a tracer for heavy metals in food. In addition, spirulina, a dried biomass of cyanobacteria, can be used as a coloring agent, antioxidant, thickener and water-binding agent in cosmetics. Therefore, cyanophycin and related products should be given more attention, and more product varieties should be developed to further promote the development of related industries.

Indigo blue pigment

Indigo blue is one of the oldest pigments known to mankind. It is easy to color and has a unique color. Studies have shown that indigo extract has good antioxidant, analgesic and anti-inflammatory activities [7]. China's food additive use hygiene standard GB2760-2011 stipulates that indigo can be used in a variety of foods such as candied fruits, fruit and vegetable juice drinks, and preserved fruits, with an allowable addition of 0-0.3 g/kg.

Natural indigo blue is derived from the indigo plant or woad plant, and its precursor substance is indole, which is oxidized to produce a greenish-blue color. With the extensive use of indigo blue in the food, cosmetics and dyeing industries, the demand for natural indigo blue has also increased year by year, so methods for synthesizing indigo blue through environmentally friendly means have received increasing attention. Natural plant indigo is prepared by soaking plants containing indole acid in a fermentation tank, and then oxidizing and condensing them under the action of enzymes secreted by microorganisms to form water-insoluble indigo. However, the availability of such plant raw materials is subject to factors such as season, climate, and place of origin, so microbial production of blue pigments is gradually becoming an important source of natural indigo.

At present, domestic and foreign researchers have used a variety of biotransformation synthesis enzymes to produce natural indigo blue pigments. Researchers used site-directed mutagenesis to evolve cytochrome P450BM-3 from Bacillus megaterium, obtaining a mutant enzyme with three mutation sites, and found that the mutant enzyme could catalyze the production of indigo from indole. In addition, the researchers used cytochrome P450BM-3 as a parent enzyme to obtain highly active mutant strains through error-prone polymerase chain reaction in vitro directed evolution technology. These mutant strains and derivatives that catalyze indole derivatives can produce natural indigo dyes. Therefore, further detailed analysis of strain screening is needed in the future to obtain more highly active engineered bacteria that can be used in practical production.

Anthocyanins

The word anthocyanin is derived from the Greek words “anthos” (flower) and “kyanos” (blue) and is a flavonoid compound found in plants. Anthocyanins are the largest group of water-soluble colorants and, apart from chlorophyll, the most visible colorants to the naked eye. Anthocyanins can produce a wide range of colors from reddish purple to dark blue. Generally speaking, in an acidic environment (pH<2), they appear dark red to orange; when the pH is 2–4, they appear mainly blue [9]. Anthocyanins are found in the roots, leaves or petals of plants with bright colors. Generally speaking, anthocyanins are abundant in fruits, especially berries such as blueberries, cherries, raspberries, strawberries, blackcurrants, etc.

Anthocyanins have strong antioxidant activity and also have good anti-inflammatory, anti-cancer and antibacterial activity. In addition, studies have shown that anthocyanins can also prevent the occurrence of diabetes, obesity and cardiovascular disease. Due to these properties, anthocyanins as natural colorants are expected to become an alternative to synthetic food colorants, which has aroused great interest from the food industry and consumers. However, the color and stability of anthocyanins are easily affected by many factors, such as temperature, light, metal ions, oxidants, and reducing agents. Therefore, researchers need to take certain measures to enhance the stability of anthocyanins in food matrices. Previous studies have shown that anthocyanins in purple cabbage can be complexed with iron ions to form a stable blue pigment, which provides a reference for future research on the stability of anthocyanins and the development of methods to enhance their stability.

Oyster green

Phycocyanin is a blue-green pigment synthesized by marine diatoms and is one of the few blue pigments in the animal kingdom. Phycocyanin is soluble in water and is blue under acidic conditions and green under alkaline conditions. Bruising is the main factor that causes it to turn blue [8]. The main problem with using microorganisms and algae to extract phycocyanin-based colorants is the high cost of the entire extraction process, which makes it difficult to achieve widespread application. In addition, further research is needed to determine whether oyster green can be used in food and whether it will affect human health, given that the chemical structure of oyster green is still unclear.

As society develops, consumers are increasingly keen to choose healthier products. In the food industry, natural pigments have many health benefits, and it is inevitable that they will gradually replace synthetic pigments. However, as can be seen from the research on the several Natural Blue Coloring described above, most natural blue pigments lack stability. In addition, degradation of natural blue coloring can occur at every stage, from extraction to food processing and storage of the final product. Degradation of the coloring not only damages the sensory properties of the product, but also affects its biological activity value. Therefore, the application of natural blue coloring in food is still challenging, and researchers need to continuously innovate extraction and preparation processes to develop more stable natural blue coloring that can be widely used in the food industry.

Reference:

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