How Is Natural Food Colour Used in the Food Field?

Color is the most intuitive indicator of food quality and nutritional content. The addition of color to food is intended to increase its attractiveness, compensate for the loss of color during processing, and enhance the quality of the product. Food marketing is closely linked to color. For example, the sight of an orange drink will suggest to the brain that it tastes of orange and tangerine. Natural dyes are dyes derived from natural sources such as plants, insects, animals and microorganisms. Among these natural dyes, plant pigments are the most widely used because of their medicinal value. Currently, the development of cost-effective and viable food coloring processes and their application in food processing is a challenge, but also has broad market prospects and demand.

Chemically synthesized dyes are color-stable, have a higher yield, are cheaper, but have certain safety risks [1]. Synthetic dyes are all refined from petroleum. For example, lemon yellow, a commonly used food coloring, is often used in the production of candy, ice cream, cereal, soup, jelly, cakes, drinks and other foods. It is one of the most controversial coloring additives in terms of safety. This pigment can interact with human serum proteins and may be related to children's attention deficit hyperactivity disorder [2-3]. Amaranth is also a synthetic dye that can make foods such as candy, ice cream, and drinks red in color, but it is carcinogenic [4]. Of course, this toxic effect is also closely related to the dosage of the additive, but the safe dosage for consumption is different for different people. For this reason, compared with the potential health effects of synthetic colors, natural colors are not only safer, but also have biological functions and activities, such as antioxidant and antibacterial abilities.

1 Extraction methods and sources of natural dyes

1.1 Plant pigments

Plant pigments are natural primers produced by the metabolism of plants themselves. Plant pigments mainly include three categories: anthocyanins, carotenoids and betalains [5]. Betalains are pyrrole pigments extracted from tyrosine. The extracted betalains come in red and yellow. With the addition of organic solvents before ultrasonic processing, betalains can be extracted from amaranth leaves, beetroot, cactus fruit, and dragon fruit. Beetroot pigment itself also has antioxidant and antibacterial effects. Under high temperatures and alkaline conditions, the aldehyde-diamine bond in red beetroot pigment will undergo hydrolysis and turn yellow. Lutein and carotenoids are yellow and orange pigments unique to food. They are polyene pigments[6] and can be extracted from carrots, pumpkins, peppers and tomatoes using supercritical fluid extraction[7]. Carotenoids are prone to isomerisation during food processing and storage, and can also easily lose their yellow colour due to oxidation during processing[8]. Anthocyanins are also a common type of phenolic natural pigment, and most vegetables and flowers are rich in anthocyanins [9-10]; they appear red under weak acid conditions and purple under alkaline conditions. Anthocyanins are a water-soluble pigment dye that is less stable during food processing. For details of the specific methods of extracting plant pigments, see Table 1.

In summary, plant pigments are widely available and relatively inexpensive to extract. However, most plant pigments have antioxidant activity, which makes them prone to oxidation during food processing. In addition, they exhibit different colors under different pH conditions. Foods with added natural plant pigments have more stringent requirements for storage conditions, and the addition of natural pigments also shortens the shelf life of the food. Therefore, it is difficult for plant pigments to achieve the bright and full color of chemical additives.

1.2 Animal pigments

Shelled aquatic animals can use carotenoids from algae and modify them through metabolic reactions. Crustaceans can convert β-carotene into astaxanthin and accumulate it in the exoskeleton, shell, eggs and ovaries. Through metabolic conversion, the carotenoids in their bodies change their color from yellow β-carotene to red astaxanthin [28]. In production, humans can use microwave-assisted and biological enzyme methods to extract astaxanthin from marine organisms [29]. In addition to most plants that can be used as a source of natural red food coloring, there is also a type of insect, the carmine beetle, that lives on cacti. The red pigment extracted from the carmine beetle is superior to plant pigments in terms of both thermal stability and vividness of color.

Animal pigments have the advantage of being highly stable, but their extraction costs are much higher than those of plant pigments, and the colors they produce are not as rich as those produced by plant pigments. At present, the commercial use of animal pigments is still very limited.

1.3 Microbial pigments

Natural pigments produced by microorganisms can be used to avoid seasonal influences and reduce environmental pollution caused by chemical extraction processes [30]. Anthocyanins extracted from plants, for example, are affected by factors such as region and storage conditions. By inserting a gene that produces anthocyanins into E. coli and optimizing the culture conditions, the metabolite anthocyanin can be produced directly during the metabolism of E. coli [31]. Carotenoids and astaxanthin can be extracted from microalgae. For example, using rain-born algae, nutrients can be added to the batch of its supplement to allow rain-born red blood cells to produce astaxanthin. Monascorubrin is a secondary metabolite of Monascus, which itself has antioxidant properties. Using Monascus pigment instead of nitrite to marinate meat not only keeps the meat red and fresh during the marinating process, but also reduces the toxic effects of nitrite and has a certain antibacterial effect.

At present, it is still difficult to use microbial synthetic pigments on a large scale. Further regulation and improvement is needed in terms of regulating the metabolic pathways of microorganisms, preventing them from producing harmful substances, reducing the accumulation of by-products and promoting the development of pigment synthesis pathways.

2 Separation and purification of natural food pigments

Most natural food coloring is extracted using organic solvents (methanol, ethanol, acetone). The extraction method is simple, but the substances extracted in this way are mostly mixtures, and the quality of the extracts varies. When used as a food additive, there is no accurate data to determine the quality of the extracted pigment. When organic solvents are used for extraction, the quality of the natural pigment is compromised because the extraction solvent is also a toxic substance. The extracted pigment needs to be further separated and purified to obtain a pigment of higher purity. The methods of extracting pigments focus on resin adsorption and high-speed countercurrent chromatography column separation, etc. For specific methods, see Table 2.

3 Application of natural pigments in food

Consumer concerns about synthetic food coloring have prompted the development and promotion of natural food coloring, which consumers readily associate with health. Natural food coloring can add color to food. China's national standard “National Food Safety Standard: Food Additive Use Standards” (GB 2760) allows the addition of more than 40 types of natural food coloring to food, which are commonly used in various fields such as cheese products, alcoholic beverages, and meat processing. Of course, when adding to food, the stability of the pigment itself must also be considered, and the appropriate pigment added according to its characteristics.

3.1 Baked goods

There is not much literature on the use of natural food colours in baked goods, which may be related to the fact that some plant-based colours have weak thermal stability and are not suitable for use as food additives in the processing of baked goods. Colored wheat flour is rich in anthocyanins, which help prevent various chronic diseases. Bread baked with colored wheat is rich in carotenoids, but many anthocyanins and carotenoids are lost during the baking process [44]. The red pigment extracted from cochineal insects is red under strong acid conditions and purple under alkaline conditions. However, the red pigment extracted from cochineal insects is not affected by light or temperature. and can also chelate metal ions to form carmine. Carmine is very stable and not affected by pH, and can be widely used in bread making. However, when carmine is extracted from cochineal insects, insect protein residues remain in the pigment, resulting in poor quality and an off-flavor. Studies have found that the insect protein residues in the pigment may cause allergies in humans, triggering allergic asthma, etc. [45].

3.2 Beverages

The color of a beverage is a very important visual attribute for it to be accepted as a consumer product. Different beverages have their own unique attributes. For example, milk needs to be colored with fat-soluble pigments, while fruit drinks need water-soluble pigments. This shows that the appropriate pigment needs to be selected according to the characteristics of the beverage. Anthocyanins have a rich color, and under acidic conditions, they can maintain a good red color. They are often used commercially as a food additive for yogurt. Anthocyanin-rich extracts have also been shown to have the potential to inhibit amylase degradation by about 1% at a quality concentration of 40 mg/mL [46]. Natural pigments have strong coloring power, and 0.03% to 0.04% can be added to beverages to achieve the desired color [47]. Roselle is an edible calyx that is rich in anthocyanins. Anthocyanins from roselle, which are spray-dried, are often used as colorants in beverages and gelatin desserts. Moreover, the results show that the pigments in these foods remain stable during storage for up to 4 weeks [48]. Anthocyanins are the most active in scavenging oxygen free radicals, and can inhibit lipoprotein oxidation and platelet aggregation. Anthocyanins in wine help to soften the cardiovascular system [49]. Betalains are stable at pH 3–7. Betaine extracted from red beets has an earthy taste, and betalains from cactus fruits have also become another important source of beverage coloring. Beta-carotene is a natural orange fat-soluble pigment with high vitamin activity, and is commonly used as an additive in fruit-flavored drinks.

3.3 Meat

Meat is rich in lipids, and lipid oxidation can easily lead to a decline in meat quality. During processing, meat products often need to be supplemented with red pigments to maintain their healthy color, and antioxidants are also added to extend their shelf life. Anthocyanins and betalains have natural antioxidant activity. Adding 2% betalains to minced pork can significantly enhance its antioxidant activity, and adding betalains to beef sausages and cooked ham can reduce the degree of color loss during storage [50]. Monascus pigments are mainly suitable as additives for pickled products. Adding 1.00% monascus pigment to minced meat After being stored for one or two weeks, it was found that red yeast rice pigment had significant antioxidant activity. Red yeast rice pigment has a strong antioxidant effect in reducing the formation of hydroxides during storage [51], and can also reduce the toxicity of nitrites [52]. In addition, red yeast rice pigment itself has a certain degree of antibacterial properties, so it can also extend the shelf life of food.

4 Ways to enhance the stability of natural food colors

Although natural food colors are rich in color and highly safe, they have poor light and heat stability, and are prone to discoloration during food processing, which is not conducive to the preservation of processed foods. In order to improve the thermal and chemical stability of natural pigments, researchers have developed different methods to improve this phenomenon.

4.1 Chemical methods

The functional groups of the extracted pigments that are unstable are chemically reacted to form new stable functional groups. Anthocyanins and betalains are both unstable water-soluble pigments. The sugar molecules of anthocyanins can be acylated with organic acids via an ester bond to enhance their thermal stability and photoelectric resistance, change the molecular size and polarity of anthocyanins, reduce their water solubility [53], and enhance their thermal stability during processing [54]. The thermal instability of betaine limits its application in food processing. The decolorizing enzyme of betaine can be inactivated by blanching. After decolorization, the addition of erythorbic acid to the betaine solution can help regenerate betaine, and the thermal stability of the regenerated betaine can be significantly improved [55]. Monascus pigment is insoluble in water and forms a precipitate at pH 4. Casein has hydrophilic groups. By combining monascus pigment with casein, a monascus pigment-sodium caseinate complex is formed, which stabilizes the monascus pigment and prevents it from precipitating under acidic conditions [56].

4.2 Encapsulation Microencapsulation

Encapsulation is a method of trapping the active ingredients of living organisms in a suitable carrier. Microencapsulation is the most commonly used method of encapsulation in the food industry, while nanoencapsulation is attracting increasing attention due to its high encapsulation efficiency, high stability, targeted release of the encapsulated substance and ability to encapsulate macromolecular substances. The anthocyanins in the sour cherry skin extract were microencapsulated using the freeze-drying method, with whey protein isolate and gum arabic as the coating agents. The extracted powder has good antioxidant properties, and the encapsulated anthocyanin pigment is used as a substitute for artificial coloring in jelly powder. Compared with synthetic coloring agents, jelly with 7% encapsulated anthocyanin pigment scored higher in all sensory and physicochemical evaluations [57]. Micro- and nanoencapsulation are effective platforms for protecting pigments from harmful environmental conditions and can provide targeted and controlled release [58]. Encapsulation of betaxanthin from cactus using spray drying and sub-micronization into a maltodextrin matrix not only preserves the bright yellow color of betaxanthin, but also its antioxidant activity [59].

5 Summary and outlook

Naturally-derived pigments are generally more expensive than synthetic pigments. However, due to the current trend of increasing consumer awareness of food safety and health, food colouring companies have begun to research and develop natural food colouring agents. Obtaining a stable supply of natural pigments has always been the biggest challenge facing the industry. This problem has prompted the search for novel and economically viable solutions to the production, extraction, purification and stabilization of natural food colorants. Attempts have been made to express the genes of plant pigments in microorganisms to directly obtain pigment metabolites, or to develop plants that contain richer pigments.

The vast majority of natural pigments on the market are still derived from plants, which are limited by the season and growing environment. In addition, the method of extracting from microorganisms can completely avoid the disadvantage of the uneven quality of the extracted pigments. In the future, the exploration of new production technologies for natural pigments can involve the large-scale cultivation of microorganisms. The expanded cultivation of microorganisms also requires the addition of nutrients during the production process, and further regulation of the reaction time is needed to avoid the production of more by-products while obtaining more plant pigments. Regarding the problem of the instability of the extracted plant pigments, the current traditional method can only preserve the activity of the plant pigments by changing the processing method, which limits the scope of use of natural pigments. Encapsulation, which involves coating the plant pigments with edible materials, can preserve the activity of the natural pigments while enhancing their stability. It should be noted that encapsulation will increase the production cost of food, so how to increase the yield of natural pigments in microbial propagation culture and enhance the stability of pigments at a lower cost is a key issue that we need to solve in the future research on food coloring.

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