What Are the Sources of Natural Food Colours?

Pigments are a very important element in food and play a pivotal role in the sensory quality of food. Food colours can be divided into artificial food colours and natural food colours. The extensive use of artificial colours can cause potential health hazards such as allergic reactions and hyperactivity in children [1], which is why natural colours are attracting increasing attention in the food industry. Natural Food Colours are derived from animals, plants and microorganisms in nature, and have an important role in promoting human health, including antioxidant and free radical scavenging activities, as well as antibacterial, anticancer and the prevention of some chronic diseases [2].

Natural colors can be divided into isoprene, porphyrin, flavonoid and nitrogen heterocyclic types according to their structures. However, their own structures also lead to a lack of stability in natural colors [3], which are susceptible to light, oxygen, pH and temperature. In recent years, researchers have developed a number of stabilisation techniques to address this issue, including microencapsulation, the addition of antioxidants, the addition of colour stabilisers (such as EDTA) and the chemical modification of the pigment's structural groups [4-6]. The coloring ability is another important factor affecting the application of natural colors in the food industry. This involves the interaction between natural colors and macromolecular substances in food. Studies have shown that there are covalent interactions and non-covalent interactions (hydrogen bonding, van der Waals forces, hydrophobic forces, etc.) between them, which also provide a theoretical basis for the application of natural colors in food [7-8]. In recent years, due to the emphasis on environmental protection, the application of natural colors in edible packaging has been widely studied and has become an important emerging field in food, including applications in food coatings, colored edible films, edible ink printing, and 3D printing.

This paper introduces the main categories and properties of Natural Colour based on relevant research in recent years. It also describes the stabilization of Natural Colour in the four main types of structures and the interaction mechanism with substances such as food macromolecules. Finally, it lists the new applications of Natural Food Colours in the food field and provides an outlook on the future development and application of Natural Colour in the food field, with the aim of providing a basic theoretical and applied technical reference for the application of Natural Colour in the food field, especially in edible packaging.

1 Classification and properties of natural colors

Natural colors come from a wide variety of sources in nature, mostly found in plants, animals and microorganisms. They can be divided into water-soluble pigments and fat-soluble pigments according to solubility; warm-toned pigments and cool-toned pigments according to hue; isoprene pigments, porphyrin pigments, flavonoids and other polyphenol pigments, and nitrogen heterocyclic pigments according to chemical structure [6], as shown in Table 1.

1.1 Classified by source

1.1.1 Plants

Plant pigments are produced through a series of biosynthesis processes in plants. The main types are flavonoids, carotenoids, porphyrins, and nitrogen-containing heterocyclic compounds [10], which have different chemical properties. They are distributed in various parts of the plant (sepal, petal, pollen, etc.) and play an important role in the plant, such as photosynthesis, signal transmission to the outside world, defense against natural enemies, and heat exchange with the outside world [6, 10].

1.1.2 Animals

 Natural color in animals can play an important physiological role, such as acting as a medium for transmitting signals, attracting the opposite sex, and also having antioxidant activity, protecting cell tissue from damage by eliminating harmful free radicals, etc. [11]. Pigments in animals include porphyrins, melanins, pterins, flavonoids, anthraquinones, etc. [11-12].

1.1.3 Microorganisms

Microbial pigments can be synthesized by themselves or formed during the culture process by the transformation of certain components. They are a kind of secondary metabolites. Common types include carotenoids, melanin, quinones, etc., some of which are more typical pigments such as red yeast pigment and purple bacillin [13]. Microbial pigment production is one of the emerging fields of research, and it has great potential in various industrial applications [14].

1.1.4 Minerals

Mineral pigments are crystalline elements or compounds formed by geological processes, and have a long history of use in foods, cosmetics and works of art. Mineral pigments can take on different shades depending on their chemical composition or physical structure, such as green chromates and white titanium dioxide.

1.2 Classification by solubility

 Natural Colour can be classified according to their solubility as water-soluble pigments, fat-soluble pigments and alcohol-soluble pigments. Water-soluble pigments are soluble in water; fat-soluble pigments are insoluble in water and soluble in vegetable oils; alcohol-soluble pigments are only soluble in alcohol solutions such as ethanol with a volume fraction of more than 70%. The solubility of natural pigments is one of the important reference indicators in practical applications, as shown in Table 2.

1.3 Classification by hue

Colors are classified by hue as warm, cool and other tones. In food, warm and cool tones are the main colors. Warm tones are mainly red, yellow and orange, etc., while cool tones are green, blue and purple, etc.

1.3.1 Warm tones

1.3.1.1 Red

Red hues come from a wide range of sources, including lycopene, carmine and anthocyanins. Lycopene is a naturally occurring, bioactive red pigment found in plants. It is abundant in red fruits and vegetables such as tomatoes, papaya, pink grapefruit, pink pomegranate and watermelon [20]. It is an unsaturated, acyclic carotenoid. Carmine is also a natural red pigment, extracted from the dried bodies of female cochineal insects. It is widely used in food coloring, medicine and cosmetics [21]. Anthocyanins exhibit a red hue under low pH conditions, and are therefore widely used in the food industry as substitutes for synthetic dyes, such as replacing the artificial color allura red [22].

1.3.1.2 Orange-yellow

Orange-yellow is a warm color that is widely distributed in animals and plants in nature. For example, gardenia yellow pigment is a natural coloring agent extracted from gardenia fruits [17]. Its main component is gardenoside, which has the effects of clearing away heat, promoting gallbladder function, protecting the liver, and lowering cholesterol [23]. Curcumin is a hydrophobic polyphenolic compound extracted from the food spice turmeric. It has a variety of pharmacological effects, including anti-inflammatory, antioxidant and anti-angiogenic activities. Traditionally, turmeric has been used to treat a variety of diseases, especially as an anti-inflammatory drug. Curcumin has been identified as the active ingredient in turmeric.

1.3.2.1 Green

Natural green pigments are mainly chlorophylls, which are not only used as additives in medicine and cosmetics, but also as green colorants in food. Chlorophylls selectively absorb light in the red and blue regions and therefore emit green light. Chlorophylls are expensive to produce and difficult to industrially produce, so further research is needed to explore them.

1.3.2.2 Blue-violet

Natural blue pigments are rarely used. Some pigments exhibit a blue hue at a specific pH, such as anthocyanins, which become bluer the higher the pH [25]. Anthocyanins are stable under acidic conditions, but unstable under weakly acidic and neutral conditions. In nature, they need to be glycosylated and acylated to improve their stability [26]. Gardenia blue is a natural food blue coloring agent widely used in East Asia. Historically, gardenia blue has been used as a coloring agent in food and cosmetics, and also for dyeing fabrics such as cotton, silk and wool [27]. It is currently widely used in Asian frozen desserts, candy, baked goods, jams, noodles, beverages, wines and agricultural products [28]. Natural purple pigments are a kind of pigment between red and blue, and the natural colour of purple is mostly anthocyanin. It has been reported that purple anthocyanins are mainly found in plants such as purple sweet potatoes [29], purple corn [30] and purple carrots, as well as some microorganisms that produce purple pigments, such as purple bacteria.

1.3.3 Other shades

1.3.3.1 Black

At present, the most widely used natural melanin is vegetable carbon black, which is mainly refined from the combustion and carbonization of materials such as tree trunks and shells. Plant carbon black is a black powder that is non-toxic and harmless, and is insoluble in water and organic reagents. In China, plant carbon black is mainly used in candy, biscuits, rice products, etc. Plant carbon black can also impart a variety of properties to food. Ding et al. [31] combined plant carbon black with gelatin to form a gelatin edible film, which imparts properties such as ultraviolet resistance and oxidation resistance.

1.3.3.2 White

Currently, the natural white pigments that can be selected are generally minerals, such as titanium dioxide. Because of its low solubility, titanium dioxide is also considered a relatively safe edible pigment. In food formulations, titanium dioxide is dispersed in the food in the form of particles.

1.3.3.3 Tan

For brown pigments, caramel pigments are widely used in the market. Caramel, also known as burnt sugar, is produced by heat treating various sugars. Caramel can produce a wide range of brown colors through different processing methods, such as reddish brown and dark brown [3].

1.4 Classification by structure

 The solubility and color of Natural Colors are determined by their own structure, and their chemical structure also determines their physical and chemical properties. Natural colors in nature can be divided into isoprene, porphyrin, flavonoid and other polyphenolic pigments, nitrogen heterocyclic, anthraquinone and ketone pigments according to their chemical structure. The following will focus on the representative Natural Colors of the first 4 chemical structures. The structural molecular formula is shown in Figure 1.

1.4.1 Carotenoids

Carotenoids are fat-soluble Natural Colours that are classified as isoprenoid derivatives [32] and have biological activity. They are widely found in higher plants, algae, fungi, bacteria, birds, etc. [3]. Carotenoids are divided into two main categories: one is carotenoids, which consist only of carbon and hydrogen; the other is xanthophylls, which consist of carbon, hydrogen and oxygen [6]. It has been reported that carotenoids can synthesize the precursors of vitamin A (α-carotene and β-carotene) [33]. At the same time, carotenoids have certain antioxidant activity and are essential for human life activities [3]. However, due to the rich electrons and unsaturated chemical structure in carotenoids, they can be easily oxidized and isomerized during processing and storage [34-35]. Oxidation has a more serious effect on carotenoids than does isomerization. The former can completely destroy their activity and color, while the latter only causes a decrease in activity and color saturation [4]. In plants, most carotenoids are trans-isomers, and isomerization occurs during processing and storage, with the trans-isomers changing to cis-isomers [33]. Among these, temperature, light, and acid are the main factors that cause carotenoids to shift from the trans-isomer to the cis-isomer [36].

1.4.2 Chlorophyll

Chlorophyll is the most widely distributed green pigment in the plant kingdom and is a derivative of the pyrrole class. The structural feature of pyrrole is a five-membered ring composed of four carbon atoms and one nitrogen atom. Chlorophyll is mainly divided into chlorophyll a and chlorophyll b, which differ in the seventh position of the structure, with chlorophyll a consisting of —CH3 and chlorophyll b consisting of —CHO. Chlorophyll is sensitive to temperature, oxygen, acid, light and enzymes, which can cause chlorophyll degradation and color changes to some extent [37]. Related studies have reported that conventional heating can reduce the chlorophyll content of kiwifruit by 42% to 100% [38]. Therefore, temperature is a very important factor affecting the stability of chlorophyll. It has been found that chlorophyll can also be used as a mouthwash, and oral chlorophyll can effectively prevent liver cancer caused by aflatoxin [39-40].

1.4.3 Anthocyanins

Anthocyanins are classified as flavonoid pigments, which are secondary metabolites in plants characterized by a C6C3C6 carbon backbone. Anthocyanins are widely found in many fruits and vegetables, including many berries, red cabbage, purple potatoes, pomegranates, etc. [41-42]. They can produce red, blue and purple colors in fruits and vegetables [43]. The color of anthocyanins depends on many factors, such as pH, concentration, temperature, light, enzymes, other flavonoids, and metal ions. Among these factors affecting stability, pH and temperature are the most important [44]. Anthocyanins are more stable under acidic conditions. At a pH of 1, anthocyanins exhibit a strong reddish hue; when the pH reaches 3.5, the intensity of the color display begins to decrease, and the overall color is still reddish. As the pH continues to increase, the color gradually fades, taking on a bluish hue; when the pH is greater than 7, anthocyanins begin to degrade [3, 45]. The glycosylation of anthocyanins and the number of methoxyl and hydroxyl groups in the structure all affect their color, with a higher hydroxyl content giving a blue hue and more methoxyl groups giving a red hue [44, 46]. Studies have shown that the color intensity of acylated anthocyanins can be maintained at pH 4.5 to 5 [3]; for the glycosylation of anthocyanins, the sugar molecule is usually attached to the 3-hydroxy position of the anthocyanin molecule [47]. In nature, anthocyanins are acylated and glycosylated to varying degrees, which will make them exist with higher stability.

1.4.4 Beetroot pigments

Beetroot pigments are a class of nitrogen heterocyclic water-soluble pigments. There are two types of beetroot pigments: the reddish-purple betalain, which is formed by the condensation of cyclopropane and betaine; and the yellow-orange betaxanthin, which is formed by the condensation of an amine and betaine. Betaine is an intermediate product in the formation of beetroot pigments [48]. In nature, betalains are more common. They are found mainly in plants such as urucu (an economically important root crop grown extensively in the Andes of South America), Malabar spinach, cactus fruits (found in Latin America, South Africa and the Mediterranean), red pitaya (found in Malaysia, China, Japan, Israel and Vietnam) and amaranth [49]. Among them, red beets and red pitaya are crops rich in betalains [50]. Betalains are susceptible to external environmental influences and are subject to certain limitations during processing and storage [51]. Among the many influencing factors, temperature has the greatest effect on betalains [52]. Compared with anthocyanins, the effect of pH on betalain is not significant. Betalain is stable at pH 3 to 7; however, the color of anthocyanins begins to change at pH > 3 [3–4, 6, 50]. Studies have shown that in addition to being a coloring agent, betalain also has pharmacological effects such as anti-oxidation, anti-cancer, lipid-lowering, and antibacterial, and plays an important role in human health [49].

1.4.5 Other

Anthraquinone pigments mainly include cochineal red and lac dye. Cochineal red is a red pigment extracted from female cochineal insects, and its main component is cochineal acid. This pigment is not easily soluble in cold water, but is soluble in hot water, ethanol and other solutions, and has a certain degree of stability and safety [53]. Lac dye, also known as shellac red, is a product obtained by extracting and refining lac, which is secreted by lac insects, in alkaline water. Lac dye is a bright red or purplish red liquid or powder that is acidic in appearance. It is not easily soluble in water, ethanol or propylene glycol, but is easily soluble in alkaline solutions.

Theaflavin is a polyphenolic pigment extracted from tea. It is easily soluble in water and aqueous ethanol solutions, but not in chloroform or petroleum ether. It has various health benefits, such as anti-oxidation, anti-cancer, anti-bacterial, anti-viral, anti-inflammatory, prevention of cardiovascular diseases, and weight loss and lowering of blood lipids [54].

Monascus pigment is a natural food colouring agent produced through the fermentation of Monascus. It is classified as a ketone pigment. Monascus pigment is a Natural Colour with a high safety profile. It also has physiological activities such as lowering blood pressure and blood lipids, and is therefore very popular with users both domestically and abroad.

2 Stabilisation of Natural Colour

 The poor stability of natural colors limits their use in food. Factors that affect the stability of natural colors include temperature, pH, light, oxygen, metal ions, enzymes, etc. In recent years, scholars have intensified research on the stabilization of natural colors, and have developed a large number of stabilization techniques for different types of natural colors, providing technical support for the practical application of natural colors.

2.1 Isoprene-carotenoid stabilization

Carotenoids are easily oxidized and isomerized, for example, by oxygen, light, temperature, metal ions, and peroxides. Among these, oxidation is the main cause of degradation of carotenoids. To prevent oxidation, microencapsulation and nanocapsulation technologies can be used. This technology involves encapsulating active substances in a micron or nanosystem material to form an effective physical and chemical barrier to improve the active substances' resistance to harmful environmental conditions (such as light, temperature, oxygen, and adverse reactions with other compounds) [55]. RAHAIEE et al. [56] prepared chitosan-sodium alginate nanoparticles by the ionogel method to encapsulate crocin, and this technology significantly enhanced the stability of crocin in an adverse environment. Pretreatment of the raw material before extraction of carotenoids is also a very important part. Physical methods such as blanching can inactivate enzymes that are detrimental to the pigment, such as lipoxygenase. There are also chemical methods, such as the addition of antioxidants (such as citric acid and ortho-phenylene triol), which can also reduce the rate of oxidation of the pigment [4].

2.2 Stabilization of pyrrole-chlorophyll

There are many factors that affect the stability of chlorophyll, among which acids and enzymes are the main ones. The stability of chlorophyll can be improved by inactivating unfavorable enzymes, so its stability can also be improved by blanching pretreatment [57]. At the same time, the adverse effects of acids should be controlled. Alkaline substances (such as KOH and NaOH) can be added to neutralize the acids [58]. During storage, chlorophyll should be stored in the dark at low temperatures. This can effectively reduce the damage to the pigment caused by ultraviolet light and maintain its stability. Metal ions replace magnesium in chlorophyll to form more stable metal chlorophyll salts. Wang Fenglan et al. [59] used CuSO4 and zinc acetate to treat chlorophyll in Bauhinia variegata, and the results showed that both reagents could stabilize the color of chlorophyll. This also proves that both Mg2+ and Cu2+ can protect chlorophyll.

2.3 Flavonoids—stabilization of anthocyanins

Anthocyanins from different plant bodies have different structures and different stabilities. Anthocyanins are relatively sensitive to pH, temperature, light, enzymes and other flavonoid substances, which can affect their stability.

A study by CHUNG et al. [60] confirmed that the addition of gum arabic (0.05%–5.0%) can improve the stability of anthocyanins in the presence of ascorbic acid, and the stability is highest when 1.5% gum arabic is added. The stability of anthocyanins can be enhanced by interactions with other molecules (such as amino acids, organic acids, metal ions, flavonoids, polysaccharides and other anthocyanins), as these substances act as complementary colors. i.e. some co-pigments (such as metal ions, polysaccharides and other flavonoids) form supramolecular assemblies with anthocyanins. Co-pigmentation is a method that can enhance the color stability of individual anthocyanins [61]. CHUNG et al. [62] studied the effect of different pectins and whey proteins on the color stability of anthocyanins in purple carrots and concluded that denatured whey protein had the best effect on stabilizing anthocyanins. GRIS et al. [63] studied the interaction between anthocyanins and caffeic acid in Cabernet Sauvignon grape extract and showed that the addition of caffeic acid significantly enhanced the stability of anthocyanins. The stability of anthocyanins can also be improved by metal ion-anthocyanin molecular complexes. The most common metals in anthocyanin-metal complexes are copper, iron, aluminum, magnesium and potassium [64]. Microencapsulation technology is also used to improve the stability of anthocyanins. TAN et al. [65] reported the use of catechins to regulate the co-pigmentation and encapsulation of anthocyanins in an anionic polyelectrolyte complex composed of chondroitin sulfate and chitosan. The study showed that the co-pigmentation effect combined with microencapsulation technology significantly improved the stability of anthocyanins.

2.4 Nitrogen heterocycles—stabilization of betalain

Beetroot pigments are affected by many external factors, such as temperature, light, pH, metal ions, etc. Their stability can be improved by increasing the concentration, and they are stable at high levels of acylation and glycosylation, as well as in dark and cold environments [50]. Studies have shown that adding antioxidants (such as ascorbic acid and erythorbic acid), stabilizers (EDTA) [5, 66], cyclodextrins [67] and other compounds can also stabilize betalain. Betalains can also be made more stable by blanching to inactivate undesirable enzymes. However, temperature also has an effect on betalains, and adding organic acids (such as ascorbic acid) can regenerate the pigment, but only betaxanthin and not betacyanin [4].

3 Interaction between natural colour and macromolecules in food

 The adhesion of Natural Colour to the inside and outside of food is an important factor in determining its application performance, which involves its interaction with food macromolecules such as proteins and polysaccharides. Natural Colour can bind to these macromolecules through covalent and non-covalent interactions (hydrogen bonding, hydrophobic interactions and van der Waals forces, etc.) and adsorb on their surfaces.

3.1 Water-soluble Natural Colour and the interaction with macromolecular substances

Water-soluble natural colors can interact with hydrophilic macromolecules. Non-covalent interactions such as hydrogen bonding, hydrophobic interactions, and van der Waals forces are the main interactions between small organic molecules and macromolecules such as proteins [68], and there are also covalent interactions between them [7]. In recent years, the interaction between them and proteins has been widely discussed. WANG et al. [8] studied the interaction between rice protein and asparagus leaf pigment. After the interaction, the antioxidant activity and free polyphenol content of the pigment were significantly reduced. The results showed that asparagus leaf pigment reacted with rice protein through hydrophobic and hydrogen bonding. Anthocyanin is a small molecule that can bind to proteins to form complexes. Jiang Lianzhou et al. [69] found that there is a strong interaction between soy protein isolate and anthocyanin, and the two can form a complex with a binding site similar to 1. Zhang Guowen et al. [68] studied the interaction between mulberry pigment (a small molecule with pharmacological activity) and proteins, and the results showed that mulberry pigment and bovine serum albumin can interact through van der Waals forces and hydrogen bonds. In addition, Deng Fanzheng et al. [70] investigated the mechanism of action between the food colouring agent cherry red and proteins by adding different types of surfactants, and demonstrated that there is a strong interaction between the pigment and the protein. Covalent bond action is a relatively strong binding force. Studies have shown that there is also a covalent bond between polyphenolic pigments and food macromolecules, and that the covalent bond structure may be produced by oxidation and nucleophilic addition processes [7].

Similarly, there is also an interaction between natural colors and polysaccharides. Many natural colors are linked to sugar substances in the vacuoles of plant cells [10]. BOWLES et al. [71] demonstrated that enzymes are involved in transferring sugar residues to the pigments of plant cells, and that the binding of sugar increases the stability of the pigment to a certain extent. Liu Lizeng et al. [72] investigated the adsorption mechanism of starch and red yeast rice pigment, and the results showed that there is physical adsorption between the red yeast rice pigment molecule and the starch particles, mainly by hydrogen bonding.

 Natural Color has a large number of weak bond interactions and potential covalent bond interactions with proteins and polysaccharides. The binding mode and strength between them also reflect the coloring ability of Natural Color, which can provide theoretical reference for the processing and application of Natural Color in related products.

3.2 Lipid-soluble Natural Color and the interaction between macromolecular substances

According to the principle of like dissolves like, fat-soluble pigments are insoluble in water, alcohol, etc., and can only be dissolved in oil. However, many applications require them to be combined with hydrophilic substances, so certain treatments are required to make the fat-soluble pigments able to combine with hydrophilic substances.

Natural chlorophyll is not easily soluble in water, but by replacing the magnesium ion in chlorophyll with a copper ion to make copper chlorophyll sodium, it can be dissolved in water. L6PEZ-CARBALLO et al. [73] used sodium copper chlorophyllin to bind gelatin, and the results showed that the addition of sodium copper chlorophyllin enhanced the antibacterial properties of the gelatin film. DE CARVALHO et al. [74] reported the use of microencapsulation technology to encapsulate lycopene, making it easier to disperse in water and combine with gelatin. RESZCZYNSKA et al. [75] used molecular spectroscopy to study the interactions of three carotenoids (carotene, lutein and zeaxanthin) with bovine serum albumin. A bovine serum albumin solution was prepared in PBS (pH 7.4), and then dissolved the carotenoids in tetrahydrofuran (which has a high solubility threshold for carotenoids, is miscible with water and does not cause structural changes in the protein). The carotenoid solution was then injected into the protein solution at 37 °C, with continuous shaking for 1 h to ensure thorough mixing. The results showed that there was interaction between the carotenoids and the protein, and that they could bind to each other. In practical production, it is often hoped that the solubility properties of fat-soluble pigments can be converted accordingly. This can be achieved through chemical modification, microencapsulation technology, emulsification, etc., so that fat-soluble Natural Colour can be flexibly applied in food production.

4 New applications of Natural Food Colours in edible packaging

Due to the interaction between natural colors and some macromolecules in food, this provides a basis for their use in the food industry, including in edible packaging (e.g. edible films, coatings, etc.). They can be combined under certain conditions and distributed inside the product or adsorbed on its surface to achieve the purpose of showing the color, as shown in Figure 2. 4.1 Application of Natural Food Colours in Food Coatings In recent years, the application of Natural Colour-based food coatings has attracted increasing attention due to their green and healthy properties. The coating can provide color, flavor and protect the food inside. It can be made into a smooth and even hard or soft coating. MANDATI et al. [76] developed colored hard chewing gum and candy products, in which the flavors and colors in the coating were separated to prevent the pigments from interacting with other substances and reducing their stability.

HITZFELD et al. [77] microencapsulated annatto orange and added it to edible coatings in the form of a dispersion or powder. The coatings prepared in this way can be used for confectionery (chocolate beans, etc.) and the coatings can coat the confectionery to give it a reddish-orange color. As it is necessary to maintain the stability of the annatto orange, the pH of the formulation should be controlled between 5 and 8. Maintaining the stability of natural color is an important factor in the application of coatings. Therefore, in actual production, it is necessary to adapt the natural color to the formulation environment as much as possible. Different types of products have different requirements for pigments. For example, the acidity and alkalinity of the product and the solubility conditions require the selection of natural pigments that are suitable for the product [78]. For coating preparation, the replacement of artificial colors with natural colors is an important challenge, because not only must the stability of the pigment in the system be ensured, but also the color must be adapted to the colors on the market. [79] The appearance of color is very important for confectionery foods, and the replacement of artificial colors with natural colors provides a guarantee for the safety of confectionery foods. However, due to the inherent instability of various natural colors, further modification research is required.

4.2 Application of natural food colors in the preparation of edible films

In order to enhance the sensory effect of the color of edible films, the combination of natural colors and edible films has become the object of research. Colored edible films will provide people with more attractive sensory colors, which will also increase people's desire to buy to a certain extent. BURGUETE et al. [80] invented an artificial casing for the preparation of stuffed meat products. The artificial casing contains reducing sugars, which give the finished stuffed meat products a pleasant golden brown color. SOBRAL et al. [81] studied the addition of copper sodium chlorophyllin to gelatin films to investigate the effect of the pigment on the properties of the film and to make the product more attractive in appearance. The combination of lycopene and gelatin film can give the gelatin film a certain color characteristic on a transparent basis [74]. In foreign research, colored edible films have been reported, but research on colored edible films in China has barely begun. For the future development of edible films in China, the combination of non-toxic, green, and naturally derived pigments with physiological activity and edible films will be widely welcomed by health-conscious people.

4.3 Natural Food Colours in Edible Ink Printing

In recent years, printing with edible ink made from natural colors has become a research hotspot. Edible inks are non-toxic, brightly colored, and edible, and have become the first choice for food and pharmaceutical packaging. Printing with edible ink can engrave patterns and text on the surface of food and medicine (capsules, tablets), etc. This type of food not only increases its attractiveness to children, but also effectively reduces the pollution caused by traditional printing on food packaging. Edible inks are mostly composed of pigments, binders, solvents and additives [82].

Shastry et al. [83] reported high-resolution inkjet printing technology on edible substrates. The edible ink formulation contains pigments, fats and waxes. The edible substrate can be candy blocks with hydrophobic surfaces (e.g., wax-polished candy). Powar et al. [84] used beets and other ingredients to make colored herbal inks. which is characterized by the addition of pharmacological activities to the ink, such as blood pressure reduction, cardiovascular protection, vasodilation and antibacterial properties. LIU et al. [85] extracted purple kidney bean pigments and used them to prepare edible inks. The results showed that the prepared edible inks had good color development on different substrates. In addition, WU et al. [86] studied the electrochemical writing method of embedding anthocyanins in a chitosan/agarose hydrogel. The writing substrate was a polysaccharide film. Unlike traditional printing, this experiment used a stainless steel wire (instead of a pen) as the cathode in contact with the hydrogel and wrote on the polysaccharide film. The characteristic of this electrochemical writing is that the anthocyanin will respond to changes in color with changes in pH. Based on the green concept, the new environmentally friendly ink is replacing traditional ink as the future development trend, and the combination of digital printing technology and edible ink has laid a solid foundation for its application. In China, edible ink is still in the exploratory stage, but due to people's increasing awareness of healthy eating and aesthetic requirements, this type of edible ink will be widely researched and applied in the market.

4.4 Natural Food Colours in 3D Printing

3D printing technology uses the preparation principle of “layer-by-layer manufacturing and stacking”, which is convenient and fast [87]. 3D food printing technology also has these characteristics. The technology is mainly divided into four categories: selective heat air sintering, hot melt extrusion, binder jetting and inkjet printing [88]. Inkjet printing is a method of depositing liquid materials in layers, and when multiple layers are superimposed, a three-dimensional object is formed [89]. The printing material can be mixed with natural food colors to give it certain color characteristics [87, 90]. The advantages of 3D food printing include the ability to customize food designs, simplify the supply chain, and broaden the range of available food materials. However, further breakthroughs are expected in the accuracy, precision, and printing speed of 3D printing technology. The application of 3D printing technology in the food sector will promote the design and development of new food products.

5 Outlook

With people's increasing demand for health and environmental protection, natural pigments have played an increasingly important role in food packaging. In recent years, the application of natural colors in edible packaging has attracted widespread attention. However, there are still huge challenges in these practical applications, such as how to maintain and stabilize natural colors for a long enough period of time, and how to solve related problems such as their weak coloring power and color mismatch. At present, research in these areas is still in its infancy, and more basic and applied research on Natural Colour is needed in the future. With the development of science and technology, Natural Colour will be increasingly applied in the emerging field of edible packaging due to its potential value for human health, non-toxic and harmless nature, and ability to give foods a variety of colors, thus helping to promote the rapid and stable development of the entire healthy food industry chain.

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