How to Improve the Stability of Natural Colours?

Colorants can be divided into two categories: natural colors and synthetic colors. Synthetic colors have the advantages of being low-cost, stable, and easy to use, and have been widely used, occupying a pivotal position in the market. However, as research into their use has progressed, it has been found that many synthetic colours that were once allowed to be used can cause harm to the human body[1] , such as diarrhea, cancer, and mutations[2, 3]. Therefore, many synthetic colours have gradually been banned from use in the food and pharmaceutical industries. Compared to synthetic dyes, natural colors are derived from nature and have the advantages of being highly safe, non-toxic and having natural hues. Many natural colors also have a certain degree of biological activity and can prevent and treat certain diseases. Therefore, the development and application of natural colors has gradually attracted more and more attention. However, their overall stability is poor, which greatly limits the industrial development and application of natural colors. This paper reviews the main factors affecting the stability of natural colors and summarizes and analyzes methods for improving the stability of natural colors, with the aim of providing theoretical and methodological references for research on improving the stability of natural colors.

1. Main factors affecting the stability of Natural Colours

Natural Colours can be divided into three main categories according to their source: plant pigments, animal pigments and microbial pigments. According to their chemical structure, they can be divided into pyridine pigments, anthocyanin pigments, flavonoid pigments, carotenoid pigments, naphthoquinone pigments, etc. According to their solubility, they can be divided into water-soluble pigments and fat-soluble pigments, etc. [4, 5]. Comprehensive literature reports that most natural pigments are relatively unstable. The main factors affecting the stability of natural colors are summarized and analyzed, including pH, metal ions, light, temperature, oxidants and reducing agents.

1.1 pH

Many natural colors are sensitive to changes in pH, and the hue can change significantly. For example, Fan Chunmei et al. studied the turmeric pigment and found that at pH 2, the water-soluble turmeric pigment formed a yellow precipitate and the absorbance decreased significantly. At pH 3–7, the color did not change significantly, and the color was lemon yellow, and the absorbance changed very little, indicating that the pigment is relatively stable under these conditions; when the pH is 8, the color is orange-yellow and the absorbance increases; when the pH is ≥9, the color is reddish-brown, indicating that the pigment changes greatly under alkaline conditions [6]. Chen Jie et al. studied purple sweet potato pigments and found that the pigment was dark red at pH 2; it turned purple when the pH increased to 6; and it gradually turned blue when the pH increased to 9. As the pH increased, the maximum absorption wavelength also shifted towards the long wave direction, showing a tendency of blue shift[7]. Li Jinxing et al. studied anthocyanins and found that when the pH is ≤3, the pigment is more stable, and the retention rate after 10 days is still above 83%; when the pH is ≥4, the retention rate of the pigment drops below 80% after 2 days. Therefore, it is proposed that anthocyanins should be stored under conditions of pH ≤3 [8].

1.2 Metal ions

Many metal ions can also affect the stability of natural colors, some of which can protect the color, while others can cause the pigment to fade. Yu Wei et al. studied lutein and found that different metal ions at a concentration of 0.5 g/L each had a certain difference in their effect on the stability of lutein. Na+ and Zn2+ had a smaller effect on the stability of the pigment, while Cu2+ and Fe3+ had a greater effect. The addition of these two metal ions caused a significant decrease in the retention rate of the pigment[9]. Li Jinxing et al. studied anthocyanins and found that at metal ion concentrations below 0.1 mol/L, different concentrations of Na+, K+, Ca2+, and Cu2+ had no significant effect on the stability of the pigment. Mg2+ at concentrations below 0.05 mol/L resulted in a higher preservation rate than the control group, while Mg2+ at concentrations up to 0.1 mol/L resulted in a lower preservation rate, indicating that low concentrations of Mg2+ have a protective effect on the pigment. Compared with the control group, the preservation of anthocyanins with the addition of Fe3+ 1 mol/L Mg2+  reduced the retention rate of the pigment, indicating that low concentrations of Mg2+  have a protective effect on the pigment. Compared with the control group, the retention rate of anthocyanin with added Fe3+  decreased significantly, and the damaging effect of Fe3+  on the stability of the pigment increased with increasing concentration.

1.3 Light

Many natural colors fade in the presence of light. These natural colors are photo-instable. Qiao Hua found that the content of red yeast rice pigment decreases under natural light or ultraviolet light, and light can promote the occurrence of the fading reaction [10]. Chen Guanlin's research found that both outdoor natural light and indoor diffused light can accelerate the degradation of the red pigment in dragon fruit, and the stronger the light intensity, the less stable the pigment [11]. Li Yuekun et al. studied lutein and found that under natural light, this pigment degrades rapidly; however, under conditions of light-protected storage, the degradation rate of this pigment slows down significantly. Therefore, it is proposed that lutein should be stored in dark conditions [12].

1.4 Temperature

When Natural Colours are used for food coloring, many require heat treatment, so attention needs to be paid to the effect of high temperatures on the stability of the pigments. Many natural pigments can fade under high temperatures and are therefore thermally unstable. Chen Jie et al. studied purple sweet potato pigments and found that the retention rates of the pigments after being treated at 40, 60, 80 and 100 °C for 6 hours were 91.47%, 84.65%, 59.23% and 43.23% respectively, indicating that the retention rate of the pigment decreased with increasing temperature. 23%, indicating that as the temperature increases, the retention rate of the pigment also decreases accordingly. When the treatment temperature exceeds 80 °C, the temperature has a greater effect on the pigment. Gao Yurong et al. studied the red pigment of red yeast rice and found that after being refrigerated for 7 days, the retention rate was still above 90%. However, high-temperature treatment had a significant effect on the pigment. Treatment at 100 °C for 0. 5h, the retention rate was only 61.8%. High temperatures can reduce the stability of this pigment[13].

1.5 Oxidants and reducing agents

Oxidants, reducing agents and other factors can also affect the stability of many natural colours[14]. Li Wei et al. studied melanin and found that with an increase in the mass concentration of hydrogen peroxide, the absorbance of the pigment solution showed a significant downward trend, and the oxidant had a certain damaging effect on the pigment. With an increase in the mass concentration of ascorbic acid, the absorbance of the pigment solution showed a significant downward trend, and strong reducing agents also had a certain damaging effect on the pigment [15]. Niu Shiquan et al. studied the production of blue pigments and found that after adding hydrogen peroxide, the absorbance of the pigment solution dropped sharply and then stabilized, indicating that oxidants have a strong destructive effect on the pigment [16]. Wang Xiaoting et al. studied the pigment of walnut green peel and found that as the concentration of hydrogen peroxide increased, the absorbance of the pigment solution decreased and the color gradually lightened, indicating that oxidants have a certain destructive effect on the pigment [17].

2 Methods for improving the stability of natural colors

The overall stability of natural colors is relatively poor, which greatly limits the development and application of natural pigments. Taking certain measures to improve the stability of natural colors can significantly expand the scope of their application and achieve good economic benefits. Analyzing and summarizing the literature reports, the current methods for improving stability mainly include adding stabilizers, microencapsulation, modifying the molecular structure of the pigment, and improving the processing and storage environment of the pigment.

2.1 Adding stabilizers

According to comprehensive literature reports, adding a certain amount of special chemical substances during the processing and storage of Natural Colours can delay the fading of Natural Colours and improve its stability. At present, the chemical substances that can be used are antioxidants and preservatives.

Antioxidants such as β-carotene, ascorbic acid and erythorbic acid can all delay the fading of natural colors and improve their stability, thereby making them easier to preserve. Sun Hong, male, and others found that both beta-carotene and sodium erythorbate have a protective effect on red yeast rice red pigment, with the protective effect of beta-carotene > sodium erythorbate [18]. It has also been reported that adding the right amount of ascorbic acid can improve the stability of betaxanthin [19]; similarly, adding isocitric acid can also improve the stability of betaxanthin, but ascorbic acid has a better stabilizing effect on the pigment than isocitric acid.

In addition, different stabilizers can also be used in combination to form a complex color protection agent to improve the stability of Natural Colours. Xu Chengjian et al. found that the stabilizer combination of sodium ascorbate + citric acid + vitamin B can improve the stability of Natural Colours in coloured wheat. After heating for 1 h, the pigment retention rate is as high as 96.3%, and the pigment solution has good stability [20]. Sodium D-isoascorbate and sodium benzoate can both improve the stability of betalain. When the two are mixed in different proportions to form a complex color protection agent, it can also 3%, and the pigment solution was stable [20]. Sodium erythorbate and sodium benzoate can both improve the stability of betalain. When the two are mixed in different proportions to form a complex color protective agent, the stability of betalain can also be significantly improved. Among them, the combination of 0.5% sodium erythorbate and 0. 0.05% sodium benzoate is the best combination, and the color retention rate is 60% higher than when no additives are used, which is a significant result [21].

2.2 Microencapsulation

Microcapsules are tiny containers with a polymer membrane. Microencapsulation technology involves embedding and encapsulating a specific solid or liquid in a microcapsule to form a solid particulate product. This technology isolates the embedded substance from the external environment, which can cause instability, and releases the embedded substance only under certain conditions, thereby improving the stability of the substance[22] . In terms of improving the stability of natural colors, microencapsulation technology has the advantages of reducing the diffusion of pigments into the environment, weakening the influence of the external environment on pigments, and improving the solubility of poorly soluble pigments [23]. These characteristics can effectively improve the stability of natural colors and expand the scope of their application.

Zhou Danhong et al. studied amaranth red pigment and found that using gum arabic, β-cyclodextrin, and sucrose (1:1:1) as a composite wall material and microencapsulating the pigment can improve the stability of the pigment and extend the storage time [24]. Zhaofang Liu and others found that microencapsulating the pigment with microporous starch and gelatin as the wall material, with orange peel pigment as the core material, can improve the stability of the pigment to light, temperature, pH, etc. [25]. Aizhi Han and others found that using gum arabic and β-cyclodextrin as the wall material and spray drying to encapsulate anthocyanins which can reduce the impact of external conditions on anthocyanins and improve their stability[26] . Ravichandran et al. found that microencapsulation of betalain using xanthan gum as the encapsulating material and spray drying can improve the stability of the pigment[27] . Hu Tingting et al. found that microencapsulation improves the stability of astaxanthin in aqueous solutions and does not affect the biological activity of the pigment [28]. In addition, when microencapsulating natural pigments, some antioxidants can be added to improve the stability of the pigments [29].

The above studies show that microencapsulation technology is a relatively effective method for improving the stability of natural colours.

In addition, encapsulation technology can also improve the stability of pigments to a certain extent. Encapsulation technology refers to the complete or partial encapsulation of one molecule in another to form a molecular capsule. Due to its special structure of “outer hydrophilic and inner hydrophobic” and its excellent non-toxic properties, cyclodextrin can be used to encapsulate a variety of objects. Therefore, the use of appropriate methods to prepare the inclusion complex can improve some of the properties of the object. For example, Stella et al. studied betalain and found that encapsulating the pigment with cyclodextrin to form a complex can improve the stability of the pigment [30].

2.3 Modification of pigment molecular structure

Structural modification of the unstable groups of Natural Colours molecules can effectively improve the stability, colouring power and solubility of Natural Colours, and has good application prospects. Yang Yun used metal ion modification to convert rutinoside to its metal salt, effectively improving the colorimetric value and stability of the pigment [31]. Wang Xiaoshan found that modifying the structure of chlorophyll by replacing the central magnesium atom with copper and hydrolyzing the ester group to a free carboxyl group to form copper chlorophyll can effectively improve the stability of chlorophyll [32]. In addition, Donald K and others found that acylation modification of Natural Colours, such as the use of carrot cells to culture anthocyanins, and the addition of styrene acid and other aromatic acids during the culture process, resulted in new monoacylated anthocyanins. The acyl group of the acylated anthocyanin has a better interaction with the chromophore, thereby improving the stability of the anthocyanin [33].

The stability of pigments is closely related to their chemical structure. To improve the stability of natural colors, it is best to start with the relationship between the stability and structure of natural colors and modify the molecular structure of natural colors to achieve the goal of improving the stability of natural colors. However, at present, the research on the chemical structure of many natural pigments and the mechanism of discoloration and fading of natural colors is not very clear, so there are still many limitations on the promotion and application of this method.

2.4 Improving the processing and storage environment of pigments

Numerous reports have shown that Natural Colours are highly stable in environments such as dark, low temperature, and vacuum packaging. Therefore, during the processing and storage of Natural Colours, methods such as dark, low temperature, and vacuum packaging should be used as much as possible [34]. Wu Jingping studied the strawberry red pigment and found that it is relatively stable at low temperatures, while high temperatures promote its degradation[35] . Zhao Zhenzhen studied the red pigment of the dragon fruit and found that it is relatively stable in the dark, while sunlight or indoor natural light can cause it to fade[36] .

Oxidants, metal ions, etc. can also affect the stability of many natural colors, so contact with these substances should also be avoided during the processing and storage of these natural colors. Oxidants such as H 2 O2 can cause aliphatic natural colors to fade quickly, which shows that aliphatic natural colors have very poor antioxidant capacity. Contact with oxidizing substances should be avoided during their processing [37]. Beet red pigments are sensitive to metal ions. Metal ions such as Cu2+ and Fe3+ can change the color of the pigment, discolor the solution or cause the pigment to fade [38]. Therefore, contact with these metal ions should be avoided during the processing and storage of the pigment.

Aromatic Natural Colours and aliphatic Natural Colours differ in terms of their mode of action, behaviour and main influencing factors. For example, the colour loss of aromatic Natural Colours is mainly caused by structural rearrangement or reaction with metal ions to form complexes, while the colour loss of aliphatic Natural Colours is mainly caused by hydrolytic rearrangement or photochemical oxidation. The influencing factors of the stability of these two types of Natural Colours are different. aromatic Natural Colours are susceptible to factors such as pH and metal ions, while aliphatic Natural Colours are susceptible to factors such as light and oxygen. Therefore, the precautions to be taken during processing, transportation and storage are also different. The complementary properties of aromatic Natural Colours and aliphatic Natural Colours can also be used in a coordinated manner to improve the stability of both. In addition, the instability of different pigments may sometimes manifest itself in very different ways under the same circumstances. Taking advantage of these differences can also improve the stability of the pigments. For example, mixing rutinosides and anthocyanins can synthesize pigments with higher stability.

In addition, measures such as low-temperature circulation, low-temperature heating, and the development of special packaging materials can be taken to eliminate the impact of various external adverse factors on the stability of natural pigments.

3 Prospects

Natural Colours have the advantages of high safety and a wide range of sources. With people paying more and more attention to health, the development and utilization of Natural Colours and improving their stability have gradually become research hotspots.

At present, there are many types of Natural Colours being researched and developed, but there are very few studies on methods to improve the stability of Natural Colours. Generally, the stability of Natural Colours is improved by improving the processing, storage and transportation environment, adding stabilizers, and microencapsulation. The research is limited to the impact of external conditions on the stability of Natural Colours, but rarely involves the study of chemical kinetics, nor does it link the chemical structure of natural pigments to their reaction processes. This fails to solve the problem at root and therefore does not improve the stability of natural colours. Therefore, in further research, the composition and chemical structure of natural pigments should be analysed as clearly as possible, and the causes of pigment instability should be fundamentally explored and targeted improvements made, in order to improve the stability of natural colours and expand their scope of application.

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