7 Natural Dyes from Plants

1 Introduction

Food coloring improves the color of food and is an important component of food additives. Food coloring is divided into two types: synthetic and natural. With the development of technology, it has been discovered that many varieties of synthetic coloring have serious chronic toxicity and carcinogenicity, causing widespread consumer concern. However, domestic and foreign research has found that natural pigments are not only highly safe and have a soft hue, but some also have certain physiological activities and are considered functional natural food coloring. Therefore, the development and utilization of natural pigments has become a hot research topic at this stage. This paper mainly provides a brief overview of the seven plant pigments that have been studied more, with the aim of providing theoretical support for researchers engaged in related research.

2 Research overview of seven plant pigments

2.1 Mulberry red pigment

Mulberries are commonly known as mulberry dates, mulberry fruit, or mulberry berries. They are the mature aggregate fruit of Morus alba L., a plant in the mulberry genus of the mulberry family. Mulberry berries are purplish red or purplish black (some are also white), with an oily, elastic texture and a slightly sour and sweet taste [1]. Mulberry red pigment is extracted from natural mulberry fruit and is a kind of anthocyanin pigment[2]. Anthocyanins exist in the form of glycosides, i.e. anthocyanins, in their natural state. Anthocyanins have a typical C6 - C3 - C6 carbon skeleton structure and are therefore considered to be a kind of flavonoid. It can reduce the fat content of serum and liver, and has anti-mutagenic and anti-tumor effects [3]. Mulberry pigment shows some instability in heating and ultrasound treatment in different concentrations of hydrochloric acid, citric acid, tartaric acid and ascorbic acid, but the degree of instability varies with the type and concentration of acid, and the change is complex. Comparatively speaking, the pigment in ascorbic acid solution is most unstable to heating and ultrasonic treatment, while the pigment in hydrochloric acid solution is relatively stable to heating treatment; the pigment in citric acid solution is relatively stable to ultrasonic treatment [4].

Li Xinlei et al. [5] determined the optimal conditions for the extraction of mulberry pigments through single-factor and orthogonal test studies: 80% ethanol, a material-to-liquid ratio of 1:3 (g/mL), an extraction temperature of 30°C, and an extraction time of 0.25 h. The order of influence of each factor was: material-to-liquid ratio > ethanol concentration > extraction time > extraction temperature. Xiao Gengsheng et al. [6] studied the dynamic changes of the main pigments during the ripening of mulberries, using high performance liquid chromatography (HPLC) to detect the main anthocyanin components during the ripening process of mulberries, which mainly include cyanidin-3-O-glucoside (C3G), cyanidin-3-O-rutinoside (C3R) and pelargonidin-3-O-glucoside (Pg3G). 3 The changes in the three components are as follows: C3G and C3R are the two main anthocyanin components in mulberries throughout the ripening period, and C3G tends to gradually increase, while C3R first increases and then decreases slightly, and then rapidly increases in the later stage. Pg3G is not detected at the beginning of the ripening and growth of mulberries, and then first increases and then decreases, and then rapidly increases in the later stages. In production, the characteristics of the content of the main pigments in mulberries and the accumulation of anthocyanin compounds can be used as one of the reference factors for determining the appropriate time to harvest mulberries.

2.2 Grape skin pigments

Grape skin pigments are natural anthocyanin pigments that are safe, non-toxic and contain certain nutrients. They have the effect of anti-oxidation and scavenging free radicals, and have certain medicinal and health benefits. They can also be used as colorants in food and cosmetics[7]. Some literature reports that the best process conditions for extracting natural pigments from the skins of Kyoho grapes were obtained using an orthogonal test: anhydrous ethanol as the extracting agent, 5.00 g of raw material, 35 mL of extracting agent, an extraction pH of 3, an extraction temperature of 80°C, and an extraction time of 1 h. The order of influence of each factor is: extraction time > amount of extraction agent > extraction pH > extraction temperature [8].

Wang Chunrong et al. [9] discussed the extraction conditions of pigments from the skins of Kyoho grapes. The results showed that 70% ethanol + 0.5% citric acid (volume ratio 5:1) was the most efficient for extracting grape skin pigments, followed by 80% ethanol + 0.5% citric acid (5:1).  5% citric acid (5:1); using 70% ethanol + 0.5% citric acid (5:1) as the extraction solution, the mass of grape skins to extraction solution is 1:10 (g/mL), the optimum temperature is 65°C, the optimum extraction time is 90 min, The optimum pH for extraction is 2.0. Studies on the stability of grape skin pigments have shown that acidity has a significant effect on pigment stability and has a significant color-enhancing effect on pigments; low temperatures are conducive to the storage of pigments; prolonged exposure to sunlight will gradually degrade pigments; the metal ion Fe3+ has a significant effect on pigment stability; the additive H2O2 has a significant effect on pigment stability; vitamin C aqueous solution, sucrose aqueous solution, and sodium benzoate have a small effect on pigment stability [10]. Li Yanmei et al. [11] studied the antioxidant activity of rose-scented grape skin pigments and found that rose-scented grape skin pigments have strong reducing power and can effectively scavenge hydroxyl radicals; they also have a good inhibitory effect on spontaneous lipid peroxidation in mouse livers.

2.3 Tangerine peel pigment

Tangerine peel pigment is a type of natural pigment that is widely used. It is mainly found in the peel of plants such as tangerines, mandarins, pomelos, citrons, sweet oranges, limes, kumquats, and gardenias. It is rich in active substances such as tangerine peel pigment, tangerine peel flavonoids, and tangerine peel polysaccharides. It has important development and utilization value as a food coloring agent [12]. Research on the extraction method of tangerine peel pigments has shown that the ultrasonic-assisted extraction method is a more ideal method for extracting tangerine peel pigments in industrial production. The optimal process conditions are: ultrasonic frequency 40 kHz, ethanol concentration 6 5%, liquid-to-solid ratio 1:10, extraction temperature 60°C, extraction time 30 min, extraction times 2 times, and the yield of tangerine peel pigment under these conditions is 6.03% [13]. Geng Jingzhang [14] believes that the optimal process conditions for the extraction of tangerine peel pigments are: an ultrasonic frequency of medium frequency (47.6 KHz), an extraction solvent of 55% ethanol, an extraction time of 15 min, an extraction temperature of 60°C, and a material-to-liquid ratio of 1:15. There have also been many reports on the activity of orange peel pigments.

Li Lingxu et al. [15] extracted alcohol-soluble pigments, ether-soluble pigments, alcohol-water-soluble pigments and water-soluble pigments from orange peels, and used the mycelial growth rate method to preliminarily determine the antibacterial activity of different solvent extracts of orange peels against seven pathogenic fungi such as apple rot fungi. The results showed that ether-soluble pigments and alcohol-water-soluble pigments had different degrees of inhibitory effects on the seven plant pathogenic fungi. Wang Hongtao [16] determined the antibacterial effect of orange peel pigments on common microorganisms and the effect of commonly used food additives and light on the stability of pigments through single factor experiments to explore the antibacterial and stability of orange peel pigments. The study found that orange peel pigments have different degrees of inhibitory effects on common pathogenic bacteria, yeasts and molds.

2.4 Chestnut shell pigment

Chestnut shell pigment is a natural brown pigment with good water solubility, strong coloring power and stable properties. It has a certain degree of antioxidant and antibacterial effects, and is currently one of the few natural food colors with stable properties. It has high development value. Zhang Yaping [17] established the extraction conditions for chestnut shell pigment by analyzing the factors affecting the extraction of chestnut shell brown pigment: the mass fraction of NaOH was 2%, the extraction temperature was 80°C, and the extraction time was 3 h. Zhou Guoyan et al. [18] extracted pigments from chestnut shells using ethanol and ultrasound methods, compared the two methods, and studied the antibacterial properties and applications of the pigments in chestnut shells. The results showed that ultrasonic waves have the advantages of saving time, energy and a high extraction rate. The optimal process parameters for the ultrasonic-assisted extraction method were 40% ethanol volume fraction, 200 W ultrasonic power and 8 min of action time. Chestnut shell pigments have an inhibitory effect on Bacillus subtilis, Escherichia coli, Aspergillus niger and Penicillium. They also have a certain preservative effect on apple juice.

2.5 Black rice pigment

Black rice is a very distinctive type of rice in China's rice seed resources. It is rich in natural color, aroma, nutrition and therapeutic properties. Black rice is not only rich in protein, 17 amino acids, fat, vitamins, minerals and 14 elements such as Fe, Zn and Cu, but also contains black rice pigment, which has important medicinal value. Numerous studies have shown that black rice pigment is an anthocyanin pigment, a plant polyphenol compound, which has the effect of reducing the incidence of coronary heart disease, improving visual acuity, and has antioxidant and anticancer activity [19]. Wang Fengjie et al. [20] conducted a systematic study on the stability of black rice pigment, which showed that black rice pigment is sensitive to direct light and oxidants, but not to low temperatures and reducing agents. The effects of metal ions Ca 2+, Cu 2+, Zn 2+, Mg 2+, and Na + on its stability are not obvious.

Wu Suping [21] used black rice as a raw material and used ethanol as an extraction agent to extract black rice pigments, and studied the process and its process conditions. The optimal extraction process parameters were obtained through single factor and orthogonal experiments: ethanol volume fraction 50%, grinding degree 50 mesh, liquid-to-material ratio 1:5, extraction time 30min, extraction temperature 80°C, extraction pH=3. The stability of black rice pigment was also studied. The results showed that KMnO4 had a greater effect on black rice pigment, while Vc and citric acid had no significant effect. The results of the changes in pigment and amino acid content during the germination of black rice showed that the color value of black rice pigment extract was positively correlated with the temperature; the batch of extraction and whether the sample was degreased also had a significant effect on the color value of the extract [22]. In addition, black rice pigment has a certain reducing power, and has a strong scavenging effect on hydroxyl radicals and DPP H, as well as a certain degree of inhibitory effect on superoxide anions. Within the experimental concentration range, the maximum scavenging rate of hydroxyl radicals was 90.42%, and the maximum scavenging rate of DPP H radicals was 84.68%, indicating that black rice pigment has broad application prospects as a natural pigment with antioxidant properties [23].

2.6 Mangosteen shell pigment

Mangosteen is an evergreen tree in the family Clusiaceae. Its fruit is nutritious and has the effects of relieving fever, dissolving fat, moisturizing the skin and lowering internal heat. The fruit shell is rich in pigment components and can be used as a natural pigment. Zhang Bin et al. [24] investigated the effects of light, temperature, redox agents, pH and food additives on the stability of mangosteen shell pigment. The results showed that the maximum absorption wavelength of the pigment in the visible light region was 478 nm, and 70% ethanol was the best solvent for extraction. The pigment is an alcohol-soluble pigment suitable for use in acidic, neutral and slightly alkaline conditions. It has strong oxidation resistance and a certain degree of heat resistance. Metal ions Ca 2+, Cu 2+, Mg 2+ and Na + and food additives such as sodium benzoate, citric acid, Vc and sodium chloride reagents have little effect on the stability of mangosteen shell pigment. Ultraviolet light, outdoor sunlight, Fe3+, reducing agents, sodium bicarbonate and glucose have a certain color-reducing effect on this pigment.

Peng Wenshu et al. [25] studied the stability and antibacterial activity of the pigment extracted from the mangosteen fruit shell under different conditions. The mangosteen pericarp pigment was treated with different pH temperatures and different concentrations of metal ions, oxidants, reducing agents, and common preservatives. The results showed that the mangosteen pericarp pigment was more stable at pH < 6; the color enhancement effect was enhanced after the temperature exceeded 80 °C; various metal ions had little effect on it; the mangosteen pericarp pigment was easily oxidized and reduced; and high concentrations of preservatives had a greater effect on its stability. Bacteriostatic experiments show that mangosteen shell extract has a strong bacteriostatic effect on moulds, bacteria and yeasts. The inhibition effect increases with the increase of pigment quality concentration, and the bacteriostatic effect is in the following order: Streptococcus viridans > Bacillus octopodis > Bacillus subtilis > Saccharomyces cerevisiae > Aspergillus niger > Shigella dysenteriae > Escherichia coli. The results show that the pigment has good stability and antibacterial activity and can be used as a natural plant pigment in food, beverage additives and pharmaceutical industries.

2.7 Purple sweet potato pigment

Purple sweet potato pigment (PSPC) is a natural pigment extracted from the tubers and leaves of purple sweet potatoes. It has a bright and natural color, is non-toxic, has no particular smell and has various nutritional, pharmacological and health-preserving functions [26]. It has been reported in the literature that a higher extraction yield can be obtained using a 0.2% hydrochloric acid aqueous solution as the extraction solvent, a liquid-to-solid ratio of 1:5, an extraction temperature of 50°C, and an extraction time of 2h. The crude extract is filtered and adsorbed using PDA-100 macroporous adsorption resin, and then resolved in 70% ethanol. The concentrated and dried eluent can be used to obtain a powdered product of purple sweet potato pigment with a high color value of Em530 nm= 100 [27].

Luo Yuezhong [28] used domestically produced purple sweet potatoes as raw material to study the conditions for extracting pigments with solvents such as alcohols and organic acid solutions. The effects of factors such as the concentration of the extraction solution, the material-liquid ratio, the number of extractions and the extraction time were investigated. Through single-factor experiments and orthogonal experiments, determined that the optimal ultrasonic extraction process is as follows: pre-soak the purple sweet potato sample for 4 h, use citric acid with a concentration of 10%, a material-liquid ratio of 1:30, an ultrasonic power of 300 W, an extraction time of 25 min, and extract 3 times. The pigment extraction effect from purple sweet potatoes is the best.

Xue Qiang et al. [29] reported that the best process conditions were an extraction solution pH of 3, an extraction time of 120 min, an extraction temperature of 60 °C, a material ratio of 1:20, and an extraction yield of 17.3 mg/100 g. Purple sweet potato pigments are relatively stable to temperature, light, and metal ions. Regarding the activity of purple sweet potato pigment, Han Yongbin et al. [30, 31] discussed the antibacterial mechanism of purple sweet potato anthocyanins from a molecular perspective. First, the migration rate of Escherichia coli and Staphylococcus aureus DNA was studied using gel blocking experiments. Second, ethidium bromide was used as a fluorescent probe to study the changes in ultraviolet spectra and fluorescence intensity between purple sweet potato pigments and Escherichia coli and Staphylococcus aureus DNA systems. The above research explores the effects and mode of action of purple sweet potato pigments on bacterial DNA, which helps to understand the antibacterial mechanism of purple sweet potato pigments at the molecular level and reveal the interrelationship between the structure and function of purple sweet potato pigments.

3 Conclusion

Plant-derived natural pigments are generally secondary metabolites of plants, with low content and poor stability. Therefore, selecting suitable natural pigments, developing new varieties of natural pigments with high stability, and exploring new sources of natural pigments have become urgent issues that researchers need to solve. China has abundant plant resources. It is of great practical significance to actively develop and utilize these valuable plant resources and conduct in-depth research on the extraction methods of natural pigments and their related activities.

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