7 Natural Plant Based Food Coloring

With the development of science and technology and the progress of society, people are paying more and more attention to food safety issues. The majority of consumers are increasingly concerned about the harm of synthetic pigments contained in food, which has led to an increasing focus on natural pigments that are highly safe and have certain physiological functions. The development and application of natural plant pigments has become a common concern among scientific and technological workers in various industries. People are trying to obtain natural pigments from various plant resources and explore their physiological activities to alleviate and solve various problems caused by synthetic pigments. This article provides a brief overview of seven plant pigments currently under research, with the hope of providing theoretical guidance for researchers working with natural plant pigments.

1 7 Natural Plant Based Food Coloring

1.1 Potato pigments

Potato (Solanum tuberosum L.), also known as yam, potato, or yam egg, is a high-yielding nightshade crop that is nutrient-rich, easily digested and absorbed, highly adaptable, and can be used for both food and cooking. It is one of the world's top ten nutritious foods [1]. The Black Beauty potato has a blackish-purple skin and flesh. It is a new variety of potato bred through hybridization. According to expert appraisal, this variety is not only rich in nutrients and high in calcium and potassium, but also contains nutrients such as anthocyanins. Long-term consumption has various effects, such as weight loss and beauty, nourishing the stomach, strengthening the spleen and promoting urination, detoxifying and anti-inflammatory, lowering blood sugar and blood lipids, etc. Cao Hong et al. [2] used a 1.1% hydrochloric acid-ethanol solution as the extraction solvent for the pigments in the “Black Beauty” potato. The best extraction results were obtained under the conditions of a liquid-to-liquid ratio of 1:40, an extraction time of 1 h, and an extraction temperature of 50 °C. The optimal extraction process conditions are: temperature 50 °C, extraction time 1 h, liquid-to-solid ratio 1:40, and hydrochloric acid concentration 1.1%.

Subsequently, Cao Hong et al. [3] studied the stability of the “Black Beauty” potato pigment. The results showed that the pigment is stable under acidic conditions; light and high temperatures accelerate pigment degradation; within the experimental addition range, metal ions K+, Ca2+, Al3+ had no effect on the pigment, while Na+, Mg2+, Cu2+ had a color-enhancing effect on the pigment, and Fe3+ had a detrimental effect, changing the color of the pigment solution; additives such as sucrose, citric acid, and sodium benzoate had no effect on the pigment; H2O2 and Na2SO3 had a significant effect on the stability of the pigment.

Li Caixia et al. [4] used a 0.1% HCl-ethanol solution to extract the “Black Beauty” potato pigment under constant temperature and shaking conditions, and used spectrophotometry to determine the change in absorbance at the maximum absorption wavelength of the pigment under different environments. The results showed that the pigment is a water-soluble anthocyanin pigment, and the pH value has a significant effect on the stability of the “Black Beauty” potato pigment. The pigment is more stable under acidic conditions; the metal ion Al3+ has a color-enhancing effect on the pigment, while Na+, K+, Ca2+, Mg2+, and Zn2+ have basically no effect on the stability of the pigment, while Cu2+ and Fe3+ have a significant adverse effect on the stability of the pigment; the pigment is highly resistant to radiation, but not to natural light or high heat, and has very poor oxidation resistance; sucrose, VC and β-cyclodextrin at low mass fractions have little effect on the stability of the pigment; citric acid at a certain mass fraction has a color-enhancing effect, while potassium sorbate has a certain effect on the stability of the pigment.

1.2 Purple corn pigment

Purple corn is a species of corn in the grass family, native to Peru. Purple corn pigment is a kind of anthocyanin extracted from purple corn kernels. Anthocyanin pigments are a type of water-soluble natural pigment widely found in plants. They have many physiological functions, such as scavenging free radicals, resisting oxidation, and resisting lipid peroxidation. They are also known for being highly effective, low in toxicity, and highly bioavailable. They are a natural edible pigment resource with medicinal value. Zhang Yajun et al. [5] used a single-factor test to initially determine the influence range of the four factors of ethanol concentration, citric acid concentration, liquid-to-material ratio and extraction temperature on the extraction of purple corn core pigment. On this basis, an orthogonal test was carried out to determine the optimal extraction conditions for corn pigment.

The results showed that among the four factors, the liquid-to-material ratio had the greatest influence on pigment extraction, followed by extraction temperature, ethanol concentration and citric acid concentration. The optimal extraction conditions are: ethanol concentration 60%, citric acid concentration 0.8%, liquid-to-solid ratio 1:10, and extraction temperature 80°C. In terms of pigment stability, Zhang Yajun et al. [6] determined the maximum absorption peak of purple corn anthocyanin pigment by colorimetry and analyzed the physical and chemical properties of the pigment. The results showed that the maximum absorption peak of purple corn anthocyanin pigment was 526 nm; purple corn pigment was stable to salt, sucrose, glucose, vitamin C, citric acid, acetic acid, and Cu2+, Mg2+, Ca2 +, K +, Al3 + are stable; purple corn pigment is sensitive to light and oxidants (H2O2) and reducing agents (Na2SO3). Regarding antioxidant research, it has been reported in the literature that this pigment has strong in vitro antioxidant capacity, and its ability to scavenge free radicals (DPPH· and ·OH, O2·) is significantly better than that of the positive control ascorbic acid. The pigment crude extract at 0.04 mg/mL has a DPPH· scavenging rate of 89.88% and a ·OH scavenging rate of 84.87%; at 0.035 mg/mL, the scavenging rate of O2· is 85.82% [7].

1.3 Lycium ruthenicum pigment

Lycium ruthenicum Murr. (black fruit wolfberry) is a perennial shrub wild plant endemic to the arid northwest region of China. It belongs to the Solanaceae family and the genus Lycium (Lycium L.). Tibetan medicine calls it “Pangma”. The ripe fruit is used in medicine. The pigment in its fruit is a type of anthocyanin pigment that is nutrient-rich and has the function of being used both as medicine and food. It has great market development value [8]. Zhang Yuande et al. [9] used a microwave-assisted method with pigment content as an indicator. The effects of five factors on the extraction of blackberry medlar anthocyanin pigments were investigated using single-factor and orthogonal experiments: extraction solvent, radiation power, extraction time, material-to-liquid ratio and soaking time. The results showed that the effects of each factor on the anthocyanin content of black fruit medlar were as follows: liquid-to-material ratio > ethanol concentration > radiation power > extraction time > soaking time; the optimized extraction conditions for anthocyanin from black fruit medlar were: extraction solvent 75% ethanol, radiation power 70 W, extraction time 20 min, liquid-to-material ratio 1:50, soaking time 20 h. Under these conditions, the anthocyanin pigment extraction rate was 15.32%, and the total anthocyanin content was 936.27 mg/100 g. In addition, Chen et al. [10] used ultraviolet-visible spectrophotometry to determine the proanthocyanidin content in black fruit wolfberry pigments, and the result was that the proanthocyanidin content in black fruit wolfberry pigments was 22 g/100 g.

1.4 Red amaranth pigment

Red amaranth (Alternanthera bettzickiana L.) is a perennial herb in the Amaranthaceae family. Its stems and leaves are purplish red and rich in anthocyanins. Red amaranth is attractive in its bright and enticing color. It has the advantages of being highly adaptable, widely distributed, highly productive, and high in anthocyanin pigments. Liu Deliang [11] found that The optimal extraction conditions for red amaranth pigment are: an aqueous hydrochloric acid solution with a volume fraction of 2% as the extracting agent, an extraction temperature of 30 °C, an extraction time of 1.5 h, and a liquid-to-solid ratio of 1:20. Common food additives have no significant effect on the pigment, while salt has a certain color-enhancing effect. The oxidant H2 O2 and the reducing agent NaHSO3 both cause the pigment to degrade, and with the increase of H2 O2 and NaHSO3 concentration, the degradation rate of red amaranth pigment accelerates. Metal ions, Mg2+, Cu2+, Zn2+, Ca2+, K+, and Na+ have no significant effect on red amaranth pigment, while Ba2+ has a color-enhancing effect on the pigment, and Al3+ can cause the pigment to fade.

1.5 Dragon fruit pigment

Dragon fruit (Hylocereus undatus L.) is the fruit of an annual succulent plant in the family Cactaceae (aeataeeae) of the genus Hylocereus (Hylocereus undatus). It is also known as red dragon fruit or honey fruit. It is cultivated in Hainan, Yunnan, Guangdong, Guangxi and other places in China. Dragon fruit is rich in nutrients and has unique functions. It contains plant albumin, which is rare in general plants, as well as abundant cellulose and water-soluble dietary fiber. It is also rich in a large amount of betaine pigments. The color of the skin and flesh ranges from rose red to purple red, making it a good raw material for extracting natural pigments [12]. Liang Binxia et al. [13] studied the effects of the type of extraction solution, liquid-to-material ratio, extraction time, extraction temperature and pH on pigment extraction, and determined the optimal process conditions: freeze-storage of the pitaya peel, extraction solution of deionized water, liquid-to-material ratio of 5:1, extraction time of 30 min, extraction temperature of 50 °C, The pH of the extract was 6. The results showed that a 75% ethanol solution gave the best results. The maximum absorption wavelength of the dragon fruit peel pigment was 536 nm. The pigment was stable under acidic conditions but not light stable.

1.6 Red raspberry pigment

Red raspberries, also known as raspberries, are plants in the family Rosaceae. Their ripe fruits are rich in red pigments and are a good source of natural pigments. The red pigments in red raspberries are present in the form of anthocyanins, which can prevent peroxidation in the body[15].

Sun Xiyun et al.[16] used red raspberry fruit as a raw material to extract natural pigments. purified using a macroporous resin method. After purification, the sample was separated using paper chromatography. The components were preliminarily identified using UV-Vis spectroscopy and paper chromatography. The results showed that HPD-700 macroporous adsorption resin is suitable for purifying red raspberry pigments. The adsorption effect is better when the pH of the crude extract is 2. Using 60% ethanol as the eluent, the elution volume was 4 BV, the flow rate was 0.6 mL/min, and the elution was carried out with 60% ethanol as the eluent. The elution effect was better; four components were separated by paper chromatography, and the preliminary identification was cornflower-3-glucoside, cornflower-3-rutinoside, cornflower-3-sophoroside, and cornflower-3-glucose-rutinoside, respectively. Wang Feng et al. [17] used an ultrasonic method to study the extraction and stability of the pigments in red raspberry fruit. The results showed that the maximum absorption wavelength of red raspberry pigments was 510 nm, and the optimal extraction conditions were: an ethanol solution with a volume fraction of 50%, a liquid-to-material ratio of 1:10, an extraction temperature of 40 °C, an extraction time of 30 min, extracted twice; pH has a significant effect on the color of red raspberry pigment. It is stable at temperatures below 50 ℃ and in the dark. Red raspberry pigment has a certain antioxidant capacity. Sucrose has no significant effect on the pigment. VC has a degrading effect on the pigment. Fe3+ ions have a protective effect on red raspberry pigment, stabilizing its structure.

1.7 Blueberry pigment

Blueberries are plants of the genus Vaccinium in the family Ericaceae. Their fruits are berries that are blue in color, making them one of the rare blue foods. Blueberries have delicate flesh and a delicious sweet and sour taste. They not only contain nutrients such as vitamin C, but are also rich in natural blue pigments. The main component of blue pigments is anthocyanins, which are a type of antioxidant that protects the human body from free radical damage and greatly enhances immunity [18]. Yang Xuefei et al. [19] optimized the ultrasonic extraction process of blueberry pigment based on single factor experiments and used the central composite design method, and analyzed the stability of the pigment. The results showed that the optimal conditions for the ultrasonic extraction of blueberry pigment were 45% ethanol volume fraction, material-to-liquid ratio (g/mL) 1:13, ultrasonic temperature 55°C, pH 4.5, ultrasonic power 450 W, ultrasonic time 50 min. Under these conditions, the extraction rate of blueberry pigment was 274.36 U/g. Blueberry pigment is heat-resistant and has high stability under conditions of metal ions such as K+, Na+, Mg2+ and food additives such as potassium sorbate, but is unstable under conditions of Zn2+, Fe2+, Fe3+, Ca2+ and alkaline environments. Wang Guyuan et al. [20] determined the optimal extraction conditions for blueberry pigments using ethanol extraction through orthogonal experiments: 70% ethanol solution as the extracting agent, extraction temperature of 30 °C, extraction time of 2 h, and liquid-to-material ratio of 1:10.

2 Conclusion

Natural plant pigments come from plants and are much safer than synthetic pigments, and therefore have more advantages than synthetic pigments. However, natural plant pigments are generally the products of plant secondary metabolism, and their composition is complex, making it difficult to completely isolate, purify and identify them. Moreover, there are many types of plant pigments, and they are complex in nature. For a single plant pigment, it has strong specificity in application and has certain limitations in scope of application. Therefore, understanding the structure and properties of the components of natural pigments, as well as their functionality and safety, and expanding the scope of application of plant natural pigments is the main task facing scientific researchers.

References

[1] Wu Xinzhuang, Li Lifeng, Zhu Hua. Overview and prospects for research on the comprehensive utilization of potatoes [J]. Food Research and Development, 2004, 25 (2): 27-29.

[2] Cao Hong, Ding Xuehai, Wang Aiguo. Study on the stability of the pigment in the “Black Beauty” potato [J]. Food Industry Science and Technology, 2011, 32(9): 354-356.

[3] Cao Hong, Ding Xuehai. Study on the extraction process of the pigment in the “Black Beauty” potato [J]. Beverage Industry, 2011, 14(2): 22-26.

[4] Li Caixia, Yang Xiaolong, Li Qiong. Study on the stability of black beauty potato pigment [J]. Food Science, 2010, 31(9): 89-94.

[5] Zhang Yajun, Liang Jiayong, Yue Suju. Study on the extraction process of purple corn cob pigment [J]. Anhui Agricultural Science, 2011, 39(7) : 4041-4042 .

[6] Zhang Yajun, Liang Jiayong, Le Suju. Stability of purple corn cob pigment [J]. Guangdong Agricultural Science, 2011 (5) : 128 -130 .

[7] Xiao Chunling, Zhang Shaoying, Sun Ying. Antioxidant activity of purple corn pigment [J]. Chinese Journal of Cereals, Oils and Foodstuffs, 2011, 26(2) : 18-22 .

[8] Chen Hongjun, Hou Xujie, Bai Hongjin, et al. Analysis of several nutrients in black fruit wolfberry [J]. Chinese Wild Plant Resources, 2002, 21 (2) : 55-57 .

[9] Zhang Yuande, Bai Hongjin, Yin Shenghu, et al. Optimization of microwave-assisted extraction of anthocyanin pigments from black fruit wolfberry [J]. Xinjiang Agricultural Sciences, 2010, 47 (7): 1293-1298.

[10] Chen Chen, Wen Huaixiu, Zhao Xiaohui, et al. Determination of proanthocyanidins in black fruit wolfberry pigment [J]. Spectroscopy Laboratory, 2011, 28 (04): 1767-1769.

[11] Liu Deliang. Study on the extraction process and stability of red amaranth pigment [J]. Hubei Agricultural Science, 2012, 51 (1): 143-146.

[12] Xiang B, Gao J R. Natural pigments [M]. Beijing: Chemical Industry Press, 2004: 139.

[13] Liang B X, Zhao W H, Bai W D, et al. Study on the extraction process of pigments from the peel of dragon fruit [J]. China Food Additives, 2011 (6): 103-108.

[14] Zhang Qianru, Yuan Wei. Study on the extraction and stability of the pigment from the skin of dragon fruit [J]. Processing of Agricultural Products, 2011 (9): 63-64.

[15] Tang Chuanhe, Peng Zhiying. Physiological functions and application prospects of natural anthocyanin pigments [J]. Cold Drinks and Frozen Food Industry, 2000 (1): 26-28.

[16] Sun Xiyun, Liu Ning, Wu Zhaoxia, et al. Purification and preliminary identification of the composition of red raspberry pigment [J]. Food Industry Science and Technology, 2011, 32(2): 285-288.

[17] Wang Feng, Ming Zhe. Extraction and stability of red raspberry pigment [J]. Northern Horticulture, 2010 (20): 14-18.

[18] KALT W, DUFOUR D. Health functionality of blueberries [J]. Horti- culture Technology, 1997, 7 (3): 216-221.

[19] YANG XF, PAN LH, LUO J. Ultrasonic extraction process and stability of blueberry pigment [J]. Food Science, 2010, 31 (20): 251-255.

[20] Wang Guyuan, Gao Leng, Gong Dianpeng. Study on the extraction conditions of blueberry pigment [J]. Journal of Changchun University of Technology: Natural Science Edition, 2010, 31 (2): 202-206.