How Is Plant Food Coloring Used in Food Processing?

Food is the most basic material guarantee for human survival. The development of green food is a need driven by people's growing environmental awareness, health awareness and improving living standards. Among these, colorants play an important role in improving the appearance and quality of food. Compared with synthetic pigments, green and healthy natural plant food coloring has become a hot spot for market development and application in the health industry.

According to the analysis in the “2022-2027 China Natural Food Color Industry Market Competition Pattern Analysis and Development Prospect Forecast Report” by the China Industry Research Institute, the global market size of synthetic food coloring in 2022 is estimated to be about 590 million US dollars, while the market for natural food coloring is expected to reach 1.54 billion US dollars as early as 2021, and is expected to grow at a compound annual growth rate of 7.4%. Although synthetic pigments have a cost advantage and can give food and beverage products a saturated and even color, natural pigments are becoming increasingly important in the food industry as a result of stricter regulations in various countries and the growing number of consumers who are concerned about the safety of synthetic pigments. In addition, most natural Plant Food Coloring is bioactive[1] and can be used for the prevention and treatment of various diseases. It is also widely used in cosmetics and health products.

1 Classification and extraction methods of Plant Food Coloring

The main natural Plant Food Coloring used in the food industry are carotenoids, chlorophyll, betalains and anthocyanins. In order to improve the extraction efficiency of Plant Food Coloring, it is necessary to select an appropriate extraction method. Traditional methods are generally used for the extraction of plant pigments, such as Soxhlet extraction, solid phase extraction and water steam distillation. Traditional extraction methods are simple, economical and easy to use, but they have problems such as solvent residue and time-consuming. Water, ethanol and methanol are most commonly used to extract polar and water-soluble pigments, while non-polar solvents such as hexane, acetone, trichloroethylene and other organic solvents are used to extract lipophilic pigments [2]. Non-traditional extraction methods (commonly referred to as green extraction techniques) have gradually replaced traditional extraction methods. Their advantage lies in the use of less solvent and shorter time. Ultrasonic-assisted extraction, pulse electric field-assisted extraction, microwave-assisted extraction, supercritical fluid extraction [3] and so on are effective methods for Plant Food Coloring extraction.

Natural plant food coloring has different solubilities depending on its chemical composition. Lipophilic pigments are mainly carotenoids, chlorophyll and lutein; water-soluble pigments are mainly betalain and anthocyanin. Due to the diversity of plant food coloring, plant food coloring can be divided into four pyrrole derivative pigments, tetraterpenoid compounds, benzopyran derivatives, pyridine derivatives. Among them, chlorophyll is the main representative of tetrapyrrole derivative pigments, carotenoids are the main representatives of tetraterpenoids, anthocyanins are the main representatives of benzopyran derivatives, and betalains are the main representatives of pyridine derivatives.

2 Extraction of natural Plant Food Coloring

2. 1 Extraction of tetrapyrrole derivative pigments

Tetrapyrrole derivative pigments are the most abundant and widely distributed pigments in nature. Chlorophyll is a natural Plant Food Coloring with a tetrapyrrole derivative molecular structure. Chlorophyll is a magnesium porphyrin compound, a complex organic molecule. Its molecular structure contains a large four-membered ring (porphyrin ring), with a magnesium atom in the central position that is positively charged and a nitrogen atom connected to it that is negatively charged. The carbon-hydrogen side chain (phycoerythrin chain) connected to the porphyrin ring is a lipophilic fatty chain, which determines the lipophilicity of chlorophyll [4]. The structure of chlorophyll enables it to absorb and convert light energy in a specific wavelength range in the visible spectrum. It mainly absorbs red and blue light, while reflecting or transmitting green light, which is why it appears green. Chlorophyll a and chlorophyll b are the main members of the chlorophyll family. Chlorophyll is chemically unstable and can be degraded by light, temperature, pH, oxidants, etc. Chlorophyll has various uses such as blood production, providing vitamins, detoxification, and disease resistance.

Chlorophyll is found in all organisms capable of photosynthesis. Li Ping [5] studied the extraction process and stability of chlorophyll in kelp. The results showed that supercritical CO2 extraction has a low temperature, fast mass transfer rate, and the addition of ethanol as an entrainer effectively improves extraction efficiency. The use of ultrasound and microwave assisted extraction, compared with conventional methods, has a certain degree of improvement in extraction efficiency and purity, with less solvent consumption and low energy consumption.

Chlorophyll stability: The results of the study show that chlorophyll should be stored away from light and high temperatures. and the storage containers should not be made of iron, copper or aluminum. If these containers are inevitably used during processing, EDTA and sodium diacetate can be added to prevent chlorophyll oxidation. The pH value should be adjusted to 6–8 with the appropriate addition of phosphate to achieve the most stable state. The addition of antioxidants such as TBHQ, BHT, vitamin C and vitamin E can significantly improve the stability of chlorophyll in kelp. Weng Xia [6] used anhydrous ethanol as an extraction agent and ultrasonic assisted extraction of chlorophyll from wild spinach. The orthogonal test showed that the optimal process conditions could achieve a maximum chlorophyll extraction of 17.748 mg·g-1. A chlorophyll aqueous solution dispersion system was prepared by compounding with gum arabic powder and maltodextrin, which can improve the stability of chlorophyll to light.

2. 2 Extraction of polyene pigments

Polyene pigments are a type of terpene compound, also known as tetraterpene compounds. The structure of a tetraterpene compound is composed of eight isoprene units connected together. They are widely found in nature, and carotene extracted from carrots is the first tetraterpene compound to be extracted. Because the molecules of tetraterpenoids all contain a relatively large number of conjugated carbon-carbon double bonds,they are all colored substances.

Carotenoids, also known as polyene pigments, include α-carotene, β-carotene, γ-carotene, lycopene and lutein. Vegetables such as carrots contain a large amount of β-carotene. People usually consume β-carotene in food and health food. It is an orange fat-soluble compound that is the most stable natural pigment widely found in nature [7]. β-Carotene can be converted into vitamin A in the body. Vitamin A is beneficial to eye and skin health. Lutein also plays an important role in delaying eye aging and degeneration. Lycopene, the first pigment extracted from tomatoes, has three times the antioxidant effect of β-carotene. It also improves the body's immune system, fights cancer and slows down the aging process.

Ren Bingqian et al. [8] investigated the production and extraction of β-carotene. The traditional organic solvent extraction method is mature, low-investment and suitable for industrial mass production. However, there is solvent residue, the extraction rate is not high, and can also lead to β-carotene isomerization, oxidation and degradation; the column chromatography method is complex; supercritical CO2 extraction is highly efficient, has no solvent residue, and is gentle on the operating conditions; ultrasonic-assisted extraction greatly shortens the extraction time, saves solvent consumption, and avoids the damage to the active ingredients caused by high temperatures; microwave-assisted extraction can improve extraction efficiency and has less environmental impact.

Han Hao et al. [9] used pumpkin powder as the raw material to explore the optimal process for the extraction of β-carotene by ultrasonic-assisted ethanol. The β-carotene extraction yield was 23.811±0.589 mg·g-1. The results of the stability test showed that β-carotene should be stored in the dark at low temperatures, and that Zn2+ and Fe3+ had the greatest effect on pumpkin β-carotene. Li Weixue et al. [10] investigated the extraction of lutein. The test results showed that supercritical CO2 extraction combined with solvent extraction is an effective method for extracting lutein from marigolds, and the extraction effect is better than that of traditional solvent extraction.

Liu Bingxue et al. [11] used Northeast China marigolds as raw material, extracted lutein from marigolds using the ultra-high pressure method, and optimized the extraction process. Under the optimized process, the lutein extraction rate was 68.57 ± 2.31 mg·g-1. Yu Wenjing et al. [12] studied the main active substances in tomatoes. Lycopene has good solubility in supercritical CO2, and the use of supercritical fluid extraction can reduce isomerization and decomposition. Wang Haifeng et al. [13] optimized the supercritical CO2 extraction process, and the purity of lycopene reached more than 90% under the optimal process parameters. Lin Zehua et al. [14] introduced the extraction of lycopene using organic solvent extraction, supercritical CO2 extraction, ultrasonic-assisted extraction, microwave-assisted extraction, ultrasonic-microwave synergistic extraction, ultra-high pressure-assisted extraction, and high-pressure pulsed electric field-assisted extraction. Compared with organic solvent extraction, all these processes can improve the extraction rate. high-pressure pulsed electric field-assisted extraction is especially suitable for the extraction of heat-sensitive substances.

2. 3 Extraction of polyphenolic pigments

Polyphenolic pigments are represented by anthocyanins and flavonoids. The molecular structure of these pigments is characterized by the presence of 2-phenylbenzopyran; anthocyanins show different colors at different pHs; flavonoids are widely distributed in the plant world and are a large class of water-soluble natural pigments. Due to the presence of phenolic hydroxyl groups in their structure, they are generally acidic.

Proanthocyanidins are currently considered to be the most effective natural antioxidants. Black goji berries are an ideal plant for extracting proanthocyanidins. Zhang Rong et al. [15] studied the extraction process and antioxidant activity. The optimal process conditions for ultrasonic-assisted extraction were used to obtain a proanthocyanidin yield of 2.72%. The proanthocyanidins in black goji berries had a higher total reducing capacity than vitamin C, and were effective in scavenging DPPH free radicals and OH-free radicals. Zhang Huimin et al. [16] used purple grape skins as raw material and extracted anthocyanins using an ultrasonic-assisted method with 65% ethanol. Through single-factor experiments and response surface test design, the process parameters were optimized. The anthocyanin yield was 25.50 mg·g-1, and the correlation coefficient between the theoretical value and the verified test value was 99.3%. In order to explore the extraction process of flavonoids and polyphenols and their antioxidant capacity, Li Shengrao et al. [17] optimized the process parameters for the extraction of anthocyanins from blueberry using high-voltage pulsed electric fields assisted extraction by response surface methodology. The anthocyanin extraction yield was 34.20 mg·g-1. The experimental results showed that high-voltage pulsed electric field assisted extraction was efficient and had low solvent consumption. Hang Shuyang et al. [18] used the rhizome skin of Chinese yam as the raw material and used an orthogonal design to optimize the process of ultrasonic-assisted ethanol extraction of flavonoids and polyphenols from the rhizome skin. The flavonoid yield was 0.929%, and the crude extract had reducing power and total antioxidant capacity.

2. 4 Extraction of pyridine derivative pigments

Pyridine derivative pigments are mainly betalains and betaxanthin in red beets. The main component of betalains is betaine, which is a water-soluble natural Plant Food Coloring [19]. Yin D. et al. [20] optimized the extraction method of betalain from red beets, using ultrasound-assisted extraction. The relative error between the actual betalain content and the predicted value was 1.96%. Tang Ling et al. [21] summarized the research on the extraction and purification technology of betaxanthin in recent years in domestic literature. Traditional extraction techniques such as solvent extraction have low extraction efficiency and pollute the environment. Emerging technologies such as ultrasonic, microwave and other auxiliary technologies, high-pressure pulsed electric field assisted extraction technology, combined with purification processes such as macroporous resin adsorption and membrane separation, effectively improve the extraction rate of betaxanthin and have good development prospects.

3 Plant Food Coloring: Prospects for application in food processing

With people becoming more health and environmentally conscious, the nutritional value and safety of food has become a major theme in food processing and development. Compared to artificial synthetic pigments, natural Plant Food Coloring is safer, more environmentally friendly as it is biodegradable, and has gradually replaced artificial synthetic pigments as a food coloring agent. In addition, natural Plant Food Coloring, as an ingredient in food, has a variety of physiological benefits in itself. For example, anthocyanins, which are powerful antioxidants, have always been a hot research topic in the fields of botany, food science and nutrition. The length of human life directly depends on the strength of people's ability to resist oxidation and free radicals. The discovery of anthocyanins has found an effective way to fight aging.

Lycopene also has a powerful effect in scavenging free radicals in the human body. Its rate constant for quenching singlet oxygen is 100 times that of vitamin E. In addition, lycopene is a hypocholesterolemic agent that can regulate the body's cholesterol metabolism. Huang Ruoan et al. [22] found that lycopene has a certain inhibitory effect on cancers of the digestive tract, cervix, breast, skin and bladder. Traditional cancer treatments include chemotherapy and radiotherapy, which have significant side effects. Lycopene inhibits the development of cancer cells by reducing the production of oxidative products, lowering the content of inflammatory factors and regulating signal pathways.

“Eye protection medicine” Lutein is the main component of the pigment in the macula of the human eye's retina, and can improve diabetic retinopathy. Xiao Yiqin et al. [23] observed the concentration changes in type 2 diabetic patients after oral administration of lutein, and showed that the blood lutein concentration increased significantly and remained stable. No serious adverse events were observed, providing a basis for studying the safety of clinical application of this drug. Modern life is stressful and fast-paced, and emotions and unhealthy lifestyles may lead to cardiovascular and cerebrovascular diseases. Liu Yaxin et al. [24] summarized and analyzed the research on betaxanthin in cardiovascular disease, neuroprotection, cell protection, anti-inflammation, etc. The study found that betaxanthin can reduce low-density lipoprotein in the blood, increase high-density lipoprotein and vasodilation; For the treatment of chronic neurodegenerative diseases such as Alzheimer's disease, betaxanthin also has certain development potential. Studies have found that betaxanthin can inhibit the aggregation of Aβ, which causes Alzheimer's disease.

The healthy physiological effects of natural Plant Food Coloring combined with color psychology are widely used in functional foods. The market for functional foods has continued to grow in recent years. However, the widespread use of Plant Food Coloring in the food industry is still limited by yield, production costs, regulatory approval, pigment characteristics, and tolerance to environmental factors such as temperature, light, and pH. Therefore, the industry faces the challenge of maintaining the availability and stability of these functional foods, which can ultimately truly promote human health. When natural pigments are used in food, care should be taken when extracting natural pigments to ensure the safety of the extraction process and the quality of the natural pigments, using non-toxic solvents and environmentally friendly extraction techniques.

After the addition of natural pigments, food industrial processing can lead to changes, degradation, and even loss of natural Plant Food Coloring. During processing, attention should be paid to controlling the factors that affect the stability of natural pigments, such as pH, temperature, water activity, oxygen, metals, solvents, the presence of enzymes, and ionic radiation. The storage of natural pigments is also a challenge facing the food industry. The stability of natural pigments after extraction is mainly solved by effective encapsulation [25]. Encapsulation mainly enhances the stability, bioavailability, bioaccessibility, digestibility and controlled release of plant food coloring. In food formulations with added natural pigments, efficient encapsulation techniques are required to control degradation and maintain bioavailability in the final product.

Spray-drying microencapsulation is a technique that uses biopolymers to trap natural pigments to protect them from food processing and environmental factors [26]. Quoc-Duy Nguyen et al. [27] studied the effects of inlet temperature and the ratio of anthocyanins to maltodextrin on the phenols, anthocyanins, antioxidant activity and some physical properties of spray-dried hibiscus pollen. The encapsulation rate of different samples was also measured and compared. Maltodextrin was used as a carrier to microencapsulate anthocyanins by spray drying (inlet temperature 170°C). The encapsulation rate was higher than 85%, which increased the total phenolic content and solubility (94.91%) and had no significant effect on the color of the product. Encapsulation of Plant Food Coloring can also be carried out using other techniques, such as microemulsification, freeze drying, supercritical fluid encapsulation, cyclodextrin encapsulation technology, and lipid carrier technology [28].

4 Conclusion

The innovation and development of Plant Food Coloring is an inevitable trend to meet the growing consumer demand for natural and healthy foods. Since natural pigments are highly susceptible to environmental factors, the addition of natural pigments to foods still faces some challenges. Research on natural pigments has focused on the identification of new and renewable sources, structural analysis, extraction and separation methods, bioactivity, bioavailability, factors affecting stability, industrial applications, high-yield production and stable processing methods. It is also a huge challenge to develop new technologies, cost-effective methods and industrialization for the extraction of natural pigments. In addition, more research should be done on the safety, health and efficacy of natural pigments in the human body, not just in vitro studies.

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