Study on Natural Yellow Food Coloring

Natural food coloring is a product obtained by refining natural raw materials (mainly plant materials). It is a natural food additive used for coloring food. Regardless of the source of the pigment, safety is always the top priority. With the development of the economy, the continuous improvement of people's living standards, and the gradual strengthening of health awareness, products made from all-natural ingredients have gradually become the mainstream of food consumption.

1 Curcumin

1.1 Properties of curcumin

Curcumin is a natural coloring agent extracted from the Chinese medicinal herb turmeric. Research on curcumins, the main active ingredient in turmeric, which accounts for 3% to 6% of the spice, has attracted much attention at home and abroad. The curcumins mainly include curcumin, demethoxycurcumin and bisdemethoxycurcumin. Turmeric pigments are insoluble in cold water, but soluble in ethanol, propylene glycol, glacial acetic acid and alkaline solutions. They are easily discolored by iron ions and have poor light and heat stability. They have good coloring power, especially for proteins.

1.2 Health benefits of curcumin

Curcumin can metabolize mutagens and indirectly inhibit their metabolism, and has an anti-mutagenic effect. It can scavenge free radicals, reduce the production of peroxides, inhibit the production of arachidonic acid metabolites, inhibit the expression of cancer cells, and has an anti-tumor effect. It can inhibit the oxidation of lipids in the air and Fe2+, Cu2+, inhibit the oxidation of hemoglobin by nitrous acid, and prevent the oxidative damage of DNA. has an antioxidant effect; can inhibit cell oxidation, modify low-density lipoprotein (LDL), and oxidized low-density lipoprotein plays an important role in atherosclerosis, and has a hypolipidemic and anti-atherosclerotic effect. Some data show that curcumin has a good inhibitory effect on most bacteria, especially Bacillus subtilis, Staphylococcus aureus and Escherichia coli. Therefore, curcumin has various physiological functions such as anti-inflammatory, anticoagulant, anti-infective and preventing the formation of age spots.

1.3 Curcumin extraction

The extraction of curcumin from turmeric using organic solvents is currently a commonly used process at home and abroad, but the extraction agents and extraction conditions used are not consistent. For the separation of the three curcuminoids, thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC) are commonly used methods of separation. They have high separation efficiency, but the amount of separation is small and the cost is high.

2 Lutein

2.1 Properties of lutein

The main component of lutein is xanthophyl, which is a carotenoid pigment. The molecular formula of lutein is C40H56O2. The unique chemical structure of lutein not only determines its color, but also its physicochemical properties. The lutein molecule has a long chain of 40 carbon atoms, with multiple conjugated double bonds. It is these conjugated double bonds that give lutein its bright color and its ability to inhibit free radicals.

Lutein is found in the cell membrane in such a way that the hydrophobic long chain is buried in the phospholipid molecule layer, while the hydrophilic hydroxyl group remains on both sides of the membrane. This positioning allows lutein to bind to the cell membrane lipids that are highly susceptible to oxidation to the greatest extent, thereby strengthening the cell membrane. In terms of stability, free lutein is extremely unstable to heat, lutein monolaurate (ML) is slightly more stable, and dilaurate (DL) is extremely stable to heat, but ML and DL are less sensitive to UV light than free lutein.

Lutein has absorption peaks at 446 nm, 442 nm and 470 nm. It is relatively stable to heat, but should not be used or stored above 80 °C. It is not stable to direct sunlight, but is stable to scattered light and UV light of a certain intensity. It is resistant to oxidation and reduction. It is stable to acids and alkali stable, the pigment retention rate is still above 96% when the pH is 2.7–12.5; stable to EDTA and most metal ions, but sensitive to Fe2+, Al3+, and Cu2+ [1–5].

2.2 Health benefits of lutein

2.2.1 Vision protection

Lutein is a major carotenoid found in the eye, which can prevent cataracts and protect against age-related macular degeneration. According to extensive research, the main physiological functions of lutein in the eye are: it can act as an antioxidant and photoprotectant [6-8].

2.2.2 Delaying early atherosclerosis

According to recent studies, lutein has a delaying effect on the early atherosclerosis process. In addition, lutein in arterial wall cells can also reduce the oxidation of LDL cholesterol.

2.2.3 Anti-cancer effect

Recent research findings show that lutein has an inhibitory effect on a variety of cancers, such as breast cancer, prostate cancer, rectal cancer, colon cancer, skin cancer, etc. Lutein is one of the main carotenoids in human blood, and it has special biological functions in inhibiting tumor growth. In vitro studies have shown that lutein is more effective than β-carotene in inhibiting lipid peroxidation of cell membranes and oxidative damage induced by oxidation.

2.2.4 Antioxidant effect

As an antioxidant, lutein can inhibit the activity of active oxygen free radicals and prevent damage to normal cells by active oxygen free radicals. Lutein can destroy singlet oxygen, thereby protecting the body from harm. In addition, lutein can also enhance the body's immune function [9,10].

2.3 Extraction of lutein

The chemical synthesis of a single isomer of lutein has not yet been successful. Currently, only lutein extracted from natural plants contains bioactive substances with antioxidant effects. Currently, there are the following main methods for extracting lutein:

2.3.1 Drying method

A new type of rotary drum dryer has been successfully developed for drying and beating marigold flowers or marigold petals, from which lutein can be extracted. When the frequency of beating is different, the efficiency of beating fluctuates between 70% and 90%. The amount of lutein depends on the length of drying time, but for the same drying time, the lutein content extracted at 70°C is less than that extracted at 60°C.

2.3.2 Extraction method

Lutein is extracted from marigold flowers, and Qingdao High-Tech Industrial Park Qingda Natural Products Research Institute has already formed industrial production. The extraction process is: marigold flowers → fermentation → drying → granulation → hexane extraction → negative pressure evaporation and separation → lutein resin.

2.3.3 Membrane separation technology

Ceramic membrane microfiltration (MF) is used to fine filter and purify the extraction solution, and then a reverse osmosis membrane (R0) is used to concentrate and filter the solution. This process uses membrane separation technology as the main method, replacing traditional ethanol purification and evaporation concentration. The process is simple, and the pigment solution is basically operated at room temperature, which saves energy and ensures the quality of the pigment product.

2.3.4 High-performance liquid chromatography

Yun Wei et al. used high-speed countercurrent chromatography (HSCCC) to extract lutein from marigolds and determined that the optimal composition of the mobile phase was 10:3:7 (heptane:chloroform:methylene chloride). The lutein content of the extract was measured using HPLC and was found to be greater than 98.5% [11–13].

2.3.5 Microwave heating method

Yang Lifei and Deng Yu used tea as the raw material, 6# solvent as the medium, and microwave heating to extract lutein. The effect of the extraction rate on the product was studied by changing the solvent concentration, microwave power, extraction time and other conditions to obtain the optimal extraction conditions. The results showed that the material ratio (W/V) was 1:25, the time was 30s, and the microwave extraction was carried out twice, and the lutein extraction rate reached 65.45%. Compared with the traditional extraction method, this method not only saves solvent, but also greatly improves the extraction efficiency.

3 Monascus yellow

3.1 Properties of Monascus yellow

It has been reported that red yeast rice yellow pigments are mainly monascin and ankaflavin [14] (see Figure 1). Its characteristic absorption wavelength is 385 nm, and it is soluble in petroleum ether, ether, benzene, hexane, ethanol, methanol, acetone, etc., but insoluble in water [15], and is a low-polarity substance.

In 2004, Yong Smith, B., et al. from Thailand isolated two new yellow pigments, monascusones A and monascusones B, from the screened-from-the-yield yellow pigment Monascus kao liang KB20 M 10.2, and deduced their structural formulas through photochemical analysis and nuclear magnetic resonance data [16,17], as shown in Figure 2. 

3.2 Health benefits of red yeast rice yellow pigment

For decades, there have been frequent reports of red yeast rice having new and diverse medicinal and health benefits [18–20]. It has physiological activities that lower blood pressure, blood lipids and cholesterol. Red yeast rice yellow pigment is a type of natural pigment that is insoluble in water but soluble in organic solvents such as acetone and ethanol, has strong heat resistance and is highly safe for consumption [21].

3.3 Extraction of red yeast rice pigment [22]

The results of research experiments show [23]: Supercritical CO2 extraction has a low extraction rate of red yeast rice pigment, about 5%, but a high extraction rate of citrinin in red yeast rice, reaching more than 83%. Therefore, citrinin is first extracted from red yeast rice by supercritical CO2, and then red yeast rice pigment is extracted by solvent extraction, so that high-quality red yeast rice pigment with high color value and low citrinin can be obtained.

Red yeast rice was extracted with 70% ethanol at 60°C for 3 times, each time for 45, 60 and 45 minutes. The extract was filtered and concentrated under vacuum at 50°C in a rotary evaporator to a concentration of 20%. The concentrated solution was added to distilled water and allowed to precipitate. After soaking and washing, the water-soluble components were removed to obtain the alcohol-soluble components. The alcohol-soluble yellow pigment components in red yeast rice were then separated and purified using column chromatography, and their purity and photostability were characterized using high-performance liquid chromatography. The results showed that the yellow pigment in the alcohol-soluble red yeast rice extract can be separated and purified using column chromatography. The maximum absorption peak is 385 nm. Red yeast rice yellow pigment is sensitive to light. After light exposure, the molecular structure changes due to decomposition, resulting in discoloration. Therefore, red yeast rice yellow pigment should be stored away from light.

4 Saffron yellow

4.1 Properties of saffron yellow

Safflower is the dried flower of the safflower plant in the Asteraceae family. It is pungent and warm in nature, entering the heart and liver meridians, and has the effect of removing blood stasis and relieving pain. It is a good medicine for various formulas that promote blood circulation and remove blood stasis. Safflower yellow pigment is a chalconoid compound that has a variety of pharmacological effects, such as dilating coronary arteries, anti-oxidation, protecting the heart muscle, lowering blood pressure, immunosuppression and brain protection. The content of safflower yellow pigment is one of the main indicators for evaluating the efficacy of safflower.

4.2 Health benefits of safflower yellow pigment

Studies have shown that safflower yellow pigment is the pharmacological component of the Chinese herbal medicine safflower, which has no toxic side effects. It can inhibit platelet aggregation and release induced by platelet-activating factor, competitively inhibit the binding of platelet-activating factor to platelet receptors, and can be directly used in medicine, as well as in health products, food, cosmetics and textile coloring.

4.3 Extraction of safflower yellow pigment

Chinese scientists used spectrophotometry to study the effects of different solvents and extraction methods on the content of crocin in safflower. The results showed that water and 70% methanol had a good effect on the extraction of crocin, while 80% acetone had a greater effect on the content of crocin, resulting in a lower extraction rate. In comparison, the extraction of crocin using water is a better, faster and more convenient method.

5 Gardenia yellow pigment

5.1 Properties of gardenia yellow pigment

The main components of gardenia yellow pigment are carotenoid-like crocin and crocetin, as well as gardenoside, a cyclic enol ether glycoside, flavonoids and chlorogenic acid. Crocin and crocetin are rare water-soluble carotenoids with multiple conjugated double bonds in their molecules. On the one hand, they give gardenia pigments their yellow color, but on the other hand, they may also be one of the reasons for the instability of gardenia pigments. Gardenia yellow pigment is soluble in polar solvents such as water and ethanol, but not in non-polar solvents such as benzene and gasoline. The pigment is less stable under acidic conditions, but stable under neutral and alkaline conditions. Similar to other carotenoids, gardenia yellow pigment is not very lightfast. Apart from Fe2+ and Cu2+, common metal ions have little effect on gardenia yellow.

5.2 Health benefits of gardenia yellow pigment

Gardenia yellow pigment has an inhibitory effect on Staphylococcus aureus, Neisseria meningitidis, Neisseria gonorrhoeae, and Streptococcus pyogenes, and has antibacterial and antiviral effects. It has an anti-inflammatory and analgesic effect on soft tissue damage, and an anti-inflammatory effect. It can also assist in the treatment of jaundice hepatitis and has a choleretic effect.

5.3 Extraction of gardenia yellow pigment

There are many methods for extracting gardenia yellow pigment. Gardenia yellow pigment contains hydrophilic groups and is easily soluble in water, so the pigment can be extracted by soaking in water. The water extraction method involves crushing, degreasing, soaking, filtering and concentrating to extract a 20% to 50% slurry. This method has the advantages of being simple, requiring little investment and low production costs. However, the pigment produced by this method is of low purity, has poor appearance and is a 20% to 50% slurry, which is difficult to refine and has little use value.

In order to obtain purified pigment active ingredients, most domestic and foreign methods use ethanol extraction, or first extract with a water solution and then separate and purify with ethanol. Generally speaking, using ethanol as a solvent has the advantages of higher pigment purity and higher color value than using water as a solvent. However, it has the disadvantages of complex process requirements, making it difficult to produce a powder with a high color value, and high cost.

If a ceramic membrane is used to directly filter and purify the pigment extraction solution to remove suspended impurities and pectin in the extraction solution, the purity of the pigment product can be guaranteed. At the same time, the purified solution can be directly concentrated by reverse osmosis without causing serious pollution to the reverse osmosis membrane. The concentrated pigment solution is then treated using reverse osmosis, which removes a large amount of water and concentrates the pigment solution, thereby avoiding the evaporation of a large amount of water, reducing energy consumption and helping to ensure the stability of the pigment.

Gardenia yellow pigment extracted from gardenia contains a large amount of gardenoside and other impurities. The presence of gardenoside not only affects the stability of gardenia yellow pigment, but also more seriously causes the pasta dyed with gardenia yellow pigment to turn green. Therefore, the crude pigment needs to be refined. At present, the main methods for refining gardenia yellow pigment are: column chromatography, reverse osmosis, acid-base precipitation, solvent reflux, etc., but they all have disadvantages such as high production costs, organic solvent residues, and environmental pollution, which limits their widespread application. Refining the pigment using adsorption and desorption is a method suitable for industrial production.

6 Other edible natural yellow pigments

Zein: Zein is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the seed coat of many plants. It is a kind of protein that is found in the It exists in the form of natural lipids in corn endosperm and has high nutritional value. Cryptoxanthin is a precursor of vitamin A.

Red date pigment: red dates have a high pigment content and a simple extraction process [26]. It is safe and non-toxic, resistant to acids and alkalis, and has good stability to light, heat, oxidizing and reducing agents, some food additives and metal ions.

Sea buckthorn yellow pigment: The main component of sea buckthorn yellow pigment is carotenoid, which is mainly found in the sea buckthorn fruit peel.

Polygonum pigment: It is a kind of yellow-brown phenolic pigment extracted from the leaves of Polygonum multiflorum.

Limonium aureum, also known as yellow flower, is a perennial herb in the Plumbaginaceae family that grows in Gansu, Qinghai, and Inner Mongolia provinces. It can also produce yellow pigments.

Forsythia is also rich in yellow pigments. The pigments are insoluble in water but soluble in organic solvents.

7 Outlook on the development of plant-based food coloring

7.1 Multifunctional natural yellow pigments are the way forward

Among multifunctional food coloring, nutritional natural food coloring has attracted a lot of attention, and the most successful is carotenoid pigments. As the market for functional food additives expands, natural yellow pigments will appear as health products and will no longer be limited to coloring agents. It can be predicted that multifunctional natural yellow pigments are the future direction of development for food yellow pigments.

7.2 Strengthen research on stabilization techniques

The biggest drawback of natural yellow pigments is their poor stability, and research into stabilisation techniques is the key to their industrialisation and improving product quality. Most of China's natural pigment manufacturers have outdated technology, rudimentary equipment and poor product stability, which seriously affects the competitiveness of China's natural pigments in the international market. China's research into the stability of natural yellow pigments is also not at a high level, and strengthening research in this area is also urgent.

7.3 The role of biotechnology in the production of natural food colours

The supply of raw materials for natural pigments is easily affected by the seasons and climate, but the production of natural pigments using biotechnology overcomes this drawback. Many natural pigments that were previously produced using traditional solvent extraction methods are now being produced using biotechnology. In the future, biotechnology is bound to play an increasingly important role in the production of natural food colors, including natural yellow pigments.

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