Synthetic pigments can be divided into two categories, azo compounds and non-azo compounds, according to their chemical structure. Azo compounds can be divided into oil-soluble pigments and water-soluble pigments.

Oil-soluble pigments are insoluble in water and cannot be easily discharged from the body after entering the human body, so their toxicity is relatively large, and they are basically no longer used as coloring agents for food in various countries. For water-soluble pigments, it is generally believed that the more sulfonic acid groups there are, the faster they can be excreted from the body and the less toxic they are.

Properties of 5 kinds of synthetic pigments

1. Amaranth red

Amaranth red, also known as light acid red, molecular formula: C20H11O10N2S3Na3, molecular weight 604.5 . Chemical name: Trisodium salt of 1-(4′-sulfo-1′-naphthalene azo)-2-naphthol-3,6-disulfonic acid. Its structural formula:

It is purple-red powder, odorless. 0.01% solution in water is purple-red. Soluble in glycerol and propylene glycol, slightly soluble in ethanol, insoluble in oil.

Easily decomposed by bacteria, stable to light, heat and salt. Good acid resistance, citric acid, tartaric acid, etc. are very stable. However, it can become dark red in constructive solution. It is not suitable for coloring of fermented food because it is sensitive to oxidation and reduction.

Toxicity:
There are many toxicity test reports, and there are different statements on carcinogenicity, with suspicion of teratogenicity and carcinogenicity.ADI (Allowable Daily Intake for Human) 0～0.75mg/Kg body weight.

The maximum allowable use level is 0.25g/kg in candied cold fruits, decorative fruits and vegetables, candies, colorful dressings on pastries, fruit and vegetable juice (pulp)-type beverages, carbonated beverages, flavored beverages (fruit-flavored beverages only), prepared wines and jellies.

2. Carmine

Carmine, molecular formula C20H11O10N2S3Na, molecular weight 604.5, also known as Lichun Red 4R, chemical name is 1-(4′-sulfo-1′-naphthalene azo)-2-naphthol-6,8-disulphonic acid trinational salt. Its structural formula:

It is red powder, odorless. Soluble in water, glycerol, insoluble in ethanol, insoluble in grease. Stable to light and acid, but weak to heat and reduction. Turns brown when exposed to alkali, easily decomposed by bacteria.
ADI: Provisional 0~0.125mg/kg
GB 2760-2014 stipulates that cochineal and its aluminum color precipitate are allowed to be used in 30 kinds of food.

3. Lemon Yellow

Lemon yellow is also known as hydrazine yellow or wine sulfur, molecular formula: C16H9O9N4S2Na3, molecular weight is 534.38, chemical name is 3-hydroxy-5-carboxy-2-(p-sulfophenyl)-4-(p-sulfophenylazo)-triple nanosalt of o-azomole. Orange-yellow powder, odorless. 0.1% aqueous solution is yellow. Soluble in glycerol, propylene glycol, slightly soluble in ethanol, insoluble in oil. Stable to acid, light and heat, poor oxidation resistance. Slightly reddened by alkali, color fades when reduced.
ADI: 0~7.5mg/kg
GB 2760-2014 stipulates that lemon yellow and its aluminum color precipitates are allowed to be used in 32 kinds of food.

4. Indigo

Indigo molecular formula: C16H8O8N2S2Na2, molecular weight 466.37, its chemical name is 5,5′-indigo pigment disulfonic acid of the dinar salt. It is an indigo pigment with the following structural formula:
Indigo is blue powder, odorless. 0.05% aqueous solution is blue. Solubility in water is smaller than the previous three. Soluble in glycerol, propylene glycol, slightly soluble in ethanol, insoluble in oil. Sensitive to light, heat, acid, alkali, oxidation, salt resistance and bacterial resistance are weak, color fading after reduction, but dyeing is better.

ADI: 0~2.5mg/kg
GB 2760-2014 stipulates that indigo and its aluminum color precipitates are allowed to be used in 14 types of food.

5. Sunset Yellow

Molecular formula: C16H10O11N2S2Na2, molecular weight: 452.37
Sunset Yellow, also known as Orange Yellow, is the disodium salt of 1-(p-sulfophenylazo)-2-naphthol-6-sulfonic acid sodium salt, which is a monoazo pigment.
It is orange powder, odorless, easily soluble in water, 0.1% aqueous solution is orange-yellow, soluble in glycerol, propylene glycol, but difficult to dissolve in ethanol, insoluble in grease. Light resistance, heat resistance, acidity is very strong, alkali resistance is still good. Only reddish brown color when meet alkali. Color fades when reduced.

ADI: 0~2.5mg/kg
GB 2760-2014 states that sunset yellow and its aluminum color precipitates are allowed to be used in 36 types of food.

Table 1 Comparison of the properties of 5 edible synthetic colorings
Note: 1.0~2.0 means stable; 2.1~2.9 means moderately stable; 3.0~4.0 means unstable; above 4.0 means very unstable.

Use of synthetic colors
Precautions and Methods of Preparation

1. Preparation of pigment solution

Direct use of pigment powder is not easy to distribute evenly in food, and may form pigment spots, so appropriate solvents to dissolve, the pigment will be formulated into a solution for application. General use of the concentration of 1 ~ 10%, too thick solution is also difficult to regulate the color tone. Prepared solution can not be left for a long time, otherwise there is pigment precipitation. Carmine will become black after long time, the preparation water should be boiled and cooled before use. Avoid the use of metal utensils.

2. The choice of color and color combination

The choice of hue should take into account the consumers’ love and knowledge of food color, i.e., it should be similar to the original color of the food or the hue that is consistent with the name of the food.

In order to enrich the color spectrum of edible synthetic pigments to meet the needs of food processing and production coloring, five kinds of pigments can be mixed and blended in different proportions. Theoretically, by the red, yellow, blue three kinds and the original color, you can match a variety of different chromatograms, the basic color matching method is as follows:
Color mixing, due to the solubility of various synthetic pigments in different solvents is different, therefore, the same proportion of the pigment mixture, in different solvents, may appear different shades and intensity. Therefore, color blending should be done according to the actual need.
Table 2 Ratio reference for mixing different shades of food colorings
Table 3 Reference for color mixing of commonly used synthetic pigments

In addition, some coloring foods, due to the evaporation of water, resulting in the concentration of pigments on the surface of the “concentrated impact”. In addition, the stability of various pigments to light is different, and the speed of fading is also different. Therefore, we must be flexible through the test.