There are two general types of staining: the pigment is dissolved or dispersed in liquid or paste-like foods; and it stains the surface of the food.
The staining property of the same pigment is different for different dyeing substrates, and the staining property of different pigments for the same dyeing substrate is also different, such as erythrosine has a better staining property for protein substrates, while lemon yellow has a worse staining property.

(D) Firmness
Firmness is the color stability of the dyed material or pigment resistance to changes in the surrounding environment. Firmness of the pigment is an important indicator of the quality of the pigment.
Firmness depends mainly on the chemical nature of the pigment and the nature of the dyed material. However, the use of improper pigment varieties or improper operation is also easy to reduce the firmness. Firmness is a comprehensive standard, with the following indicators.
1、Heat resistance 2、Acid resistance
3, alkali resistance 4, oxidation resistance
5、Reducibility 6、Ultraviolet (sunlight) resistance
7、Salt resistance 8、Bacteria resistance
Since most of the food processing, heat treatment, so the coloring agent is required to have a certain degree of heat resistance. The heat resistance of colorants is related to coexisting substances such as sugar, salt, acid and alkali. When coexisting with the above substances, it will change color and fade.

Lemon yellow, sunset yellow heat resistance is stronger; indigo, carmine heat resistance is weaker.
Most of the general foods are in the acidic range, such as fruit foods, candy, drinks, lactic acid fermented foods and vinegar foods, etc. Colorants may form pigment acids in the acidic environment and easily discolor or precipitate, so the colorants are required to have a certain degree of acid resistance.
The acid resistance of indigo is weak; the acid resistance of lemon yellow and sunset yellow is strong.
Adding alkaline puffing agent cake food, in addition to high-temperature processing, there are alkaline substances, which requires a certain degree of alkaline resistance of the pigment.
Lemon yellow has a strong alkali resistance; carmine has a weak alkali resistance.
Therefore, the coloring agent is required to have better antioxidant properties, anthracene pigment antioxidant is stronger, azo pigments or other pigments are generally weaker, which nature of fruit juices, margarine and so on have a greater impact.
Colorants added to food can be subjected to reduction and fade. This phenomenon can be in the fermented food processing, or in food manufacturing, storage and other processes due to the role of microorganisms and can also be caused by metal containers (iron, aluminum, etc.) and acid reaction, or metal containers and salt potential difference.

In addition, ascorbic acid and sulfites and other additives also have a reducing effect. Anthracene pigments are quite stable, while indigo and azo pigments are unstable.
The use of translucent film packaging food, ultraviolet light can deteriorate the quality of food. The UV resistance of colorants varies considerably depending on the nature of the water used to manufacture the food (pH, hardness, heavy metal ion content, etc.) and the type of substances that coexist with the pigment. Indigo is less UV resistant; lemon yellow and sunset yellow are more UV resistant.
For pickled products, salt resistance is required for colorants due to the high salt used. Lemon yellow in the salt concentration of 20 ° Be above is still more stable; indigo in 1 ~ 2 ° Be that is not stable.
The stability of different colorants to bacteria is different. Lemon yellow, sunset yellow is more resistant to bacteria; indigo is weaker.

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