First, the importance of color retention and its characteristics

1. Preservation of color of agricultural products is the primary issue for preservation. Now that many agricultural products have lost their marketability, they begin with discoloration.
The first problem of improper storage and handling of fresh produce is discoloration, which begins with discoloration. Especially fresh produce such as vegetables and fruits. Once discolored, its value is greatly reduced. The price will plummet, and it won't even be acceptable to consumers.

2. The preservation of agricultural products has become a current problem in the circulation of commodities.
The discoloration of agricultural products will result in discoloration from the moment they are picked to the circulation, and the quality and value of these products will be seriously reduced.
For example, in the mid-1990s, Hami melon from Xinjiang received a batch of foreign orders. The sample and order location was in the origin of Xinjiang, and the delivery was in Shenzhen. Foreign merchants took pictures according to the samples provided at that time, first in the contract. Indicate acceptance by sample. Then Xinjiang's supply was convenient for receiving delivery preparations. After packaging and transportation, it took more than half a month to arrive at Shenzhen's delivery site from the date of shipment. Foreign merchants also immediately conducted inspections in Shenzhen, but later the foreign merchants received goods and samples. Color does not match and reject. After difficult negotiations between the two parties, foreign investors reluctantly agreed to receive the goods and reduced the original purchase price by 30%. As a result, they caused huge losses to Xinjiang suppliers.
From this example we can see that the preservation of color is extremely important. In fact, there are many examples in this area. We will not give examples here.

3. Characteristics of color retention of agricultural products The color retention of agricultural products is affected by many factors, mainly reflecting the following characteristics.
1 The color change speed is fast, and the color retention measures are required in a timely manner, starting from picking at the production site.
(2) The discoloration time is short, and it will be completely different in a very short period of time. Therefore, the color retention process needs to be performed quickly in a short time.
3 The discoloration influence factors of different products are not the same.
4 The color change speed of different primary colors is not the same.
5 Different environmental conditions have different effects on discoloration.
6 Green is the most volatile and unstable species. Therefore, vegetable preservation is the most difficult.

Second, color retention techniques and methods

Techniques for color retention of agricultural products. According to its intrinsic mechanism of discoloration, it can be summarized as follows: (In the specific use, several technical methods can be comprehensively considered for different species)

1. Control of moisture retention technology The effect of moisture on the stability of pigments varies greatly depending on the nature of the pigments. Carotenoids are generally very stable in living bodies, but they are very unstable when they come in direct contact with oxygen after drying. Freeze-dried carrots, if exposed to oxygen, turn into white carrots in a short period of time. Chlorophyll hardly decomposes in an absolutely dry state, but when the water reaches 6% to 8% or more, it decomposes rapidly and clearly. Especially in the presence of light and oxygen, the decomposition rate is accelerated and fades in a short time. The anthocyanidin pigment is also very stable in absolute dry conditions, but if the water content increases, it is susceptible to fading due to pH.
The above is the study of the color change of different foods in different moisture content, but as a preservation of agricultural products, the main target is vegetables rich in moisture and mostly green, which increases the possibility of discoloration. Therefore, when controlling moisture, contact with oxygen and light should be minimized.

2. Control of temperature retention technology Temperature has a great influence on the color of agricultural products and can be considered as one of the key factors. The first method of freshness preservation of any agricultural product is to reduce the temperature, but the purpose of the package is to circulate, such as supermarkets, etc. Therefore, only proper temperature control is possible. Chemical deterioration of food, in general, the higher the temperature, the faster the reaction. Dry foods will fade if they absorb moisture and cause browning at the same time. Browning has a great relationship with temperature, and it often loses its original color and flavor due to browning at high temperatures.
As a color preserve for fresh produce, it is also possible to reduce the temperature and control its temperature within a certain range. This is an ideal color retention method. This can be achieved by adding heat absorbing material to the refrigerated cart or package.

3, oxygen removal and color retention technology Oxidation is an important reason for discoloration of agricultural products. No matter if it is fresh agricultural products or processed agricultural products, it is no exception.
Pigments that are most likely to cause oxidation in natural pigments are carotenoids and myoglobin. Fig. 1 shows the stability of carotenoids during storage of tomato powder. As can be seen from the figure, the drying method and the packaging method are different. The effects of oxygen are also different, so the stability of carotenoids varies.

Fig. 2 shows packaging of cooked corned beef using various plastic films having different gas permeability, and observing the relationship between the skin red protein content and the air permeability of the packaging material and the storage temperature. As can be seen from the figure, the higher the oxygen permeability, the faster the decomposition of the pigment and the greater the dependence on the temperature.

In Figure 2, K-glass paper is high-barrier glass paper, KON is high-barrier nylon, ON is nylon, and PE is polyethylene. PET is polyester, and OPP is biaxially oriented polypropylene.
For other pigments, although the degree of difference, but will be accelerated by the role of oxygen decomposition, especially in the case of light irradiation, its decomposition is more significant. It can be seen that oxygen is a major factor that needs special consideration in maintaining food quality.
The above two figures are based on experiments using processed agricultural products (food) on the changes between color and oxygen, and inferred that fresh agricultural products with extremely high water content will have similar oxidation patterns. Therefore, in fresh-keeping packaging, it is necessary to separate oxygen or remove oxygen.