Wall corrosion

1. Factors affecting corrosion in tank

(1) oxygen and oxygen are strong oxidants for metals. In cans, oxygen shows strong oxidation in acidic medium. Therefore, the residual oxygen content in the tank is a decisive factor for the corrosion of the inner wall of cans. The more oxygen content, the stronger corrosion.

(2) Canned acidic or high acidic fruit food, the higher the acid content of fruit corrosive.

(3) different types of raw materials have different corrosiveness to tinplate.

(4) low methoxyl pectin and low methoxyl pectin can accelerate the corrosion of tin. Therefore, the activity of pectinase should be destroyed rapidly in the process of fruit processing to prevent pectin decomposition due to the action of causal pectinase and promote corrosion by producing low methoxy pectin or galacturonic acid.

(5) Nitrate ion canned food caused by the presence of nitrate, which caused rapid tin dissolution corrosion and occurred in some canned fruits. Especially in the case of more residual oxygen in the tank and lower pH value of the medium (below PH 5, the amount of tin dissolved due to nitrate radical increased significantly), the corrosion rate was accelerated.

(6) anthocyanin pigment cherry and berry fruits contain anthocyanin pigment. These pigments are also highly corrosive to empty tanks.

(7) canned fructose or canned fruit can sometimes be subjected to severe corrosion. This is due to the coking of carbohydrates.

(8) Sulfur and sulphide-containing pesticides sprayed on fruits during the growing season sometimes contain sulphur. When sulphur or sulphide is mixed into cans, it is easy to cause wall corrosion.

Nitrate in canned food also has corrosion effect on the tank wall.

2, preventive measures

(1) The fruit sprayed with pesticides before dishes can be cleaned and disinfected. The fruit can be soaked in 0.1% HCL for 5-6 minutes, and then cleaned to help remove pesticides.

(2) For the fruits with more air, it is better to take the air extraction treatment to minimize the content of air (oxygen) in the raw material tissues, thereby reducing the concentration of oxygen in the tank.

(3) in order to prevent the top gap of the tin from being too large, the sugar solution must be filled up.

(4) syrup in the syrup should be boiled to remove SO2 from the sugar solution.

(5) positive and negative inversion of cans to reduce corrosion of tank walls.

(6) The storage environment temperature of canned products should not be too high and the relative humidity should not be too high to prevent internal corrosion and external wall corrosion.

(7) According to the corrosive properties of different kinds of raw materials, tin-plated sheet steel with different corrosion resistance and tin plating quantity is selected to make cans, and the tin layer damage is prevented during the canning process.

Two. Hydrogen expansion tank and perforated corrosion of canned fruits

Generally canned fruit is the easiest hydrogen expansion can, the reason is that acid and iron play a role in releasing hydrogen gas caused expansion can. Tin-plated steel sheet with dew point or coated iron film with many pores is the main reason for concentrated corrosion perforation. Canned fruits such as bayberry, cherry and strawberry are more varieties of hydrogen expansion tanks. In order to prevent hydrogen expansion tank, it is necessary to use tinplate with less iron.

Three. Discoloration

Fruit discoloration often occurs during processing and storage. The main reason for discoloration is enzymatic browning and non-enzymatic browning. Non enzymatic browning includes Maillard reaction and ascorbic acid oxidation. In addition, some metal ions (such as iron, tin, copper, etc.) and anthocyanin pigments are also factors causing discoloration.

Preventive measures include the following:

(1) use anthocyanins and low tannin materials.

(2) In the process of processing, for some easily discolored varieties such as apples, pears, etc., peeled and cut, quickly soaked in dilute salt water (1% - 2%) or dilute acid color protection. In addition, when the fruit block is evacuated, the fluctuation of the vacuum degree in the suction tank and the fruit block from the water surface are prevented.

(3) Blanching at suitable temperature and time can destroy enzyme activity and remove air from raw material.

(4) The addition of ascorbic acid in the injected sugar water can prevent the discoloration of apples, pears and peaches. However, attention should be paid to the dehydrogenation of ascorbic acid, resulting in corrosion of air tanks and non enzymatic browning.

(5) Malic acid, citric acid and other organic acids in aqueous solution can not only protect the color of semi-finished products, but also reduce the PH value of canned contents, thereby reducing the rate of enzymatic browning. Therefore, raw material peeling, slicing, soaking in 0.1% - 0.2% citric acid solution and sugar water adding appropriate amount of citric acid will have the effect of preventing browning.

(6) the sugar water should be boiled and ready to use. If acid is needed, but acid should not be added too early to avoid excessive conversion of sucrose, otherwise excessive conversion of sugar will easily produce non-enzymatic browning when encountering amino acids.

(7) In the process of processing, prevent the fruit from directly contacting with iron, copper and other metals, and pay attention to the heavy metal content of processing water should not be excessive.

(8) sterilization should be sufficient to kill microorganisms such as acid bacteria and prevent rancidity.

(9) control the storage temperature of the storehouse, the temperature is low, the browning is light, and the browning is accelerated at high temperature.

Four, bacterial canisters and sour fruits with low acidity often have bacterial dilatation tanks.

Preventive measures are:

Adding appropriate amount of acid to reduce the content of the PH value; production process microbial contamination; shorten the process to maintain the freshness of raw materials and semi-finished products; the use of appropriate sterilization conditions.