Why can canned food be stored for a long time without preservatives?

In the 1970s and 1980s, it was such a pleasure and pride to have tomato marinated noodles in winter! At that time, many families bought tomatoes when they were in large quantities and made tomato sauce, which was kept for winter use. The method of making tomato sauce at home is very simple, which is to wash the tomatoes thoroughly, heat them to remove the skin, put them into a saltwater bottle, then plug in a rubber stopper, insert a needle, steam them in a steamer for about 30 minutes, remove the needle, and cool them. This way, the tomato sauce can be stored until winter and eaten again, with excellent flavor and color. The principle of making canned food is the same as that of making tomato sauce at home. The raw materials are processed, placed in cans (iron cans, glass bottles, or soft packaging bags), vacuumed, sealed, and then heated for sterilization. After cooling, the canned product is produced. After the food is loaded into the can, it undergoes processes such as exhaust, sealing, and sterilization, and the container is completely sealed in an anaerobic environment. Sealed food that reaches this sterile state has no conditions for microorganisms (bacteria) to grow and reproduce, and bacteria outside the can cannot enter the can. This way, the food will not decay and there is no need to add any preservatives. Besides, canned food has a history of over 200 years, and there was no mention of preservatives at that time. Canned food is safe, nutritious, convenient, and convenient. It is very popular in Europe, America, and Japan, with China exporting approximately 3 million tons of canned food annually. The majority of aerospace and submarine food is canned food. In fact, canned food is closely related to our lives. Sweet corn, tuna, mushrooms, tomato sauce, fruit grains in yogurt, and decorative fruits in Birthday cake in the catering industry are all made of canned food. Babao Congee is a typical canned food in the market.