It refers to the degree to which the air pressure in a can is lower than atmospheric pressure. In order to prevent the cans from expanding due to the expansion of the air in the can during the high-temperature sterilization process, and to inhibit aerobic bacteria, vacuuming is required before the can body is sealed. There are currently two main methods. The first is to directly use an air extractor to vacuum and seal. The second is to spray water vapor into the headspace of the tank, then seal the tube immediately, and wait for the water vapor to condense to form a vacuum.