Steam retorts (sterilization kettles) effectively sterilize canned foods through the combined action of high temperature, high pressure, and a sealed environment.

1. High-Temperature Sterilization: Saturated steam provides a consistent, uniform temperature above 100°C. This heat denatures the proteins and inactivates the enzymes of microorganisms (bacteria, yeast, mold), destroying them. To eradicate heat-resistant bacterial spores (e.g., Clostridium botulinumspores), even higher temperatures (typically above 121°C) and sufficient processing time are required.

2. Pressure Raises Boiling Point: Inside the sealed retort, increasing pressure raises the temperature of the steam above 100°C (e.g., to about 121°C at 0.1 MPa pressure). This significantly improves sterilization efficiency, shortens processing time, and is effective against thermophilic spores.

3. Ensuring Can Safety: Canned food, after sealing, creates an airtight environment. Steam sterilization is performed aftersealing, destroying all microorganisms inside. Combined with the hermetic seal, which prevents recontamination, this achieves commercial sterility. This allows canned food to be stored safely at room temperature for extended periods without spoilage.

In summary, a steam retort uses a high-temperature, high-pressure saturated steam environment to completely eliminate microorganisms inside the can. The integrity of the can's seal then maintains that sterile state, which is the core principle of canned food preservation