All raw foods contain bacteria. If the food sits around the bacteria will grow and spoil it.
Some bacteria produce gas which will make a sealed can swell or even blow up
Some bacteria produce a deadly posion.
When a can is sealed no more bacteria can get in and the cook kills those on the inside.
If all bacteria are kills the canned food will keep for a long time.
Heat kills bacteria. The higher the temperature, the shorter the time. For instance, a cook of 20 minutes at 250o F. dose as good a job as 10 hours in boiling water.
Why use steam pressure retort for processing?
With steam under pressure we get a high temperature, but this must be pure steam, not steam and air.
Air is no good for carrying heat, in fact it make a good insulator. Steam from a tea kettle with a temperature of 212o F. will burn you quickly. Over air at 300o F will just feel warm
In a full retort all the air must be blow from among the cans before starting the processว
We do this by letting steam in at one end of the retort and air out through the other end. This is called VENTING. Steam must come in FAST to blow out all the air and the vent pipe must be BIG so the air can get away easily.
The vent valve must be WIDE OPEN during venting. Or there will be a traffic jam. And all the air won’t get out.
When all the air is out of the retort it’s time to vent but not before. Venting requirements for removing all air from your retorts are available – check with the management for these details.