The saturated steam process is the oldest method of in-container sterilization. Since air is considered an insulating medium, saturating the retort vessel with steam is a requirement of the process. It is inherent in the process that all air be evacuated from the retort by flooding the vessel with steam and allowing the air to escape through vent valves. There is no overpressure during the sterilization phases of this process, since air is not permitted to enter the vessel at any time during any sterilization step. However, there may be air-overpressure applied during the cooling steps to prevent container deformation.
In order to successfully complete this sterilization, both time and temperature conditions established in the filed process must be met simultaneously. At the end of the scheduled vent time, the retort must be at, or above, vent temperature. If these conditions are not met, the vent time may be extended so that the vent temperature can be reached, without creating a deviation. Should the actual process be advanced to the next process step without satisfying both conditions simultaneously, then an Uncleared Deviation condition is created.