The Water Immersion Process is the most widely accepted method of sterilizing product using an overpressure process. The Water Immersion process is similar to a saturated steam process in that the product is totally isolated from any influence of cooling air – the product is totally submerged in water. But, it is different from Saturated Steam in that air can be introduced into the vessel during sterilization.

Overpressure is provided by introducing air (or steam) on top of the water. In some instances, air is added to the steam (which then heats the air). The heated air agitates the water as it flows to the surface and serves to pressurize the process load.

The water immersion still retort was first designed for use in processing glass jars. Water immersion still retorts are now used to process a variety of containers requiring an over-pressure. In some cases the water immersion retort may be used to process small metal containers as well.

If the water immersion retort is used to process metal containers there is some concern with the rusting of containers caused by the addition of air to the processing water.

Basically there are four principle differences in a still retort equipped for pressure processing in water as compared to a still steam retort:

1. Compressed air must be provided to the retort to create the overpressure during processing and cooling.

2. A pressure control valve must be added to the retort

3. The temperature of the retort must be controlled independently of the pressure

4. The containers are processed and cooled under water.

The water level in the retort must be maintained to provide water above the top level of the containers at all times. It is recommended that the containers be covered by at least 6 inches of water.

If the water level falls below the level of the top containers those containers exposed to the steam/air mixture in the top of the retort must be identified and set aside for reprocessing or evaluation by a processing authority.

A water level indicator is required on all water immersion retorts. This can be in the form of a water glass sight tube, pet cocks, or mechanical indicator. A low-water alarm (visual & audible) is suggested