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 over pressure 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-over pressure applied during the cooling steps to prevent container deformation.


The steam-air ratio should be the same as that used during studies to determine temperature distribution in the retort.The steam-air retort uses a mixture of steam and air, which is added to create an over-pressure in the retort. The air over-pressure allows thermal processing of a wide variety of containers including: glass, metal cans, rigid plastic, and flexible pouches. Steam-air mixtures, although in a gaseous state similar to 100% steam, when used as a heating medium behave more like water. The heat capacity of the steam-air is very small when compared to pure steam. As the steam condenses and gives off heat the air remains. Because the retort uses air in the steam mixture there must be a method of mixing the air and steam to prevent the formation of air pockets in the retort and to provide for the rapid movement of the heating medium over the container surface. Air may provide insulation around the container preventing the heat from reaching the container walls in an efficient manner. A large fan is normally used to rapidly mix the steam-air mixture and to force this mixture to flow through the retort and containers during processing.

It is very important to maintain the correct steam-air ratio in this type of retort. The steam-air ratio is normally determined by the processing temperature and the type of package being processed.The steam-air ratio is normally determined by the processing temperature and the type of package being processed.

Steam-air retorts are normally operated at steam-air ratios ranging from 75% steam/25% air to 95% steam/5% air, depending upon the air over-pressure in the retort.

The pressure in a steam-air retort must be maintained at the correct pressure to prevent container distortion and to maintain the correct steam-air ratio. If the amount of steam in the steam air ratio drops to too low a level, the energy available to heat the product is reduced. If the temperature is maintained by the steam controller the partial pressure of the steam remains constant throughout the process. The pressure of the system is controlled by releasing over-pressure through a small purge valve and adding compressed air as needed.

The percent steam can be calculated using the following formula:

Steam pressure psi (pounds per sq. inch actual) = % steam

Total system pressure psi

Steam pressure psi is determined by determining the amount of steam pressure generated at a certain temperature [e.g. 240° F generates 10 psig (pounds per sq. inch gauge)] plus 14.7 psi (atmospheric pressure at sea level) equals 24.7 psi. The total system pressure is determined by taking the gauge reading and adding 14.7 psi. A steam air system having a total gauge pressure of 15 lbs operating at 240° F would have a 83% steam, 17% air mixture.

10 psig + 14.7 psi atmospheric = 24.7 = 83% steam

15 psig + 14.7 psi atmospheric    29.7

The MBPL retort is offered in static or rotational end over end agitation models. The retort is normally operated in the following manner:

  1. Product is placed into the retort in either standard rectangular baskets or crates for metal, glass and rigid plastic containers. Special racking trays are used for flexible and other containers needing support during processing.
  2. The retort is purged for 1 to 2 minutes through a large purge (vent) valve,the bottom drain or by vacuum to remove excess air from the retort and to enhance the steam entry into the retort. Steam is introduced into the retort through steam spreaders in the bottom of the retort.
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