HYDROSTATIC RETORTS 

Equipment and procedures for pressure processing in steam in hydrostatic retorts are covered 

by 9 CFR 431.6(b)(4). 

Generally, the hydrostatic retort is usually very large and many stories high. It can be thought of 

as a still steam retort operated at a constant process temperature through which containers are 

conveyed by a continuous carrier chain at a constant rate designed to provide the correct 

process time. 

Hydrostatic retorts are manufactured by the FMC Corporation in the U.S. and by Stork and 

others in Europe. Newer designs now offer end over end or axial agitation of the product; the 

use of overpressure for the maintenance of container integrity; the ability to process glass and 

flexible pouches; and water as a heating medium in addition to steam. The systems are often 

used for high volume products which need long cook times such as condensed soups and pet 

foods. 

Hydrostatic pressure is pressure from a body of water at rest. The weight of the water causes 

the pressure. The higher the water is in a vertical column, the greater the pressure. The name 

hydrostatic is derived from the fact that the pressure in the steam dome is counter balanced by 

water in the entry and exit legs of the retort. The containers are conveyed through this steam 

dome. The higher the water level, the higher the pressure and temperature obtained in the 

steam dome. They are pre-heated in a feed leg (hot water leg) prior to entering the steam dome 

and cooled in an exit or discharge (cooling) leg after the steam dome. Containers are carried by 

chain conveyors. The water is under “hydrostatic pressure” created by pressure in the water 

legs which seals the steam in the steam chamber. 

Startup procedures for a hydrostatic retort requires venting of the retort and bringing the water in 

the feed legs up to temperature. This procedure takes a longer period than the venting of still 

steam retorts. The hydrostatic retort is normally operated for periods of up to several weeks and 

may be shut down and cooled only when required for maintenance or repairs. 

Containers are loaded into a horizontal carrier on the continuous chain and conveyed up to the 

inlet leg of the sterilizer. The inlet leg is filled with water which counterbalances the pressure in 

the steam dome. The temperature of the water increases as the container moves from the top of

the inlet leg down toward the steam-water interface at the bottom of the leg. Water temperature 

in the inlet leg may range from ambient to boiling. The feed leg may contribute to the process 

lethality by increasing the initial temperature of the product. If process lethality is claimed for the 

inlet leg of the retort, the water temperature in the inlet leg must be carefully controlled. 

There is minimal product agitation unless specifically designed into the system. Steam is 

generally used as the heating medium. There are no doors and transfer valves between the 

processing chamber (steam dome) and the atmosphere. Process times are basically the same 

as a still retort. The conveyor system is being used to increase the number of containers 

processed. Steam may enter at the top or middle of the dome. 

For example, the water height in the water legs must be 37 feet high at sea level to 

counterbalance a processing temperature of 250° F (121° C). Operating at temperatures above 

250° F will require a higher water level. The retorts can be operated below the maximum 

temperature if the pressure remains high enough to prevent water contact with the containers in 

the steam dome. 
 

 

 

 

 

 

 

 

The container is conveyed through the steam water interface into the steam dome. The number 

of times that the carrier passes through the steam dome as well as the speed of the carrier

determines the process time. Traveling from the top of the steam dome to the bottom, and viceversa, is referred to as one pass. Hydrostatic retorts with 2, 4, 6, and 8 passes are common. 

After traveling through the steam dome, the containers are conveyed into the exit water leg 

where the temperature decreases as the container passes up the leg. The cans leaving the 

steam dome are heated to a high level and give up their heat to the water in the discharge leg. 

This results in several situations depending upon the design of the retort. 

As the container exits the leg it is exposed to atmospheric pressure, and it may pass through a 

series of water spray coolers to further cool the product. The conveyor chain carries the 

containers back to near the loading station where the processed product is unloaded from the 

continuous carrier. Because the container inlet and exit are close together, care must be taken 

to ensure that unprocessed containers do not become mixed with processed containers.

Containers found on the floor or elsewhere whose status is questionable should be destroyed. 

9 CFR 431.6(b)(4)(i) requires that the MIG thermometer be installed in the retort steam dome 

near the steam-water interface. This should be the coldest spot in the retort dome. There must 

be a steam controller. 

9 CFR 431.6(b)(4)(i) requires the installation of additional temperature recorders near the top 

and bottom of each hydrostatic water leg if the process schedule specifies maintenance of 

particular temperatures in the water legs. 

An automatic steam controller must be utilized for all retorts to maintain the retort temperature 

(9 CFR 431.6(a)(3)). 

Before the start of operations, the steam dome must be vented just like a regular steam, batch, 

still, retort. The venting time and temperature must be recommended by the equipment 

manufacturer or the plant’s process authority. 

9 CFR 431.6(b)(4)(iv) requires the hydrostatic retort to be equipped with at least one bleeder 1/4 

inch or larger at the top of the steam chamber or chambers at the opposite end of steam entry. 

In addition, all bleeders must be arranged in such a way that the operator can observe that they 

are functioning properly. 

Control of the water levels in the feed and exit legs are important to maintain the hydrostatic 

pressure in the retort. The water level is normally controlled through a differential pressure 

controller which adds water when it is needed and dumps excess water from the legs. Water 

level fluctuation in the feed and exit legs may be caused by fluctuations in the feeding and 

discharge of containers. As more containers are fed into the container conveyor more water is

displaced from the legs, a lack of production results in a lack of containers in the legs and the 

water level falls. 

The container-conveyor is driven by a variable speed motor and must be checked and recorded 

at the beginning of processing and recorded at least every 4 hours. Changing containerconveyor speed changes the process time. Operating at a slower speed than in the process 

schedule will result in longer cook times which effect product quality. Operating at a faster 

speed than outlined in the process schedule will result in shorter cook times and potentially 
 

 

 

 

underprocessed product, which could indicate a food safety issue. 

Water contacting containers in the bottom loops is a serious concern. It may result in 

underprocessing. If containers contact the water, the containers must be segregated and 

evaluated by the processing authority. 

The feed leg cannot be lower than the minimum product initial temperature (IT) or the leg 

temperature becomes the IT. Critical factors must be measured and recorded in accordance 

with the method and frequency in the written procedure, as is true for all retorts (9 CFR 431.4). 

Records must include conveyor (chain) speed and be checked at least every 4 hours (9 CFR 

431.6(b)(v)).