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The Hydrostatic Retort is a continuous agitating retort. Generally the hydrostatic retort can be thought of as a still steam retort, operated at a constant temperature through which the containers are conveyed by a continuous carrier chain, at a constant rate designed to provide the correct process time. There can be a small amount of product agitation as the convey chain changes direction in the hydrostatic tower, these changes of direction of the conveyer are referred to as “passes”.

These vertical retort systems are manufactured by only a few companies worldwide. They are very large and normally extend several stories into the air. Also smaller horizontal versions  of a hydrostatic retort are available.

Containers are loaded into the horizontal carrier on the continuous chain and conveyed up to the inlet leg of the sterilizer. The inlet leg is a water filled tower that the product travels down whilst being preheated and pressurized. At the bottom of the inlet leg the conveyor passes into the steam dome where sterilization takes place. The product then exits the cooker via another hydrostatic leg that starts the cooling and depressurization process. The conveyor chain then carries the containers back to near the loading station where the processed product is unloaded from the continuous carrier.


The water usage is lower than that of conventional retorts. Unfortunately, hydrostatic retorts have a high capital cost and require very high production volumes which need long cook times.

For validation of temperature and pressure measurements, only TrackSense wireless data loggers ideally mounted inside the cans can be used. For some hydrostatic retorts and hydrolock retorts some can rotation can take place. Typically most products will be sterilized about 121.1˚C, but depending on products and tower height the temperatures can be higher or lower.


Typical products in cans, trays, bottles and pouches processed in Hydrostatic Retorts:


  • Pet foods
  • Soups
  • Vegetables
  • Fruits
  • Canned specialty products


Temperature Distribution & Heat Penetration Tests

The Temperature Distribution test is designed to ensure that the pre-heating-, steam- and cooling zones are at a uniform temperature. The purpose of a Heat Penetration test is to determine the heating and cooling behavior of the product in a specific hydrostatic retort, for establishment of a safe thermal process. The purpose is also to evaluate process deviations, to find the “cold spot” in the product, and to ensure that all products receive the heat scheduled.


When performing Temperature Distribution Tests the TrackSense temperature loggers are placed in a can perforated to a sufficient extent, to allow easy access of steam/air/water.

New hydrostatic retorts can be presumed to require temperature distribution studies, and machines that have undergone repairs or redesign, can also be presumed to require temperature distribution studies.



Many products are produced in Hydrostatic Retorts using different processing parameters, resulting in development of products with unique identities. There are several critical factors that have to be monitored and evaluated periodically:

  • Acceptable heat in the retort affecting the safety and quality of the products
  • Fo-Value calculations of the product
  • Internal and external pressure measurements.
  • (For some hydrostatic and hydrolock retorts number of can rotations (RPM) during processing).
  • Internal mounting of wireless data loggers.
  • 21CFR 113.40(f) (1) requires that at least one Temperature Indicating Device (TID or ETI) should be installed in the retort steam dome near the steam-water interface. This should be the coldest spot in the retort dome.
  • If the thermal process is based on lethality gained in the feed or exit water legs a TID is required to be installed near the bottom temperature recorder in each water leg.

Heat Penetration

Measurements in the “cold spot” of the product is a major source of errors, and therefore Ellab offer a wide range of fittings securing accuracy and reproducibility for every batch. In convective products cold spots can potentially move inside containers as they change direction in the retort and this might have to be considered.

In Hydrostatic Retorts, ideally only the smallest wireless data loggers such as TrackSense® Mini and Micro mounted inside the product container can be used. It may be possible to use externally mounted loggers also.

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Internationally Recognized Guidelines

Before doing any measurements, it is a good idea to use equipment which is complying to:


  • ISO 9001 Certification
  • FDA 21 CFR 113 Thermally Processed Low-Acid Foods
  • 21 CFR part 11 Electronic Signatures
  • CE & UL Approvals
  • cGMP or GAMP5
  • ISO/IEC 12207 Software life cycle processes
  • IFTPS Guidelines
  • Guidelines issued by local authorities

Recommended Test Equipment

The calibrated system should be equipped with sufficient channels, to accurately monitor and record temperature and  pressure – within the process delivery system.
Use TrackSense wireless data loggers of sufficient accuracy and in sufficient quantity, to adequately monitor the process water and product temperatures within the retort.
The data loggers should record the temperatures and pressure at sufficient intervals, not to exceed one minute, throughout the test.
Equipment and procedures for pressure processing in steam in hydrostatic retorts are covered by US legislation 21CFR 113.40(f) of the lacf regulations.
Check the accuracy of the sensors against a reference instrument (ETI). Any single sensor should be within 0.3°C of the reference.
The maximum deviation between any  of the  sensors should not be more than 0.6°C.

Hydrostatic Retort Validation


  • Initial temperature should be recorded
  • Depending on size and conveyor speed 6-20 temperature loggers are recommended
  • Temperature Loggers must be spread over the whole width of the carriers
  • At least one Pressure logger is recommended
  • A sampling time of 1-15 seconds
  • The TrackSense wireless loggers should be introduced into the carriers with intervals giving a representative mapping of the retort
  • The “cold spot” detected is in the geometric center for conduction heating cans.
  • Bring the retort up to the same temperature and pressure as designed for the actual processing parameters before any testing commences.


Choosing the System

In continuous Hydrostatic Retorts a cable based system using thermocouples cannot be used, so therefore a wireless data logging system like TrackSensePro should be used. This system is logging the data, which are downloaded to the computer after the process.

In order not to disturb the heat penetration of the product, it is important the data loggers used, are as small as possible, why TrackSense MINI and MICRO Loggers mounted inside the cans are ideal to measure temperature and pressure.

Standard TrackSense Pro loggers could also be used. The logger would be mounted externally through the flat lid of the can using a GKJ packing gland to secure the measuring point in the cold spot of the can. Dual sensors allow simultaneous heat penetration and temperature distribution testing across the carrier bar.

To validate the processes according to predefined acceptance criteria, ValSuite software is used.

Documentation – Standard ValSuite Reports

  • Comments: Specifications and test equipment
  • Limit Report: Temperature, Pressure, F0-Values
  • RPM Report
  • Time Event Markers: Initial temperature, Preheating-, Sterilization- and Cooling zones
  • Advanced Validation Report: Temperature, Pressure, F0-Value evaluation of the whole process criteria’s with pass/fail indication
  • Statistic Reports: Min, Max, Average, Delta of all parameters
  • Heat Penetration: Ball Calculations
  • Word Documents

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ValSuite® Software Reports

Hydrostatic Retort

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