Retorts, Common Questions and Answers

Sometimes also called an Autoclave or Sterilizer, a Retort is a pressure vessel used in the food manufacturing industry to “commercially sterilize” food after it has been placed into its container and the container has been hermetically sealed.  More on commercial sterilization later.  The method of sterilization is a thermal based process, or the application of high heat to the product.  The high temperatures required in a sterilization process destroys pathogenic organisms that may be present in/on the container and/or food product, and is well above the boiling point of water at normal atmospheric pressure.  So, the retort is by design a “pressure vessel”.  Thermal processing/sterilization of shelf stable, low acid foods (i.e.: defined as products with a finished equilibrium pH greater than 4.6 and a water activity greater than 0.85) is usually performed at temperatures at or above 250°F or 121°C, give or take ~10°F/5.6°C.  The higher the temperature, the shorter the time the product needs to be exposed to heat.  While there are several categories of retorts, for the purposes of our discussion we will be referring to batch type retorts only.

Batch retorts are available in several configurations.  First, they can be static or agitating style machines.  A static retort is typically used for conductive (i.e.: viscous) heating products where the product does not benefit from any kind of movement during the process (heat or cooling).  Agitating style retorts are typically used for convective (i.e.: “flowable liquid”) type products such as dressings and sauces that benefit from some product movement (i.e.: ‘stirring”) in the container during the process.  Either from a process stand-point, to improve the rate of heat transfer into/out of the container, and/or from a product quality stand-point (i.e.: to shorten the exposure time to heat).  Agitating style retorts can utilize various methods of agitation depending on the orientation of the product container.  Vertically oriented containers, such as cans or glass/plastic bottles, are typically agitated in a rotary fashion, either axially or end-over-end.  With horizontally oriented containers, such as flexible pouches and semi-rigid bowls and/or trays (which contain product that benefits from agitation) the preferred method is horizontal reciprocation.  Gentle Motion® and SHAKA® are two forms of such agitation.

Batch retorts can employ various methods of process delivery.  Some of which also utilize overpressure or counter-pressure to help protect the integrity of the container during the process (i.e.: to keep the package from bursting as temperature and pressure builds inside the container during the process).  Rigid containers, such as steel cans, can withstand large differences between the pressure inside and outside of the container, and therefore these types of containers typically do not require overpressure.  They can be processed in a 100% saturated steam environment without the use of overpressure during the heating phases.  On the other hand, more fragile flexible and semi-rigid containers cannot withstand high pressure differentials, so air is introduced into the retort to provide overpressure to maintain package integrity during the process.  These types of containers require more sophisticated overpressure process delivery methods such as water spray, water cascade or water shower, water immersion or steam-air type systems.  Because air is an insulator, a means of stirring or mixing the process media in the retort is required to avoid cold spots in the machine, thus ensuring good temperature distribution throughout the retort and product load.  This mixing is accomplished by the different water flow methodologies mentioned above, or via a fan in the case of steam-air retorts, and/or via mechanical rotation of the insert/drum in the case of agitating style machines.

Overpressure is also important in the cooling phases of a retort process because as cooling water is introduced into the retort it collapses the steam created in the heating step(s).  Without sufficient introduction of air overpressure during cooling, the pressure in the retort can suddenly drop due to steam collapse thus creating a vacuum situation in the retort.  If this happens the pressure differential between the outside environment and the temperature/pressure environment inside the container becomes too great thus causing the container to burst (otherwise known as “buckling”).  Precise control of overpressure during the initial phases of cooling is important to avoid the above situation but ramping that pressure down in the latter stages of cooling is important as well to avoid crushing the container (or otherwise known as “paneling”) as the temperature and pressure inside the container subsides.  While the retort process inactivates or destroys bacterial pathogens, it does not destroy all microscopic spoilage organisms. Thermophiles are bacteria that can withstand temperatures well above typical retort temperatures.  For this reason, the product must be cooled down to a temperature below that at which these organisms will reproduce, thus causing thermophilic spoilage.