Frequently Asked Questions

1. What is a retort (also referred to as an autoclave)?

A retort is an enclosed vessel that operates above atmospheric pressure thus allowing steam or water within the vessel to reach elevated temperatures. Packaged food or beverage products that are placed into the retort will thereby be subjected to the super-heated temperatures, thereby inactivating any vegetative or non-vegetative spores and pathogenic microorganisms in the package/product so that the food or beverage product doesn’t spoil, even when stored in ambient conditions. The process is called commercial sterilization, and it renders the packaged product shelf-stable (i.e.: no need for refrigeration).

2. How do retorts work?

Retorts are loaded with containerized food and beverage products which are held in baskets or trays. Pressurized (saturated) steam is either directly injected into the retort process chamber or is used to heat the process medium which can be water or air, which in turn heats the product inside the container. Some containers require additional pressure (known as overpressure or counterpressure) during heating to prevent deformation of the container as pressure builds within it, and to prevent damage to the hermetic seal. After the product in the container has reached the desired sterilization time and temperature, cold water is then used to bring the containerized product back down to a safe temperature. The initial cooling step(s) may also be pressurized with air to continue protecting the containers from damage while the pressure inside the container is reduced.

3. Why are retorts used?

Retorts are used to commercially sterilize food/beverage products for human and animal consumption, and to render the packaged food shelf stable. A retort is designed to operate at temperatures that exceed that which can be achieved at atmospheric pressure. The minimum temperature and pressure that most retorts operate at is at least 250°F/121°C and about 15 p.s.i./1 bar. Some process temperatures and pressures are higher or slightly lower.

4. What products need to be processed in a retort?

A food or beverage product needs to be processed in a retort if it has the following characteristics:

  1. It has a stabilized pH of 4.6 or higher. The term for this is “low-acid foods”. Foods with a pH below 4.6 are too acidic to support microbial growth and reproduction.

  2. The product’s water activity “aW” is above .85. Water activity as defined by the U.S. FDA is: “The moisture condition of a product measured as the equilibrium relative humidity (ERH) expressed in percentage or as the water activity expressed as a decimal”. In an environment with water activity below .85 a microbial cell cannot absorb nutrients or remove waste through its cell wall.

5. What are the different types of retorts?

Retorts are classified by category – Batch (loaded and unloaded with packaged product before and after each process) or continuous (a constant feed of containers through the retorts heating and cooling chambers). They are also classified by the type of process they employ. Common retort processes used in the food manufacturing industry are saturated steam, steam/air, water spray, water cascade or shower and water immersion as well as new innovations such as ImmersaFlow. Lastly, retorts can be either static or agitated. Static retorts do not use any methods of forced convection to facilitate heat into or out of the product. Heat is either conducted through high viscosity products such as refried beans or pumpkin puree, or by convection through the product in currents created by the heating and cooling of a low viscosity liquid (e.g.: vegetables in a brine solution). Agitated retorts have mechanical systems that move the containers in an a rotary fashion (axial or end-over-end) or horizontal reciprocation at low or high frequencies.

6. How do I pick the correct retort?

The correct retort for a given application is determined by five primary factors. First being the food or beverage product itself. Second factor is the container used for the product. Third consideration is the duration of the thermal process. Fourth is mandate time. This is to prevent incipient spoilage and prevent the minimum initial temperature from dropping below the scheduled process requirement. The fifth and last (but not least) factor considered is the is the speed of the production line expressed in containers per unit time. This production parameter when considered with the third and fourth considerations will determine the amount of retort capacity and the size and quantity of retorts needed.

7. What sizes of retorts are available?

Production batch retorts are available in varying diameters usually between 42” (1,100mm) and 80” (2,000mm). Lengths can vary between one or two baskets/positions to over twelve baskets/positions.

8. How long do retorts last?

If properly maintained, retorts can last for decades. Retorts are typically made from either carbon steel or stainless steel. Rust (oxidation) will deteriorate the former while stress corrosion cracking (SCC) causes expensive or irreparable damage to stainless steel vessels. SCC is caused by having high concentration of chlorides in the cooling water which will negatively react with stainless steel at high temperatures and pressures, particularly attacking the material at high stress points (i.e.: weld joints). Improperly installed retorts (i.e.: restricting expansion and contraction as the vessel heats and cools) can also cause damage to the retort and/or retort door(s).

9. How are containers moved through a retort operation?

In a batch retort operation, the type of material handling equipment needed is determined by the type and volume of containers moving through it. Rigid containers such as cans and glass bottles/jars are loaded into baskets by sweep-type loaders and unloaders. Fragile and semi-rigid containers such as pouches and plastic bowls, cups, trays or pouches are loaded into retort trays by robotic systems or gantry-type loaders/unloaders. Plastic bottles are loaded and unloaded from baskets and specialized cartridge like trays by fragile container handling systems. Depending on the throughput needs and the diameter of the retorts, the product is either manually moved to and from the retorts by multiple operators pushing baskets or tray stacks of product on carts/trolleys. Or automatically transported to and from the retorts and loading and unloading system by specialized conveyors and/or shuttles, thus making the entire retort room fully automated and capable of being operated by one or two people.

10. How can retort processing be more sustainable?

There is really no way of getting around the fact that retorts consume large amounts of energy (heating) and water (cooling). There are however ways to “recycle” these resources in most cases. For example, Steam Heat Recovery Systems recapture the steam vented during the vent stage of a steam retort, thus creating a hot water source that can be used elsewhere in the plant. Water Recovery Systems reuse the hot and temperate water that is used to cool the product after the heating steps of a retort process.