From filling to door opening: the heat transfer, thermodynamics and process engineering principles behind every stage of a Steam & Water Spray (SWS) retort cycle.
This article explains the physical principles behind every stage of the retort thermal process.
It follows the actual sequence of a thermal process, introducing the relevant concepts of heat transfer, thermodynamics and process engineering as they become necessary. The reader will gain a clear understanding of not only how a retort operates, but why each stage, component and control strategy is essential to achieve safe, repeatable and efficient thermal processing.
Filling
The retort cycle starts by filling the vessel with process water through the water fill valve until it reaches the maximum operating level, which is set below the product load and the steam injection pipes.
Unlike full immersion retorts, the purpose of the process water in a is not to immerse the containers, but to provide the water required for the recirculation system. During the heating phase, this water is pumped through the spray nozzles, where it promotes the mixing of steam and air, creating a homogeneous thermal environment and improving heat distribution throughout the load.
Because the process water continuously circulates through the pump, valves, piping and spray nozzles, its quality is essential for long-term equipment reliability. Water hardness and pH should comply with Surdry recommendations to prevent scaling, corrosion and premature wear of hydraulic components. Refer to the Surdry Manual for the recommended water quality specifications.
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Heating: Steam Injection, Venting and Pressure Control
The heating phase begins with the injection of saturated steam into the retort through the steam pipes to increase the retort temperature up to the sterilization temperature defined in the process recipe. Steam flow is regulated by a proportional control valve under PID control to ensure a smooth temperature rise.
Why steam heats so efficiently
Unlike hot air, steam is an extremely efficient heating medium because of its latent heat of condensation. Steam does not transfer most of its energy simply because it is hot, but because it changes phase from vapor to liquid when it comes into contact with colder surfaces. During condensation, a large amount of thermal energy is released almost instantaneously and transferred to the surrounding environment.
Venting: removing the air
At the beginning of the heating phase, the retort still contains the air trapped during loading and filling. Since air is a non-condensable gas, it does not release latent heat and significantly reduces the efficiency of heat transfer. Air pockets may also remain trapped between containers, creating localized cold regions where heating is slower than in the surrounding areas. For this reason, an initial venting stage is performed to remove as much air as possible before the recirculation system starts operating.
As steam continuously condenses, the resulting condensate accumulates inside the retort, causing the process water level to gradually increase. To maintain the correct operating level, the level control system automatically opens the water level control valve whenever necessary.
Recirculation: the key to a homogeneous environment
After a short period of steam injection, the recirculation pump is started. The pump draws process water from the bottom of the retort, forcing it through the filter, flowmeter and spray nozzles located around the chamber, continuously mixing steam and air, eliminating thermal stratification and reducing temperature gradients inside the retort. This homogeneous environment is one of the key principles behind the Steam & Water Spray (SWS) technology.
For the recirculation system to operate correctly, the pump must always receive a continuous column of water. If the water level falls below the minimum required level, air may enter the pump suction. The presence of air reduces the hydraulic stability of the pump and may lead to cavitation, a phenomenon in which vapor cavities form and subsequently collapse inside the impeller. Cavitation reduces pump performance, generates vibration and noise, and can progressively damage the hydraulic components.
During the heating phase, the bypass valve installed in the recirculation circuit remains fully open, allowing the entire water flow delivered by the pump to circulate through the spray system. This maximizes the recirculation flow rate and ensures continuous mixing of steam, air and process water inside the retort. From a heat transfer perspective, increasing the water flow enhances the convective heat transfer coefficient at the package surface, reducing thermal resistance and promoting faster and more uniform heat distribution throughout the load. In other words, more water flow means heat reaches the packages more easily.
Overpressure: protecting the package
As the product temperature increases, the pressure inside the package also increases. Liquids expand when heated, while the gases contained in the package headspace follow the ideal gas law, increasing their pressure as temperature rises. If the external pressure inside the retort were not increased accordingly, the pressure difference across the package walls could cause deformation or permanent damage, particularly in flexible and semi-rigid containers.
To prevent this, compressed air is progressively introduced into the retort during the heating phase. Unlike steam, compressed air contributes pressure but virtually no heating capacity. Its purpose is to generate the additional external pressure, commonly referred to as overpressure, required to balance the increasing internal package pressure and maintain package integrity throughout the thermal process. The pressure control system continuously regulates the combined contribution of steam and compressed air to ensure that both the required process temperature and the appropriate package protection are maintained.
Holding Phase: Heat Penetration and Thermal Lethality
Once the retort has reached the sterilization temperature, both temperature and pressure are maintained under precise control by the PLC. At first glance, it may appear that the entire load has already reached the required processing conditions. In reality, however, only the retort atmosphere has reached thermal equilibrium. The product itself is still absorbing heat and its temperature continues to increase progressively.
Thermal energy must travel from the steam-water environment, through the package wall and finally into the food until it reaches the slowest heating location inside the container. This process, known as heat penetration, continues throughout the early stages of the holding phase. Consequently, the product temperature always lags behind the retort temperature, and the process cannot be considered complete until the coldest point of the slowest heating package has received the required thermal treatment.
The time required for the slowest heating location to reach the sterilization temperature is known as the [ENLACE: Come-Up Time (CUT)]. Once the Cold Spot has reached the target temperature, the product begins to accumulate the required microbial lethality, expressed as the [ENLACE: F₀ value]. The holding phase therefore continues until both conditions have been satisfied: the Cold Spot has reached the sterilization temperature and the required F₀ has been accumulated to ensure commercial sterility.
WConvection vs. conduction: why products heat differently
Although every package inside the retort is exposed to the same processing conditions, not all products heat at the same rate. The reason is that heat does not travel through every food in the same way. The heating mechanism depends mainly on the physical characteristics of the product, particularly its viscosity and its ability to move when heated.
Low-viscosity products, such as milk, juices, soups or liquid sauces, are free to circulate inside the package. As the product in contact with the package wall is heated, it becomes less dense and rises, while the cooler product moves towards the wall to replace it. This continuous internal movement distributes thermal energy throughout the product and is known as convection. Because heat is transported by the moving liquid, convection-heated products usually reach the target temperature relatively quickly.
In contrast, highly viscous or solid products such as pâté, mashed vegetables or meat products cannot circulate inside the package. Since there is little or no internal movement, thermal energy can only travel from the hotter regions to the colder ones by passing from molecule to molecule. This mechanism is known as conduction and is considerably slower than convection. As a result, conduction-heated products require longer heating times and generally represent the most demanding applications in thermal processing.
The Cold Spot
Regardless of the heating mechanism, every package contains a location that heats more slowly than any other. This location is known as the Cold Spot and represents the critical point for thermal process design. In conduction-heated foods, the Cold Spot is usually located close to the geometric centre of the container. In convection-heated products, internal fluid movement shifts the Cold Spot away from the centre, typically towards the lower portion of the package. Since food safety depends on the least heated location, all thermal process validation studies are based on monitoring the temperature history of the Cold Spot.
Thermal inertia
A further characteristic common to all products is thermal inertia. Food does not instantly follow changes in retort temperature because thermal energy requires time to penetrate through both the package and the product. Even after the retort has stabilized at the sterilization temperature, the product core continues to heat up. Similarly, when cooling begins, the centre of the product remains significantly hotter than the surrounding retort atmosphere and continues accumulating thermal lethality for a short period. This thermal inertia explains why the product temperature always responds more slowly than the retort temperature.
Cooling
The cooling phase begins immediately after the required thermal process has been achieved. At this point, the retort atmosphere is at sterilization temperature and pressure, while the product core still contains a considerable amount of stored thermal energy. The objective of the cooling phase is to remove this energy while ensuring package integrity and minimizing unnecessary thermal exposure.
Steam condensation and pressure drop
The first physical phenomenon occurring during cooling is the rapid condensation of the steam present inside the retort, and the resulting pressure drop. As cooling water begins to remove heat from the recirculating process water, the steam temperature decreases and large quantities of steam condense almost instantaneously into liquid water. Since steam occupies a much larger volume than liquid water, this causes a rapid reduction in the volume of the gaseous phase and therefore a strong tendency for the retort pressure to collapse.
If this pressure drop were not controlled, the internal pressure of the packages would remain significantly higher than the pressure inside the retort, creating a large pressure differential capable of deforming flexible containers, buckling metal cans or even damaging glass containers. For this reason, compressed air continues to be injected during the initial cooling stage, replacing the pressure lost by steam condensation and maintaining the required overpressure until the product temperature decreases.
Throughout the cooling phase, steam condensation continuously increases the amount of liquid water present inside the retort. The automatic level control system compensates for this effect by draining excess water whenever necessary.
Controlled cooling through the heat exchanger
Cooling is performed by circulating the process water through a plate heat exchanger. However, maximum cooling is not applied immediately. During the first minutes of cooling, the bypass valve progressively regulates the proportion of recirculating water passing through the heat exchanger. This controlled reduction in water temperature prevents excessively rapid steam condensation and allows pressure and temperature to decrease in a controlled and stable manner. Once the risk of pressure collapse has disappeared, the bypass valve gradually directs a larger proportion of the flow through the heat exchanger to minimize the cooling time.
End of Cycle
Once the cooling endpoint has been reached, typically corresponding to a product discharge temperature below approximately 38 °C (depending on the validated process), the thermal process is considered complete.
The residual compressed air inside the retort is used to assist the drainage of the process water. The drain valve is opened while maintaining part of the residual air pressure, allowing the compressed air to push the process water out of the vessel.
Once the process water has been removed, the remaining pressure is gradually released until the retort reaches atmospheric pressure, ensuring safe conditions before the door interlock is released and the retort can be opened.
Want to know how these principles apply to your product and packaging? Talk to a Surdry retort expert