Shelf stable food packaging has significantly evolved during the past 20 years, from the standard metal can and glass jar, to value-added polymeric and composite pouches, bowls, cups, and trays. Because of this progression, the ordinary saturated steam or water immersion retort has been found to be inadequate for delivering an optimum thermal process to these new containers. Retort technology has kept pace with the container evolution, advancing to more complex overpressure sterilizers that use a variety of heating and cooling mediums coupled with overpressure to deliver the required thermal process while ensuring package integrity is not compromised.
Not all retorts are created equal when it comes to processing flexible packages. Standard offerings from many present retort manufacturers include batch Steam/Water Spray, Water Cascade, Water Immersion, and Steam/Air models, all capable of overpressure processing. Also available to the food industry are some continuous horizontal sterilizers capable of saturated steam processing with axial rotation, and hydrostatic sterilizers capable of overpressure. Because of the inflexibility and limited ability to optimize thermal processes, the continuous retorts are a dying breed best suited for commodity style products packed in metal cans (i.e. vegetables in brine, pet foods). This paper will forego the pros and cons of the continuous models due to the considerably large capital expenditures associated with these units and the belief that batch retorts offer the greatest flexibility when it comes to food preservation. In addition to the old standby – saturated steam retorts, Allpax offers all of the batch models. Most are available as rotary or non-rotary models.
Some retorts are capable of delivering multiple process modes, not locking the food manufacturer into a large capital investment that may have limited capability with different package designs. Maybe one day you are processing metal cans that do not require overpressure, and the next day pouches, requiring overpressure. The multimode retort designs provide the flexibility to adapt to changes in production schedules – Saturated Steam for metal double-seamed cans and those containers not requiring overpressure, and Steam/Water Spray for flexible packages.
Overpressure Retort Designs
The one common connection for all modern overpressure retorts is the need to mechanically move the heating and cooling media. Generally this is accomplished with a pump in Water Immersion, Steam/Water Spray, and Water Cascade retorts, or with a high velocity fan in the case of Steam/Air retorts. The premise is to maintain a sufficiently high enough flow rate throughout the retort to ensure temperature equilibrium. The objective is to make sure every container sees the same temperature profile while providing a counterbalance to the pressure created inside the package.
Probably the most important attribute of any retort is its ability to provide a uniform thermal treatment to all containers. This is best accomplished by directing the heating and cooling media parallel to the main heat transfer surfaces of the container. For example, the main heat transfer surface of most cylindrical containers when the can height exceeds the diameter is the sidewall of the container – compared to the container’s ends. For pouches and low profile bowls and trays, the tops and bottoms of these containers will transfer most of the heat energy to the container. In both cases, the optimum flow of the heating and cooling media will be parallel to these surfaces.
So which retort is best suited for which package? Is there a “one retort that fits all”?
The answers to these questions could certainly draw the ire of retort manufacturers that limit their product line to a single retort model. This is not the intent of this paper. If political correctness, or in this case, retort correctness, were the objective, then the imparting of relevant information would not be accomplished. With this said, I give to my humble opinions based on almost 25 years of hands-on experience in retort design and testing of every retort mentioned, using a wide variety of container types and sizes.
First, I want to list some of the common features of these retorts.
- All are horizontal batch sterilizers
- All use some mechanical device to circulate the heating medium (pump or fan)
- Most use compressed air injected directly into the process vessel to create overpressure
- All modern versions of these vessels use advanced controls
- All use steam as either the process medium, or to heat the process medium
Water Immersion Processing
This retort type has been in use for decades, well before the mixed media units. These come in both vertical and horizontal models. The vertical models typically use compressed air injected at the bottom of the shell to promote circulation, and to create overpressure. The horizontal models typically use a pump to circulate the process water from bottom to top, with either air injected above the water level to create overpressure, or steam pressure in a separate storage vessel to accomplish this.
The operational principle of the vertical model is quite simple. For the horizontal models, it is more complex. For vertical retorts, the shell is filled with process water, and some degree of preheating this water takes place before the baskets of product are lowered into the sterilizer. The water level should be above the topmost containers. After loading the retort, the top lid is closed and steam mixed with air is introduced at the bottom of the retort. As previously stated, the compressed air injected at the bottom of the shell has a twofold purpose – to promote circulation, and to create overpressure. These retorts are ideally suited for cylindrical containers standing upright in the baskets because the water flow is parallel to the main transfer surfaces of the container. Containers having these surfaces perpendicular to the flow of water (i.e. pouches, trays, and flat metal cans) will provide more resistance to flow, and by doing this, hot and cold spots inside the retort load develop.
For horizontal models, the baskets are rolled into the empty retort, the door is closed, and typically preheated process water is transferred from a separate storage vessel. Once the proper water level is attained (above the topmost containers), a circulation pump is energized drawing the process water from the bottom of the vessel, sending it through a mixing chamber where steam is directly injected, or through a heat exchanger where indirect heating of the process water occurs. The heated process water is then returned to the top of the retort shell, or through a number of inlet ports enveloping the retort baskets.
Overpressure in the most common horizontal models can be more complex than in the vertical units. The most common method of creating overpressure is through the use of a second vessel piggybacked onto the process vessel. This second vessel, referred to as the storage vessel, serves two purposes. First, it is used to preheat the process water to very high temperatures, reducing the length of the Come-Up Time. This superheated hot water is transferred to the lower process vessel once the retort door is closed. The storage vessel should be of the size that its water capacity is slightly greater than that of the process vessel when the process vessel is filled with containers. Once the transfer of preheated water is complete, there must be a residual amount of this water remaining in the storage vessel to ensure proper pressure control. Via a connection valve, the storage and process vessels remain “connected” during all of the heating steps, and the initial cooling step. Steam pressure above the residual water in the storage vessel is maintained at the overpressure desired in the process vessel, and this overpressure translates down from the storage to the process vessel via the connection valve.
The less common type of horizontal water immersion retorts are simpler in operation, and become isolated from the preheat storage vessel once the water transfer is complete. In these retorts, overpressure is accomplished by the introduction of compressed air into the headspace above the water level. As with the dual tank versions, once the proper water level is attained, a circulation pump is energized drawing the process water from the bottom of the vessel, sending it through a mixing chamber where steam is directly injected, or through a heat exchanger where indirect heating of the process water occurs. The heated process water is then returned to the top of the retort shell, or through a number of inlet ports enveloping the retort baskets.
Most of the horizontal water immersion retorts will have 1 – 3 rows of inlet ports where the water reenters the vessel at the top of the vessel. Depending upon how well these ports envelop the retort baskets will determine the optimal container geometry for processing in these retorts. Those that introduce the water above the baskets are best suited for cylindrical containers, for the same reason mentioned in the vertical retort section. For horizontal retorts that have a multitude of entry ports along the top and the sides, the package geometry becomes less of an issue provided the entry force of the water is sufficient to penetrate the load.
Rotation of the retort load facilitates temperature distribution inside these retorts because movement of the containers inside the process water benefits the mixing of this water. Rotation reduces some of the limitations on container geometry seen with many of the other process modes that I address in this paper. The most common rotary horizontal retorts are of the water immersion type. Water provides buoyancy to the load, creating less stress on the rotating mechanism. Rotating the retort load has many positive benefits, one of which is to provide better heat transfer because the system is not totally dependent on a circulation pump to adequate achieve an isothermal condition inside the process vessel.
Water Cascade Processing
This retort type is often referred to as cascade, trickle, shower, or raining retorts because the process water showers over the retort load.
The operational principle is simple – Process water is drawn from the bottom of the retort, circulated through an external heat exchanger where the water is indirectly heated, and redistributed at the top of the retort. The process water flows across a perforated water distribution plate spanning the top of the retort, and cascades into the product baskets. The distribution plate is typically the same width as the retort baskets, and the same length (front to rear inside the retort) as the combined length of all the baskets representing a full load. The number and size of the holes in the top distribution plate is critical to ensuring the process water flows evenly across the entire load. And in most systems, the 4 vertical sides of the retort baskets are solid, which helps channel the process water through the basket. Water level is maintained below the bottom of the retort baskets, and the perforated basket bottoms should be of the design that they do no restrict the flow of the water out through the load. The following is a simple depiction of this retort.
Using the premise that heat transfer is optimized when the heating and cooling media flow is parallel to the main transfer surfaces of the container, then these retorts are ideally suited for cylindrical containers (cans and glass jars) standing upright in the retort basket. Containers that have these surfaces perpendicular to the flow of the cascading water (i.e. pouches, trays, and flat metal cans) will generate more resistance to flow, and by doing this, hot and cold spots inside the retort load develop.
Also with these retorts, due to the dynamics of flow and the fact that the process water is heated externally, the containers at the top of the baskets heat faster than those at the bottom. As the heated water flows into the basket, the first containers to contact this water absorb much of this heat energy. As the water flows down through the basket, there is less available heat for the containers at the basket bottom, particularly during the come-up and early part of the cook hold step. During cooling, the containers at the top cool faster than those at the bottom, so there is some “balancing” of the heat received by all containers. However, for those processors that rely on heating-only lethality when determining the length of the thermal process necessary for Commercial Sterility, then those containers at the top of the basket will most certainly receive more of a heat treatment before the bottom containers reach the target lethality.
This is not a common retort type in the US (less than a handful of processors use them), although they are common in Europe.
As with the water cascade retort, the operational principle is simple – Steam is injected at the bottom of the vessel, and when combined with the air trapped inside the shell when the door closes, creates a steam/air environment. Overpressure control is maintained by either venting excess pressure out of the shell, or adding compressed air into the shell.
The distinguishing feature of the steam/air retorts is the use of a high speed fan or fans to circulate the steam/air mix. This is an extremely important function to maintaining a homogeneous mix of the steam and air because if circulation fails, then pockets of air could insulate containers in the load. This could result in an under-processing condition. Also, condensate created due to the transfer of heat is expelled from the bottom of the retort’s shell.
The most common steam/air models have a single high speed fan mounted at one end of the retort. In these retorts, the steam/air mix is pulled toward the fan, and is then forced back to the opposite end of the retort shell behind solid baffles that shroud the lateral sides of the retort baskets. Once the mix reaches the end of the shell opposite the fan, the domed end-cap forces the steam/air back through the retort baskets toward the fan. In these retorts, the flow of the steam/air heating medium is horizontal or parallel to the top and bottom of the retort shell. The following depicts the basic concept of the steam/air retort:
In most of the “horizontal flow” steam/air models, the 4 vertical sides of the retort baskets are open to allow unrestricted flow of the heating medium. Because the flow is horizontal, these retorts are ideally suited for “flat” containers. Pouches and low profile trays and cans are the target containers.
The other steam/air retorts I have had the opportunity to test used multiple fans, one mounted above each basket, to circulate the heating medium. Steam is injected below the retort baskets, and the flow direction is vertical (bottom to top). The fans draw the medium toward them. As with the water cascade retorts, these retorts are ideally suited for cylindrical containers (cans and glass jars) standing upright in the retort basket. Containers that have the main heat transfer surfaces perpendicular to the flow of the cascading water (i.e. pouches, trays, and flat metal cans) will provide more resistance to flow, and by doing this, hot and cold spots inside the retort load develop.
With any of the steam/air retorts, horizontal or vertical flow, the side of the basket closest to the emission point of steam from the steam spreader(s) will generally heat faster because the containers along this side are exposed first to the heat. As the steam/air mix flows through the basket, there is less available heat for the containers at side opposite from where the steam enters the basket. As with the water cascade models, temperature eventually equilibrates sometime during the early part of the cook hold step.
Steam/Water Spray Processing
This is the most versatile of the overpressure retorts for flexible and semi-rigid packaging.
As with the water cascade retorts, the operational principle is simple – Process water is filled into the bottom of the retort, to a level in the trough below the baskets, and if a preheat steam spreader is available in this trough area, the water may be preheated to a recipe temperature prior to closing the door and processing. If there is no steam spreader in the trough, then the process will start with whatever temperature water is in the trough.
The process water is drawn from the bottom of the retort, and may be circulated through an external heat exchanger where the water is indirectly heated, and then redistributed through a series of spray nozzles positioned around the retort baskets, or it may bypass the heat exchanger and be redistributed through the spray nozzles with direct steam injection into the retort shell. The latter approach uses single or dual steam spreaders that traverse the full length of the retort shell, positioned in a void below the basket bottoms and the process water contained in the trough. These spreaders are separate from the spreader that is used to preheat the process water, and it is important to maintain the process water level below these spreaders. Allpax offers both retort styles – indirect and direct heating of the process medium.
When heating is by direct injection into the retort shell, the turbulence created by the water exiting the spray nozzles mixes with the steam and air environment to create a homogeneous heating medium.
Just as the steam/air retorts use the air contained within the retort when the door is closed and heating begins to create the overpressure environment, so does the steam/water spray model. Overpressure control is maintained by either venting excess pressure out of the shell, or adding compressed air into the shell.
The key to the steam/water spray retorts is their ability to take the process water and spray it turbulently throughout the retort. The water reenters the process shell through specially designed nozzles that “atomize” it into very small droplets or a mist. The spray pattern from the nozzles is usually conical, and this creates overlapping spray patterns throughout the retort. In some steam/water spray retort models, there are virtually no dead spots as demonstrated below.
The early models typically had 1-3 spray bars suspended above the retort baskets, with emission nozzles spaced 6-12 inches apart, directing the spray across the top of the retort load. I believe there were similar limitations with these early models in regards to optimal container orientation like that with the water cascade retorts.
The Allpax steam/water spray models allow the user to customize the number and location of the spray bars, essentially encircling the load for maximum penetration. It is because of this multidirectional approach to heating medium penetration that I believe these retorts offer the highest degree of versatility for just about any package design or geometry. The water spray is directed from the top as well as both sides of the baskets, with the injected steam rising from below the baskets. Also, condensate created due to the transfer of heat is expelled from the bottom of the retort’s shell.
Cooling is typically done through an external plate heat exchanger, and again, cooling medium penetration is from 3 sides of the retort basket, minimizing hot spots that can occur with single direction cooling mediums.
Rotary versus Non-Rotary versus Gentle Motion Retorting
The thermal process and finished product quality can be greatly enhanced for some containers and products by moving the containers during the cook / cool cycle. Movement or agitation of the containers forces convection heating of the product inside the container. The level of convection is dictated by the headspace or void inside the container, the viscosity of the product, the container’s geometry and orientation inside the retort basket, and the type of movement applied to the container.
One of the more common types of forced convection batch processing uses end-over-end (EOE) rotation of the containers. Historically, metal cans, polymeric bowls, and glass jars have been targeted for EOE rotation because of the ease in controlling the headspace, and rigidity of the container helps to maintain this void. However, recent tests we conducted on pouched products subjected to lower rotation speeds (<10 RPM’s) proved that the products in these containers can also benefit from EOE rotation provided the headspace (typically measured as level of residual gas) is controlled
In the EOE process mode, the rigid cylindrical containers are usually placed upright inside the retort baskets, and the containers are rotated as shown below.
The mechanical action of the retort’s rotation mechanism rotates the baskets EOE at speeds ranging from 2-25 RPM’s. This forces the headspace bubble inside the container to move from the container wall through the center of the container, forcing hot and cooler product to thoroughly mix. The mixing greatly reduces the length of time required to produce a sterile product. Process reductions of as much as 60% below non-rotary processing are possible.
EOE is generally better suited for water immersion processes because the water provides buoyancy to the retort load, and this relieves some of the stress placed on the drive mechanism and the trunion rollers supporting the load.
Gentle Motion processing is another form of container agitation processing. This is a relatively new technology in which the cylindrical containers are placed in baskets or in the case of pouches or trays, placed in racks that are stacked to make a basket, and a reciprocating forward and backward motion is applied to the retort load. As with EOE processing, Gentle Motion processing can induce convection in the product, but at a much reduced rate compared to EOE. The reciprocating motion is programmable to speeds of 0-60 strokes per minute. A stroke is defined as one complete forward and return movement of the retort load.
However, unlike EOE processing, Gentle Motion processing takes place in a Saturated Steam, Steam/Water Spray, or Water Cascade environment. These modes maximize the penetration of the heating medium through the retort load. Recall that Saturated Steam processing is best suited for metal containers employing a double-seam closure, whereas Steam/Water Spray and Water Cascade processing can be used for all container types.
Retort pouches and low-profile trays are best suited for Gentle Motion processing because this process mode is less abusive on these fragile containers compared to EOE processing, and the low-profile geometry requires less vigorous agitation to force convective heating inside the containers. It appears based on limited testing as of the time of this paper that Gentle Motion processing does not create enough turbulence inside cylindrical containers oriented in the upright position to produce a benefit in process time savings.
A retort is generally the most expensive piece of equipment in a processing plant and usually the longest lasting. We commonly see retorts that have been in use for well over 40 years. For that reason many companies are moving to multi process mode retort systems. They know what containers and products they are running today but in the fast changing market-driven industry where appearance and convenience are everything, production plants have no idea what products and containers are around the next corner. Multi process mode retorts give them that flexibility.
I understand there can be exceptions to any generalization. While the generalizations made here are made on solid repetitive testing of these retort types, they are not immune to exceptions. Even without supporting tests, logic would tell you that the best approach for attaining adequate temperature distribution at the time the cook hold step begins is to provide the least amount of restriction to the flow of the heat medium. That is what I have tried to call attention to in this paper. There are ways to overcome obstacles to flow, but given the choice of having those obstacles or not, I think you would choose the latter.