Hydrostatic Retort Cutaway

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jor breakthrough. This allowed a concurrent development of devices permitting the exposure of the filled and sealed containers to steam pressures above atmospheric, namely, processing temperatures of 120°C instead of only 100°C (boiling water). Because the thermal inactivation of food spoilage organisms is a function of both time and temperature, the higher temperature under pressure allowed a very considerable reduction in the time necessary to ensure product sterility. Equally important was the marked improvement of the canned food quality. Commercial equipment systems in use today for heat sterilization of canned foods are described in the following sections.

Retort Processing

Batch Systems. There are two fundamentally different process methods by which canning is accomplished in the food industry. These two methods are known as retort processing and aseptic processing. In retort processing, foods to be sterilized are first filled and hermetically sealed in cans, jars, or other retortable containers. Then, they are heated in their containers using hot steam or water under pressure so that heat penetrates the product from the can wall inward, and both product and can wall become sterilized together. In aseptic processing, a liquid food is first sterilized outside the container by pumping it through heat exchangers that deliver very rapid heating and cooling rates. Then, the cool sterile product is filled and sealed in a separately sterilized package under a sterile environment at room temperature. Thus, retort processing can be thought of as in-container or in-can sterilization, and aseptic processing can be thought of as out-of-container sterilization.

In retort processing (in-can sterilization), the food to be sterilized is first filled and hermetically sealed in rigid, flexible, or semirigid containers such as metal cans, glass jars, retort pouches, or plastic bowls or trays that are placed within large steam retorts, sometimes called autoclaves. These are pressure vessels that work like giant pressure cookers, as shown in Figure 1. Once the retorts are full of containers to be sterilized, the retort doors are closed tightly and the air is replaced by hot steam under pressure to achieve temperatures above the atmospheric boiling point of water. A common retort temperature for sterilizing canned foods is 121°C (250°F), at approximately 1 atm of added internal pressure. After the containers have

Hydrostat Retort System
Figure 1. Empty batch retort with doors ajar showing interior rails for entry and exit of wheeled crates used in loading and unloading operations. Source: Courtesy FMC Food Process Systems Division, Madera, Calif.

been exposed to the sterilizing temperature for sufficient time to achieve the desired level of sterilization, the steam is shut off, and cooling water is introduced to cool the containers and reduce the pressure, thus ending the process. Once the retort pressure has returned to atmosphere, the doors can be opened, and the processed containers are removed for labeling, case-packing, and warehousing to await distribution to the marketplace.

Although the unloading and reloading operations are labor intensive, a well-managed cook room can operate with surprising efficiency. The cook room is the room or area within a food canning factory in which the retorts are located (Fig. 2). Some cook rooms are known to have more than 100 retorts operating at full production. Although each retort is a batch cook operation, the cook room as a whole operates as a continuous production system in that filled and sealed unsterilized cans enter the cook room continuously from the filling line operations, and fully processed sterilized cans leave the cook room continuously en-route to subsequent case packing and warehousing. Within the cook room, teams of factory workers move from retort to retort to carry out loading and unloading operations, and retort operators are responsible for a given number or bank of retorts. These operators carefully monitor the

Hydrostat Retort System
Figure 2. Exterior view of operating batch retort (above), and commercial system of batch retorts showing automated loading/ unloading operations in a typical food processing plant cook room (below). Source: Courtesy FMC Food Process Systems Division, Madera, Calif.

operation of each retort to make sure the scheduled process is delivered for each batch. For convection-heating products that benefit from mechanical agitation during processing, agitating batch retorts are available that accomplish axial rotation of the cans or end-over-end agitation by rotating the entire crate during processing operations (3,4).

Continuous Retort Systems. Continuous retort operations require some means by which cans are automatically and continuously moved from atmospheric conditions into a pressurized steam environment, held or conveyed through that environment for the specified process time, and then returned to atmospheric conditions for further handling operations. The most well-known commercially available systems that accomplish these requirements are the crateless retort, the continuous rotary cooker, and the hydrostatic sterilizer (3,4).

Crateless Retorts. A crateless retort system is, in a sense, an automatic cook room in that the system is made up of a series of individual retorts, each operating in a batch mode, with loading, unloading, and process scheduling operations all carried out automatically without the use of crates. An individual crateless retort is shown schematically in Figure 3. When ready to load, the top hatch opens automatically, and cans fed from an incoming conveyor literally fall into the retort, which is filled with hot water to cushion the fall. Once fully charged, the hatch is closed while the incoming conveyor diverts the flow of cans to another retort that is ready for loading. Steam entering from the top displaces the cushion water out the bottom. When the cushion water has been fully displaced, all valves are closed and processing begins. At the end of the process time, the retort is refilled with warm water and the bottom hatch, which lies beneath the water level in the discharge cooling canal, is opened to let the cans gently fall onto the moving discharge conveyor in the cooling canal.

Crateless Retort System
Figure 3. Operating schematic of a crateless retort showing "free fall" discharge of sterilized food containers onto submerged cooling canal exit conveyor.

After all cans are discharged, the bottom hatch is reclosed, and the retort is ready to begin a new cycle. A commercial system of crateless retorts would consist of several such retorts in a row sharing a common infeed and discharge conveyor system to achieve continuous operation of any design capacity.

Continuous Rotary Cookers. The continuous rotary pressure sterilizer or "cooker" is a horizontal rotating retort through which the cans are conveyed while they rotate about their own axis through a spiral path on a revolving reel mechanism. Residence time through the sterilizer is controlled by the rotating speed of the reel, which can be adjusted to accomplish the specified process time. This, in turn, sets the line speed for the entire system. Cans are transferred from an incoming can conveyor through a synchronized feeding device to a rotary pressure sealed transfer valve, which indexes the cans into the sterilizer while preventing the escape of steam and loss of pressure (much like a revolving door). Once cans have entered the sterilizer, they travel in the annular space between the reel and the shell. They are held between spines on the reel and a helical or spiral track welded to the interior shell wall. In this way, the cans are carried by the reel around the inner circumference of the shell imparting a rotation about their own axes, while the spiral track in the shell directs the cans forward along the length of the sterilizer by one can length for each revolution of the reel. At the end of the sterilizer, cans are ejected from the reel into another rotary valve and into the next shell for either additional cooking or cooling (Figs. 4 and 5).

Most common systems require at least three shells in series to accomplish controlled cooling through both a pres-

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Figure 4. Schematic of helical path traveled by food containers moving through a series of rotary cooker/cooler shells in a continuous rotary sterilizer system.


Figure 4. Schematic of helical path traveled by food containers moving through a series of rotary cooker/cooler shells in a continuous rotary sterilizer system.

Continuous Sterilizer
Figure 5. Continuous rotary sterilizer system with cutaway views showing rotary pressure-seal valves and internal helical spline and real conveying mechanism. Source: Courtesy FMC Food Process Systems Division, Madera, Calif.

sure cool shell and an atmospheric cool shell following the cooker or sterilizer. For cold-fill products that require controlled preheating, as many as five shells may be required in order to deliver an atmospheric preheat, pressure preheat, pressure cook, pressure cool, and atmospheric cool. By nature of its design and principal of operation, a continuous rotary sterilizer system is manufactured to accommodate a specified can size and cannot be easily adapted to other sizes. For this reason, it is not uncommon to see several systems in operation in one food canning plant, each system dedicated to a different can size filling line.

Hydrostatic Sterilizers. These systems are so named because steam pressure is controlled hydrostatically by the height of a leg of water. Because of the height of water leg required, these sterilizers are usually installed outdoors adjacent to a canning plant. They are self-contained structures with the external appearance of a rectangular tower as shown in Figure 6. They are basically made up of four

Figure 6. Exterior view of continuous hydrostatic sterilizer. Source: Courtesy FMC Food Process Systems Division, Madera, Calif.

chambers: a hydrostatic bring-up leg, a sterilizing steam dome, a hydrostatic bring-down leg, and a cooling section.

The principal of operation for a hydrostatic sterilizer can be explained with reference to the schematic in Figure 7. Containers are conveyed through the sterilizer on carriers connected to a continuous chain link mechanism that provides positive line speed control. This provides residence time control to achieve specified process time in the steam dome. Carriers are loaded automatically from incoming can conveyors and travel to the top of the sterilizer

Hydrostatic Retort
Figure 7. Schematic of continuous conveyor path traveled by food containers through inlet water leg, sterilizing steam dome, outlet water leg, and cooling section of a continuous hydrostatic retort.

where they enter the bring-up water leg. They travel downward through this leg as they encounter progressively hotter water. As they enter the bottom of the steam dome, the water temperature will be in equilibrium with steam temperature at the water steam interface. In the steam dome, the cans are exposed to the specified process or "retort" temperature controlled by the hydrostatic pressure for the prescribed process time controlled by the carrier line speed. When cans exit the steam dome, they again pass through the water steam interface at the bottom and travel upward through the bring-down leg as they encounter progressively cooler water until they exit at the top. Cans are then sprayed with cooling water as the carriers travel down the outside of the sterilizer on their return to the discharge conveyor station.

Aseptic Processing

Aseptic canning systems have rapidly developed in recent years primarily to allow for the marketing of shelf-stable foods in novel or more economical packaging systems that cannot withstand normal retort processing conditions. The primary goal in earlier development work on aseptic canning systems was to use the high temperature-short time (HTST) benefit of aseptic processing to minimize quality losses that occur in the slow heating of foods processed in conventional retorting systems. In either case, aseptic canning circumvents the need for retort operations by sterilizing the product outside of the container through heat exchanger systems before it is filled aseptically into separately sterilized containers or packaging systems.

Heat Exchangers. The benefits of HTST processing have been known for a number of years and have led to the rapid development of aseptic canning systems wherever possible. These methods generally apply only to fluid products that can be pumped through heat exchangers capable of applying ultra-high temperature-short time (UHT) heating conditions to the product before it is filled and sealed aseptically. The general types of heat exchangers commonly used with aseptic canning systems fall into the two basic categories of direct and indirect heating. In direct heating, the product is brought into direct contact with live steam through either steam injection or steam infusion heaters followed by holding and flash cooling under controlled pressure (Fig. 8). In indirect heating, the product contacts the heated metal surfaces of a heat exchanger, which separate the product from direct contact with the heat exchange medium. Either plate, tubular, or swept-surface heat exchangers are most often used for this purpose (Fig. 9). The residence time experienced by the product as it flows through an insulated holding tube or holding section between the heating and cooling heat exchangers accomplishes the necessary process time for delivering the specified sterilizing value or lethality, and is controlled by flow rate.

Aseptic Processing Systems. Among the first commercially successful aseptic canning systems is the Dole system, illustrated schematically in Figure 10. The system was designed to aseptically fill conventional steel cans, and

Product in

Product in

Product out

Figure 8. Steam-infusion heat exchangers. Source: Ref. 5, courtesy of Crepaco, Inc.

Product out

Figure 8. Steam-infusion heat exchangers. Source: Ref. 5, courtesy of Crepaco, Inc.

made use of superheated steam chambers to sterilize empty can bodies and covers as they were slowly conveyed to the filling chamber. The filling chamber was also maintained sterile by superheated steam under positive pressure and received cool sterile product from the heat exchangers in the product sterilizing subsystem. The entire system was presterilized before operation by passing superheated steam through the can tunnel, cover and closing chamber, and filling chamber for a prescribed start-up program of specified times and temperatures. The product sterilizing line was presterilized by passing pressurized hot water through the cooling heat exchanger (with coolant turned off), product filling line, and filler heads. This startup procedure had to be repeated every time a compromise in sterility occurred at any system component. Careful


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  • pervinca
    How big are hydrostatic retorts?
    2 years ago

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