Guide to Inspections of Low Acid Canned Food 28
Manufacturers - 2
provide for circulation of the water during thermal processing and cooling, in addition to crate supports.
Containers are normally added to vertical retorts that contain water at or near the same temperature as the initial temperature of the product. If product in glass jars are subjected to water which is too cold, thermal shock may break the containers. If the water in the retort is more than 15° F above the temperature of the container the glass container closure may vent product. Water must be added to horizontal retorts after the containers are loaded. A separate warm water supply at or near the initial temperature of the product which can be used to fill the retorts aids in more rapid production of glass containers. If the water added to the retort is below the initial temperature of the containers measures must be taken to insure that the initial temperature in the container is not lowered, or that the temperature of the water is used as the initial temperature. Several retort systems use hot water storage tanks located above the processing shell. The temperature of the stored water is dependent upon the container to be processed, and may range from several degrees above the thermal processing temperature for metal cans to only a few degrees above the initial temperature of glass containers. Following processing, a portion of the processing water is forced back into the upper storage drum and reheated for processing the next batch.
Cooling water is normally added to the circulation pump line on horizontal retorts or through a cooling ring above the containers in a vertical retort. It is important with glass containers that the container not be subjected to a severe thermal shock to prevent glass breakage.
The overpressure must be maintained in the retort until the pressure in the container falls to levels which do not cause container failure.
Overpressure may be critical to the thermal process for containers such as the large profile half steam tray metal container and retortable flexible pouches, as the pressure maintains the container profile and holds the container against the product allowing for more efficient heat transfer from the container to the product. When over-pressure is listed as a factor critical to the thermal process a record of the process pressure must be made. The overpressure used during thermal processing should be the same as that used during establishment of the thermal process or as recommended by the firms processing authority.
It is recommended that the temperature probe in water immersion retorts be located next to the MIG thermometer, except in vertical retorts which are equipped with a combination recorder/controller. The controller bulb in these vertical retorts must be located in the bottom of the retort beneath the lowest crate rest in a such a position that the steam does not strike it. This should be the coldest spot in the vertical retort. In a horizontal retort the recorder/controller probe should be located between the water surface and the horizontal plane passing through the center of the retort so that there is no opportunity of direct steam impingement on the control bulb.
Some water immersion systems may be set up to process in both steam and water. Those systems must be carefully evaluated to insure that they meet the mandatory provisions of both sections of the lacf regulations.
CONTINUOUS AGITATING STEAM RETORTS Equipment and procedures for processing in continuous steam retorts are covered by 21CFR 113.40(c) of the lacf regulations.
These systems provide for continuous container handling with intermittent product agitation. Continuous agitating steam retorts are manufactured by FMC (Sterilmatic) in the U.S. and Europe and by Stork (Steristork) in Europe. The basic design of both systems are similar (Attachment 7).
The retort systems are made up of a series of processing vessels called shells. The arrangement and number of shells is dictated by the product, production capacity, and space limitations in the plant. Common arrangements found include: Two-shell: one pressure cooker and one pressure cooler; Three-shell: one pressure cooker shell, one pressure cooler shell and one atmospheric cooler shell; and Four-shell: preheater shell, one pressure cooker shell, one pressure cooler shell and one atmospheric cooler shell. Specialized systems may have up to seven shells. The shells can be arranged side by side or offset end to end depending upon the available plant space.
Containers are fed mechanically through the series of at least one heating and one cooling shell. Within each processing shell is an open, closely fitting rotating reel which runs the length of the vessel. Cans are carried horizontally in channels made up of angles, usually of stainless steel, welded around the perimeter of the reel. A spiral T-track is welded to the inside of the shell spaced at intervals slightly larger than the can length. As the reel turns the containers are forced against the spiral T which causes the cans to move down the length of the retort while being carried by the reel. The containers are fed into the processing shell or preheater through a pocket valve: a rotary transfer valve which is timed to the reel and designed to prevent loss of steam from the retort (Attachment 7).