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Guide to Inspections of Low Acid Canned Food 35

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controlled. The container is conveyed through the steam water interface into the steam dome. The number of times that the carrier passes through the steam dome as well as the speed of the carrier determines the process time. Traveling from the top of the steam dome to the bottom, and vice-versa, is referred to as one pass. Hydrostatic retorts with 2, 4, 6, and 8 passes are common. After traveling through the steam dome the containers are conveyed into the exit water leg where the temperature decreases as the container passes up the leg. The cans leaving the steam dome are heated to a high level and give up their heat to the water in the discharge leg. This results in several situations depending upon the design of the retort:

  1. The water in the discharge leg ranges from 215° F near the steam-water interface to 212° F near the top of the leg, and the water quietly boils and steam is discharged from the top of the leg.
  2. Cross circulation pumps are used to pump the hot water from the base of the exit leg to the base of the inlet leg and from the top of the inlet leg to the top of the exit leg, stabilizing the temperatures in the legs.
  3. Water is pumped from the base of the exit leg through a heat exchanger with the cooled water being returned to the top of the exit leg.

Any of the above three methods of operation are acceptable.

As the container exits the leg it is exposed to atmospheric pressures, and it may pass through a series of water spray coolers to further cool the product. The conveyor chain carries the containers back to near the loading station where the processed product is unloaded from the continuous carrier. Because the container inlet and exit are close together, care must be taken to insure that unprocessed containers do not become mixed with processed containers. Containers found on the floor or elsewhere whose status is questionable should be destroyed.

Control of the water level in the feed and exit legs is important to maintain the hydrostatic pressure in the retort. The water level is normally controlled through a differential pressure controller which adds water when it is needed and dumps excess water from the legs. Water level fluctuation in the feed and exit legs may be caused by fluctuations in the feeding and discharge of containers. As more containers are fed into the container conveyor more water is displaced from the legs, a lack of production results in a lack of containers in the legs and the water level falls.

Hydrostatic retorts have been installed with up to 3 carrier chains operating in the same steam dome. This allows for the production of different can sizes at the same temperature by adjusting the process time through the speed of the conveyor. Carrier chains designed for cans can normally be used to process a number of different diameter cans within limits (e.g. 211 - 303, 404 - 603).

Steam is fed into the steam dome, depending upon the manufacturer, either in the center or top of the steam dome. Standard temperature or pressure controllers are normally used to control the retort temperature. The control of retort temperature through the use of a water level float has been used on some european systems.

21CFR 113.40(f)(1) requires that the MIG thermometer be installed in the retort steam dome near the steam-water interface. This should be the coldest spot in the retort dome. If the thermal process is based on lethality gained in the feed or exit water legs a MIG thermometer is required to be installed near the bottom temperature recorder in each water leg.

21CFR 113.40(f)(2) requires the installation of additional temperature recorders near the top and bottom of each hydrostatic water leg if the scheduled process specifies maintenance of particular temperatures in the water legs.

21CFR 113.40(f)(7) requires the hydrostatic retort to be equipped with at least one bleeder 1/4 inch or larger at the top of the steam chamber or chambers at the opposite end of steam entry. In addition, all bleeders must be arranged in such a way that the operator can observe that they are functioning properly.

The carrier conveyor in the hydrostatic retort may be numbered so that the location of the carriers can be determined during processing. Carrier location becomes important if the retort temperature/pressure falls to a level where the containers contact the water. The retort should be equipped with an automatic stop if the temperature drops below the minimum filed scheduled process temperature. If the retort is stopped at that point the operator can identify those carriers in contact with the water. The containers in the affected carriers can then be set aside for reprocessing, destroyed or held for evaluation by a processing authority.

The carrier speed is controlled for each container conveyor through a variable speed motor. Carrier conveyor speed may be measured by the number of flights per minute using a stop watch or electronically by a sensing probe. Electronic measurement of the conveyor speed should be verified by using a stop watch on a routine basis. The correct container-conveyor chain speed can be determined in the following manner.

  1. The desired carriers-per-minute is determined by

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