Guide to Inspections of Low Acid Canned Food 34
Manufacturers - 2
lower drum. Stock systems can also use air for over-pressure if requested by the customer. Stock claims that when steam is used in the lower shell to provide an over-pressure that deviations are not normally caused by lower than normal water levels in the processing shell of agitating retorts, as the head space will provide a steam processing medium. There is some question as to whether the head space is pure steam or a mixture of steam/air which may not provide a uniform temperature. When lower than normal water levels are noted the manufacturer should treat the process as a processing deviation; requiring the reprocessing of the product, the destruction of the product or the review of the thermal process by a processing authority unless there is documentation on hand to support processing with lower than normal water levels.
The majority of the systems noted have been equipped with a water sight glass or mechanical indicator of water level in the retort as well as being equipped with electronic alarms for low-water levels.
Adequate temperature distribution in the batch agitating water immersion retort is dependent upon such factors as the proper functioning of the centrifugal pump, rotation of the reel, and the number of cages in the retort. A minimum number of cages full of containers (e.g. 3 cages in a 4 cage retort) is required to maintain the water level in the retort. The number of cages also effects the CUT (come-up time) to the processing temperature. If the minimum number of product filled crates are not available the firm may use ballast (e.g. containers filled with water) to fill the additional crates required. In addition the open area provided by divider plates or spacers, as well as the cages or racks themselves, can have an effect on adequate temperature distribution. Any change to a more restrictive-to-flow design in any of the above must be validated by new temperature distribution studies. Temperature distribution may be affected by container type, container size, racking configurations, number of containers in the retort, product being produced and many other variations in thermal processing. Because of the many variables that can influence temperature distribution in these retorts in some cases each retort, each containers type, each racking system and each container size will have to be evaluated to determine their effect on temperature distribution.
Delivery of the filed scheduled process is also dependent upon the control of critical factors such as: headspace, product consistency, fill-in or drained weights, vacuum in vacuum packed products, minimum net weights, percent solids and other critical factors identified in the scheduled process.
Equipment and procedures for pressure processing in steam in hydrostatic retorts are covered by 21CFR 113.40(f) of the lacf regulations.
Generally the hydrostatic retort can be thought of as a still steam retort operated at a constant temperature through which containers are conveyed by a continuous carrier chain at a constant rate designed to provide the correct process time (Attachment 9).
Hydrostatic retorts are manufactured by the FMC Corp. in the U.S. and by Stork and others in Europe. Newer designs now offer end over end or axial agitation of the product; the use of over-pressure for the maintenance of container integrity; the ability to process glass and flexible pouches; and water as a heating medium in addition to steam. The systems are used for high volume products which need long cook times such as condensed soups and pet foods.
The name hydrostatic is derived from the fact that the pressure in the steam dome is counter balanced by water in the entry and exit legs of the retort. The higher the water level, the higher the pressure and temperature obtained in the steam dome. For example, the water height in the water legs must be 37 feet high at sea level to counter balance a processing temperature of 250° F (121° C). Operating at temperatures above 250° F will require a higher water level. The retorts can be operated below the maximum temperature as long as the pressure remains high enough to prevent water contact with the containers in the steam dome.
Start up procedures for a hydrostatic retort require venting of the retort and bringing the water in the feed legs up to temperature. This procedure takes a longer period of time than the venting of still steam retorts. The hydrostatic retort is normally operated for periods of up to several weeks and may be shut down and cooled only when required for maintenance or repairs.
These retort systems are very large and normally extend several stories into the air. Containers are loaded into a horizontal carrier on the continuous chain and conveyed up to the inlet leg of the sterilizer. The inlet leg is filled with water which counter balances the pressure in the steam dome. The temperature of the water increases as the container moves from the top of the inlet leg down toward the steam-water interface at the bottom of the leg. Water temperature in the inlet leg may range from ambient to boiling. The feed leg may contribute to the process lethality by increasing the initial temperature of the product. If process lethality is claimed for the inlet leg of the retort the water temperature in the inlet leg must be carefully