Guide to Inspections of Low Acid Canned Food 39
Manufacturers - 2
- The records maintained by the firm must document that operating steps set-up to provide uniform temperature distribution in the retort during processing are being met.
Equipment and procedures for processing in steam-air retorts are not specifically covered in the lacf regulations. These systems are covered by 21CFR 113.40(j), Other Systems. "Other" systems are required to conform to the applicable requirements of the lacf regulations.
Steam-air retorts are normally batch type static or rotary, end-over-end, retorts. Steam-air has been used as the heating medium in the Hydrolock (a horizontal continuous feed water lock retort) manufactured by the Rexham Corporation, and in at least one hydrostatic retort in Japan. These retorts are however the exception and not the rule.
Known manufacturers of batch type steam retorts include: J.L. LaGarde in France (Distributed in the U.S. by Stork Food Machinery), Lubeca in West Germany, Marrodan in Spain, and Barriquand in France (Steristeam). At one time the Container Machinery Corp. built the Truxton Steristar in the U.S. The Steristar steam-air retort is now built by Malo. Custom installations of steam-air retorts have been reported and other manufacturers may exist. An example of the steam-air retort system is the LaGarde steam-air retort (Attachment 11).
The steam-air retort uses a mixture of steam and air, which is added to create an over-pressure in the retort. The air over-pressure allows thermal processing of a wide variety of containers including: glass, metal cans, rigid plastic, and flexible pouches. Steam-air mixtures, although in a gaseous state similar to 100% steam, when used as a heating medium behave more like water. The heat capacity of the steam-air is very small when compared to pure steam. As the steam condenses and gives off heat the air remains. Because the retort uses air in the steam mixture there must be a method of mixing the air and steam to prevent the formation of air pockets in the retort and to provide for the rapid movement of the heating medium over the container surface. Air may provide insulation around the container preventing the heat from reaching the container walls in an efficient manner. A large fan is normally used to rapidly mix the steam-air mixture and to force this mixture to flow through the retort and containers during processing.
It is very important to maintain the correct steam-air ratio in this type of retort. The steam-air ratio should be the same as that used during studies to determine temperature distribution in the retort.
Steam-air retorts are normally operated at steam-air ratios ranging from 75% steam/25% air to 95% steam/5% air, depending upon the air over-pressure in the retort. The steam-air ratio is normally determined by the processing temperature and the type of package being processed.
The pressure in a steam-air retort must be maintained at the correct pressure to prevent container distortion and to maintain the correct steam-air ratio. If the amount of steam in the steam air ratio drops to too low a level, the energy available to heat the product is reduced. If the temperature is maintained by the steam controller the partial pressure of the steam remains constant throughout the process. The pressure of the system is controlled by releasing over-pressure through a small purge valve and adding compressed air as needed.
The percent steam can be calculated using the following formula:
Steam pressure psia (pounds per sq. inch actual) = % steam
Total system pressure psia
Steam pressure psia is determined by determining the amount of steam pressure generated at a certain temperature [e.g. 240° F generates 10 psig (pounds per sq. inch gauge)] plus 14.7 psi (atmospheric pressure at sea level) equals 24.7 psia. The total system pressure is determined by taking the gauge reading and adding 14.7 psi. A steam air system having a total gauge pressure of 15 lbs operating at 240° F would have a 83% steam, 17% air mixture.
10 psig + 14.7 psi atmospheric = 24.7 = 83% steam
15 psig + 14.7 psi atmospheric 29.7
The LaGarde retort is offered in static or rotational end over end agitation models. The LaGarde is normally operated in the following manner:
- Product is placed into the retort in either standard rectangular baskets or crates for metal, glass and rigid plastic containers. Special racking trays are used for flexible and other containers needing support during processing.
- The retort is purged for 1 to 2 minutes through a large purge (vent) valve, the bottom drain or by vacuum to remove excess air from the retort and to enhance the steam entry into the retort. Steam is introduced into the retort through steam spreaders in the bottom of the retort.