Guide to Inspections of Low Acid Canned Food 24
Manufacturers - 2
crates. Vertical retorts are commonly found to be approximately 42 inches in diameter and to hold 3 or 4 crates of product. Horizontal retorts vary from small one crate, to large retorts holding 12 or more crates of product. Retort size normally depends upon the size of the firm, the amount of product to be processed at one time, the length of the thermal process and the closing capacity of the firm.
Still steam retorts were one of the first types of retort systems used to pressure process lacf. Steam under pressure provides a number of advantages for processing lacf products in metal containers:
- It is an excellent medium for heat transfer
- Its temperature is easily regulated
- The steam pressure required in the retort in order to achieve the required processing temperature serves to counter-balance the pressure built up inside of the container during processing.
- Steam can be easily manufactured and held in reserve for immediate use.
- The stored energy property of steam makes it a superior heating medium as compared to water or steam-air. The stored energy results from the fact that in the manufacture of steam considerable heat (970 B.T.U.'s) is required to change one pound of boiling water to one pound of steam at the same temperature. When the steam condenses on the containers during processing this latent heat is given up to the container.
It is important in the operation of still steam retorts to remove air from the retort prior to starting the process. Air in the retort acts as an insulator around the containers, preventing steam from contacting the container. Air in the presence of high heat may also cause the containers to rust. Small amounts or pockets of air in the retort can cause containers in those areas of the retort to be under processed.
Air is removed from the retort prior to processing through the retort vent and bleeders. 21CFR 113.40 describes vent plumbing and operating procedures for various vertical and horizontal still steam retorts. As long as the equipment is installed and operated exactly as per the regulations, the vent times and temperatures given are adequate to meet the requirements of the regulations for containers in jumble stack (containers randomly dropped into the crates) or off-set stack arrangements.
The National Food Processors Association Bulletin 26L recommends venting schedules similar to those found in the regulations. The recommendations found in Bulletin 26L do not, however, apply to retort systems which utilize divider plates. Bulletin 26L suggests that the venting schedules for still steam retorts using divider plates be obtained from a process authority.
Each retort system must be equipped with a steam inlet large enough to provide sufficient steam for proper operation of the retort. Steam may enter either the bottom or top portion of the retort, but must enter that portion of the retort opposite the vent. This is important to provide for adequate circulation of steam and removal of air during the venting of the retort. It is recommended that the automatic steam control valve be of the same size or larger than the steam inlet. This allows the use of the automatic steam controller during retort venting. If an automatic steam controller valve of a size smaller than the steam inlet line is used a steam by-pass may be needed during venting and come-up to provide enough steam to the retort to rapidly vent the retort. The steam by-pass may be manually operated by the retort operator or automatically operated through the retort control system. At the time that the retort reaches processing temperature the by-pass valve would be closed. Control of retort temperature during processing would be through the smaller automatic steam control valve. Use of a smaller automatic steam control valve provides for more accurate control of processing temperature with less temperature fluctuation.
Steam spreaders which are continuations of the steam inlet line inside of the retort should not be larger than the steam inlet line. Steam spreaders are required in horizontal retorts. In horizontal retorts the steam spreader must extend the entire length of the retort. The steam spreader should have perforations along the top 90° (that is within 45º on either side of top dead center of the spreader). Horizontal retorts over 30 feet long should have two steam inlets connecting to the steam spreader. In vertical still steam retorts a steam spreader is not required, however if one is used it should be operational. When used the spreader should be in the form of a cross with perforations along the center line of the pipe facing the interior of the retort or along the sides of the pipe. The steam should not flow directly onto the retort bottom (this erodes the metal) or directly onto the containers.
Vertical steam retorts are required to have crate supports to prevent the crate from sitting upon the steam spreader.
The number of perforations in steam spreaders should be such that the total cross sectional area of the perforations is equal to 1.5 to 2 times the cross sectional area of the smallest restriction in the steam inlet. The following table can be used for guidance:
NUMBER OF HOLES IN STEAM SPREADERS OF
STEAM INLET PIPE SIZES