Guide to Inspections of Low Acid Canned Food 22
Manufacturers - 2
performed for each container type (e.g. glass, metal, plastic), for each container size, for each container racking system, for each container shape, for each product produced and for each individual retort system.
Acceptable temperature ranges in the retort at the time that the process begins varies with retort systems and products. Normally you would expect to find all thermocouples (TMDs) within 1ºF of the processing temperature at the time the process timing begins for still steam retorts. For water immersion retorts you would normally expect to find all thermocouples (TMDs) at or above the set point temperature and no greater than a 2ºF degree difference between the minimum and maximum TMD. Other conditions may be acceptable to meet the requirements of adequate temperature distribution in a particular retort system. These conditions should be documented in the temperature distribution study. Equipment and procedures which fail to provide adequate temperature distribution in a retort system prior to start of the process must be modified to provide adequate temperature distribution.
There must be documentation on hand at the lacf plant, from the processing authority, which specifies the come-up procedures (e.g. venting, cut-up time) to be employed to ensure adequate temperature distribution is achieved at the time that the thermal process or sterilization cycle begins. The firm must employ the same procedures during production as were used to establish the temperature distribution in the retort.
Among the factors which may affect temperature distribution are:
- Initial temperature of the product
- Loading configuration (retort crate and retort)
- Retort geometry
- Steam spreader design
- Type of heating media (steam, water immersion, cascading water, water spray, steam air etc.)
- Circulation of heating medium
- Use of agitation
- Size, type and location of vent valve in steam retorts
- Steam header pressure
In some foreign countries lacf manufacturers attempt to establish the effectiveness of the retort by performing heat penetration tests in various areas of the retort, or by performing numerous heat penetration tests using a limited number of containers placed in different areas of the retort during several different process cycles. This method of retort temperature validation has not been accepted by CFSAN as being a valid method for performing temperature distribution studies. Variations in such things as retort load, retort cycles, product fill, and initial temperatures can cause temperature readings obtained from thermocouples inside of the containers to vary by several degrees.
A good reference for temperature distribution protocol for steam still retorts has been published by the Institute for Thermal Processing Specialists.
Heat penetration, in contrast to temperature distribution, is designed to measure the rate of heating of the food product under consideration. In these tests thermocouples or other temperature measuring devices are inserted into the containers to measure the product heating rate. Not only should the slowest heating container in the test be identified, but also the slowest heating zone within any given container. Thus it may be necessary to do a preliminary "cold-spot" study to identify the slowest-heating zone within the container prior to doing the actual heat penetration test on several containers. In addition, the performance of heat penetration work must be based upon adequate knowledge of the heat resistance of the microorganism(s) of concern in the specific product to be processed. Finally, the heat penetration test must cover all the variables expected to be encountered during the course of commercial operation in the plant.
Among the factors which may affect heat penetration results are:
- Container position, geometry and heat transfer characteristics.
- Type of heating medium
- Product factors ( e.g. fill weight, viscosity, particle size, percent solids, method of preparation)
- Equipment factors (e.g. filling method, head space, rotational speed)
The information developed during the heat penetration test is then used along with other information such as the heat resistance of the microorganism(s) in question (this can be effected by the food being tested), and the numbers of microorganisms expected to be present in the product, to calculate a theoretical process.
The theoretical process may be confirmed by using an inoculated pack. In an inoculated pack a known amount of an organism with a known heat resistance is placed into each container. The containers are then processed at various calculated process times and/or temperature(s) and examined for surviving organisms. The objective of the