Low Acid Foods

Establishing a thermal process for low-acid foods requires knowledge of both the heat resistance of microorganisms, which has been discussed previously, and the heating rate of the products. The heating rate of a product is determined by conducting heat penetration studies, which measure the changes in product temperature during processing. In addition to establishing a thermal process, the thermal process system must be operated in an appropriate manner to ensure the production of a commercially sterile product. Temperature distribution studies are conducted to establish the operating procedures for the thermal processing systems. The regulations previously discussed require that thermal processes and thermal processing system operating procedures be established by a processing authority.

Heat Penetration. Heat penetration studies involve measuring the product temperature inside test containers by placing one temperature sensor in each container. A series of tests are conducted to locate the slowest heating zone within the container and then to confirm the heating rate in the slowest heating zone. A processing authority will analyze the data from all of the heat penetration tests to determine the heating rate of the product and calculate thermal processes. The heat penetration tests should be designed to collect data under the "worst case" conditions that could be reasonably encountered in commercial production. Test design should consider product preparation, temperature sensor location, container fill procedures, container geometry and orientation, product initial temperature, retort temperature, and other possible critical factors.

Products typically heat by conduction, convection, or a combination of both. Conduction heating involves particle-

to-particle heat transfer with no gross particle movement. Products that heat by conduction are typically viscous, with little free liquid—pumpkin, stews, corned beef hash, and condensed soups are examples. Convection heating involves particle-to-particle heat transfer with particle movement. Products that heat by convection are typically thin or contain free liquid—brine-packed products such as mushrooms and green beans are examples.

Copper-constantan (Type T) thermocouples are usually the temperature sensor of choice for heat penetration testing. The time- and temperature-recording equipment used for heat penetration tests normally consists of a multichannel recording potentiometer or computer modified for data collection. The thermocouples should be connected to the recording equipment with copper-constantan thermocouple wires. The test containers may be processed in a commercial retort or in a pilot retort simulator. The thermocouple wires are normally installed through the retort wall using a stuffing box or packing gland to seal the opening into the retort.

The data must be analyzed by a processing authority. The time-temperature data from all heat penetration containers should be analyzed unless there is a problem noted with a specific temperature sensor. The slowest heating thermocouple located at the cold zone is used to define the product heating rate for process establishment. The processing authority will use this information, along with the thermal resistance data, to calculate a thermal process. Several mathematical methods have been developed to calculate a thermal process, but the scope of this chapter does not allow further discussion.

Temperature Distribution. Thermal processing systems (retorts) must be operated in such a way that the process timing does not start until the temperature-indicating device (mercury-in-glass thermometer [MIG] or equivalent) reaches the process temperature and uniform temperatures are achieved throughout the retort. Processing authorities conduct temperature distribution tests to develop the operating procedures that are necessary to establish uniform temperatures throughout the retort. For steam retorts, the operating procedure is commonly referred to as the venting procedure or vent schedule. Venting procedures are designed to remove the air present in the retort and replace it with saturated steam. Vent schedules have at least two critical components: time and temperature.

The timing of the retort process cannot begin until the scheduled process temperature has been achieved and the prescribed operating procedures have been completed. Properly designed operating procedures will ensure that the retort temperature, as indicated by the temperature-indicating device, is uniform in the retort environment.

The processing authority needs to be aware of the many variables that may affect the development of an adequate operating procedure. A few examples of these variables include container loading configuration, percent open area of the baskets and divider plates, vent pipe sizes, or pump speed.

The distribution temperature sensors (typically a continuous Type T thermocouple wire with the copper and constantan wire tips connected to form the temperature sensor) are distributed among the containers to monitor the temperatures throughout the retort load. In addition, at least one sensor is typically located next to the temperature-indicating device to serve as a reference against which all the other sensors can be compared. The temperature-recording equipment used for distribution tests is similar to that used for heat penetration tests.

The temperature distribution data are analyzed by the processing authority to establish the operating procedure, which will provide a reproducible relationship between the temperature-indicating device and the thermocouples located throughout the load of containers. Once established, the operating procedure needs to be followed each time the retort is operated to ensure adequate delivery of the scheduled process.

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