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General Method Overview

The General Method is used to determine lethality equivalents at known internal container temperatures measured directly by the use of thermocouples positioned within the test container. The General Method is a procedure used to integrate the lethal effects of a time/temperature history resulting from a thermal process at a position located in the slowest heating zone in the container. The General Method is the basic direct method for calculating the Fo value. Since the General Method uses the physically measured product temperature directly, it is the most accurate method available.

First, the time-temperature data at the slowest heating zone of a canned food must be obtained through a heat penetration test. If General Method calculation is to be used, it is very important to be aware of the following:

  1. The heat penetration test must be designed properly. For example, the condition of the thermocoupled containers must meet or exceed the "worst case" of fill, formulation, thermocouple placement, etc.
  2. Test containers must be located in the slowest heating zone of the retort (which is determined through Temperature Distribution Testing).
  3. The accuracy of the data-collection equipment is also very important since direct temperature measurement is critical if the General Method is used.

Then for each measured product temperature (thermocoupled container) in the heat penetration test, a corresponding lethal rate value is calculated using the General Method and the appropriate z-value and Tref. During the entire cycle of the retort process, the calculated lethal rate is accumulated for each thermocoupled container. The Fo value delivered to the product at the slowest heating zone is the equivalent sterilization value of that specific process condition.

CALSoft provides reports that use the General Method to determine accumulated lethality during a heat penetration test. CALSoft uses the "trapezoidal method" of the General Method. However, you must use the appropriate z-value and Tref in the calculation according to the product that is tested.