Canning and Fish Packaging Technology

Unit 4 - Thermal process for canned foods

4.8.2.Lethality of heat during heating and cooling
The lethal capacity of the heat process during sterilization process is converted to that at 121.1^oC at which temperature the lethal rate of hypothetical microorganisms(HMO) is unity (1). The lethal rate at any other temperature can be determined by using the TDT graph (Fig.1) of HMO or by using the equation given by Ball.

The above graph shows that relative to lethal rate of unity at 121.1^oC those at 91.1, 101.1, 111.1, 131.1, 141.1 and 151.1 are 0.001, 0.01, 0.1, 10.0, 100 and 1000 respectively.
The lethal value of thermal process is determined by integrating the lethal rates over the entire heating and cooling curve as measured at the centre of the container. The lethal value of one unit is the sterilizing effect achieved by holding the product at 121.1^oC for 1 minute. Mathematically it is expressed as F = LR at 121.1 x 1 =1 min
The lethal values of all time temperature combinations throughout the process can be computed and their sum gives the total lethal value of the process. It follows that a process with an F value =7 is equivalent to sterilizing effect of heating the contents of the can instantly to 121.1^oC maintaining the temperature for 7 min and then instantly cooled. While this cannot be done in practice, the lethal effect of the process has been expressed as though it is possible.