Canning and Fish Packaging Technology

Unit 4 - Thermal process for canned foods

4.8.General method of process calculation
In general method of process calculation, it is suggested that if the time required to render the food at the slowest heating region is safe, other regions of the can as well as the whole container is rendered safe. In the general method, the temperature at the cold spot is read at equal intervals of time starting from the time steam is allowed in to the retort (steam on) till can cooling. The TDT of most significant organism in that particular pack has to be determined at various temperatures throughout the heating and cooling period. The lethal rate which equal 1/TDT is calculated and these values are plotted against corresponding times to get what is known as lethal rate curve. Total lethality of the process is computed by area under the lethal rate curve.

Lethality of 1 means complete destruction of all spores in the can.
Lethality of 1 square in the graph sheet = 0.0025X5 = 0.0125
Suppose total area under the curve is 71.35 square cm, the total lethality = 0.0125 X 71.35 = 0.089

Total lethality of the process of 0.089 is much less than what is required for complete destruction of spores. Hence there is a necessity to increase the retorting time.