Canning and Fish Packaging Technology

Unit 4 - Thermal process for canned foods

4.8.3.Calculation of Fo Value
To be assured of giving commercial sterility to a product, F_o value at the slowest heating point has to be obtained by heat penetration study as explained in general method. Temperature at the cold point is measured at equal intervals of time throughout the retort come up Period till the retort reaches sterilization temperature), heating and cooling period. From the table or graph or by calculation using the formula for lethal rate, lethal rate is calculated at each temperature during the heat process. The graph is plotted by taking lethal rate of X axis and time on Y axis. The lethal value of 1 unit (X x Y) square area of the plot of the graph is then calculated. Suppose the lethal rate on Y axis for 1 cm =0.0125 and 1 cm on X axis is 10 min then the lethal value of 1 unit of the plot = 0.0125X10=0.125 measure the total number of squares present under the curve, The total number of square under the curve is say 50 sq. cm, then the lethal value of the process is 0.125X50=6.25. The F_o value of 6.25 minutes.

Suppose F value of the process required to destroy a particular organism is 8, the process time is increased on the original heat penetration curve and parallel cooling curve is drawn in such a way that the area under the curve is increased to get F_o value of 8. The F_o value required to achieve complete destruction of Clostridium bolulium is 2.5 and the heat process required is known on “botulinum cook” which is mandatory for all low acid canned foods. Many a time, this heat process is not sufficient to prevent spoilage and hence heat process with F_o value between 6-8 may be required.