Appendix K: F_o Value

The F_o value is a concept associated with the thermal destruction of microorganisms relative to food preservation and canning.

The F value is used to designate the time necessary to destroy a given number of microorganisms at a reference temperature, usually 121°C for spores or 60°C for vegetative cells. To avoid confusion the temperature can be added as a subscript to F. It represents the total time-temperature combination received by a food.

The integrated lethal value of heat received by all points in a container during processing is designated F_o. F_o is used to describe processes that operate at 121°C, which are based on a micro-organism with a z value (degrees required for the thermal death destruction curve to traverse one log cycle) of 10°C. It represents a measure of the capacity of a heat process to reduce the number of spores or vegetative cells of a given organism per container. When there is instant heating and cooling throughout the container of spores, vegetative cells or foods, F_o is derived as follows:

F_o = D_r (log a - log b)

where a = number of cells in the initial population, and b = number of cells in the final population

D_r = when D is determined at 250°F

D = decimal reduction time, or the time required to destroy the 90% of the organisms. This value is numerically equal to the number of minutes required for the survivor curve to traverse one log cycle. It is the measure of the death rate of an organism.

For example if F₂₅₀ = 3.0, this is the time in minutes at 250°F (121° C) required to achieve sterility of a suspension containing a known number of cells or spores (commercial sterility).

References

Basic Food Microbiology, second edition, George J. Banwart, 1989, Published by Chapman & Hall Modern Food Microbiology, sixth edition, James M. Jay, 2000, Published by Aspen Publishers, Inc.