Z-value
Japanese: Z値
The temperature increase required to reduce the D-value by 1/10. An indicator expressing microbial heat resistance on the temperature axis.
Z-value is the temperature increase needed to reduce the D-value by a factor of 10. For a microorganism with Z=10°C, each 10°C rise reduces its D-value tenfold.
Microorganisms with small Z-values are temperature-sensitive, dying rapidly with slight heating. Those with large Z-values are temperature-insensitive, requiring high temperatures. Spore-forming bacteria (Bacillus, Clostridium) have large Z-values and cannot be fully killed by ordinary boiling.
D-value represents 'endurance at a given temperature'; Z-value represents 'sensitivity to temperature change'—together, these two parameters completely describe a microorganism's thermal 'character.' Food safety design is an engineering approach based on such values.