Work out d value and z value instantly with clear inputs, formula shown and shareable results.
The z value is the temperature change that alters the D value tenfold: z = (T2 - T1) / log10(D1/D2). A z of 10 C means each 10 degree rise cuts D by a factor of ten. Vegetative bacteria typically show z of 4-8 C and bacterial spores 8-12 C, which is why spore destruction needs retort temperatures.
z value
z = (T2 - T1) / log10(D1 / D2)
Rate factor
Rate increase per 10 C = 10^(10 / z)
D and z values must be determined for the specific organism and food matrix. Using literature values without confirmation can lead to an unsafe process.
D fixes how long a given temperature takes to achieve a tenfold reduction; z fixes how much faster a higher temperature works. Together they allow any time-temperature combination to be compared for equivalent lethality.
Only with care. The log-linear relationship generally holds over the range tested but can break down at extremes, and shoulder or tailing behaviour invalidates simple first-order treatment.