Thermal Processing and Kinetics

For *Clostridium botulinum* spores, $D_{121.1} = 0.21\text{ min}$ and $D_{111.1} = 2.1\text{ min}$. The temperature difference is $121.1 - 111.1 = 10.0^\circ\text{C}$, and $\log_{10}(2.1 / 0.21) = \log_{10}(10) = 1.0$. The $z$-value in $^\circ\text{C}$ is: