A batch sterilizer is being operated at $121^{\circ} \mathrm{C}$ for sterilizing a medium containing microbial cells. Assume that the thermal deactivation of cells is a first order process with a death rate constant of $0.69 \mathrm{~min}^{-1}$ at $121{ }^{\circ} \mathrm{C}$. If the initial concentration of microbes in the medium is $10^{10}$ cells $\mathrm{m}^{-3}$, the time taken to reduce the microbial load to a final concentration of $10$ cells $\mathrm{m}^{-3}$ is $\_\_\_\_$ $\min$. (rounded off to the nearest integer)
