Milk can be pasteurized at 91°C for 3 s or at 73°C for 30 s to give the same sterilization value 10. Find the nearest thermal-death-time temperature constant z, in °C.
At equal lethality, thermal processing times vary as $t\propto10^{-T/z}$. Thus $\log_{10}(t_{73}/t_{91})=(91-73)/z$. The time ratio is $30/3=10$,whose base-ten log is one. Hence $z=18/1=\mathbf{18.0^\circ C}$,option **A**;the equal sterilization value cancels.
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