A suspension contains $3.6 \times 10^{5}$ spores of $C$. botulinum having a D-value of 1.5 minute at $121.1^{\circ} \mathrm{C}$ and $8.5 \times 10^{6}$ spores of $B$. subtilis having a D-value of 0.9 minute at the same temperature. The suspension is heated at a constant temperature of $121.1{ }^{\circ} \mathrm{C}$. The heating time needed in minutes for the suspension to obtain a survival probability of $10^{-3}$ for the most heat resistant organism in it is___________
A suspension contains $3.6 \times 10^{5}$ spores of $C$. botulinum having a D-value of 1.5 minute at $121.1^{\circ} \mathrm{C}$ and $8.5 \times 10^{6}$ spores of $B$. subtilis having a D-value of 0.9 minute at the same temperature. The suspension is heated at a constant temperature of $121.1{ }^{\circ} \mathrm{C}$. The heating time needed in minutes for the suspension to obtain a survival probability of $10^{-3}$ for the most heat resistant organism in it is___________