Discussion
Pasteurisation of milk can be carried out either at 91 °C for 3 s or at 73 °C for 30 s to inactivate vegetative cells of microorganism(s). The sterilisation value is 10 in both the cases. Thermal death time constant (z value) for reference temperatures of 73 °C and 91 °C, in °C, is nearest to
Answer: A
The z value is 18.0 °C.
The two treatments are equally lethal, so their times lie on the same thermal death time curve. The z value is the temperature rise that cuts the required time tenfold:
\[ \begin{aligned} z &= \frac{T_{2} - T_{1}}{\log_{10}(t_{1}/t_{2})} \\ &= \frac{91 - 73}{\log_{10}(30/3)} = \frac{18}{1} = 18 \end{aligned} \]
Here the times differ by exactly a factor of 10, so z is simply the 18 °C difference between the two temperatures. The stated sterilisation value of 10 is the same for both processes and does not change z. 1.8 and 0.9 shift the decimal point, and 9.0 halves the temperature difference.
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