Discussion
The decimal reduction time D121 and the z value of thermophilic spores in milk were found to be 60 seconds and 11 °C, respectively. Heating time required at 141 °C for a 10 log cycle reduction in population, in seconds, is __________. (Rounded off to 2 decimal places)
Answer: D
The heating time is 9.12 s.
The z value is the temperature rise that cuts the D value tenfold. From 121 °C to 141 °C is 20 °C, or 20/11 of a log cycle:
\[ \begin{aligned} D_{141} &= D_{121} \times 10^{-(141-121)/11} \\ &= 60 \times 10^{-1.818} = 0.912\ \text{s} \end{aligned} \]
A 10 log cycle reduction needs 10 D values, so the time is \(10 \times 0.912 = 9.12\) s.
0.91 s is a single D value, one log cycle only. 2.81 and 0.28 s do not follow from the given D and z values.
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