Subtopic: Kinetics of microbial death – pasteurization and sterilization of milk and other liquid foods
Question Statement:
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)
Options:
(A) 0.28
(B) 0.91
(C) 2.81
(D) 9.12
Official Answer Key:
D
Step-by-Step Solution & Explanation
Step 1 — convert the D value from 121 °C to 141 °C. The z value is the temperature rise that changes D by a factor of ten:
D_T2 = D_T1 × 10^((T₁ − T₂)/z)
D₁₄₁ = 60 × 10^((121 − 141)/11) = 60 × 10^(−20/11) = 60 × 10^(−1.8182)
= 60 × 0.015199 = 0.912 s
Step 2 — time for a 10 log cycle reduction. By definition, one D value achieves a 1-log (90 %) reduction, so:
t = n × D = 10 × 0.912 = 9.12 s
Trap: option (B) 0.91 is the D value at 141 °C — correct as far as Step 1, but it forgets to multiply by the 10 log cycles demanded.
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