Preservation of Food: Kinetics of microbial death – pasteurization and sterilization of milk and other liquid foods; preservation of food by cooling and freezing; refrigeration and cold storage basics and applications.
A batch of 10000 L milk is to be sterilized and packed in 20000 packets of 500 ml each. The mean Standard Plate Count (SPC) of Bacillus subtilis in 100 samples of fresh milk was found to be 50 ml⁻¹. The milk is to be sterilized such that each 500 ml packet is completely devoid of the organism. Minimum number of log cycle reduction for sterilization of this batch is
Options:
(A) 8
(B) 9
(C) 10
(D) 12
Official Answer Key:
B
Step-by-Step Solution & Explanation
Total organisms in the whole batch before sterilization:
N0 = SPC × total volume = 50 organisms/ml × (10,000 L × 1000 ml/L) = 50 × 10,000,000 = 5×10⁸ organisms.
To ensure that essentially none of the 20,000 packets contains even a single surviving organism, the expected number of survivors across the ENTIRE batch after sterilization must be reduced to below 1:
N0 × 10⁻ᴰ < 1 ⟹ D > log₁₀(N0) = log₁₀(5×10⁸) = 8.699.
Since D (the number of decimal/log-cycle reductions) must be a whole number and must exceed 8.699, the minimum required is D = 9. Answer: (B) 9.
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