GATE AG 2012 • Q29 NAT • 1 Mark

Section 7: Dairy and Food Engineering

Preservation of Food

Subtopic: Kinetics of microbial death – pasteurization and sterilization of milk and other liquid foods

Question Statement:

Decimal reduction times for Bacillus subtilis are 37 s and 12 s at 120°C and 125°C respectively. The temperature rise, in °C, necessary to reduce the D-value at 120°C by a factor of 10, is:

Official Answer Key:

10 to 11

Step-by-Step Solution & Explanation

Thermal death-time z-value: z = ΔT/log(D1/D2) = (125-120)/log(37/12) = 5/log(3.0833) = 5/0.4889 = 10.23°C. Since a one-log (factor of 10) reduction in D corresponds exactly to a temperature rise of one z-value, the required temperature rise = z = 10.23°C, within the accepted 10–11 range. Answer: 10–11 °C.

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