Subtopic: Kinetics of microbial death – pasteurization and sterilization of milk and other liquid foods
Question Statement:
A mixture of microorganisms, P, Q, R, and S are to be inactivated by autoclaving. Number of spores of the above organisms is 6 × 10⁵, 4 × 10⁶, 2 × 10⁴, and 9 × 10⁵ respectively. Further, their decimal reduction time
Options:
(A) P and Q
(B) R and P
(C) P and S
(D) P and R
Official Answer Key:
B
Step-by-Step Solution & Explanation
Time to reduce each organism's population from its initial spore count N₀ to the target survivor probability N = 1/10000 = 10⁻⁴ follows t = D·log₁₀(N₀/N). Computing: t_P = 2.2×log₁₀(6×10⁵/10⁻⁴) = 2.2×9.778 ≈ 21.5 min; t_Q = 1.8×log₁₀(4×10⁶/10⁻⁴) = 1.8×10.60 ≈ 19.1 min; t_R = 0.8×log₁₀(2×10⁴/10⁻⁴) = 0.8×8.30 ≈ 6.6 min; t_S = 1.6×log₁₀(9×10⁵/10⁻⁴) = 1.6×9.954 ≈ 15.9 min. R needs the shortest time (inactivated first), P needs the longest (inactivated last), matching option (B), R and P.
Appendix A — Consolidated Answer Table
Appendix B — Section-wise Weightage
Preparation Observations
• Soil and Water Conservation Engineering (Section 4) carries the single largest weightage (14 questions, 24 marks) — Fluid Mechanics, Hydrology, and Soil Mechanics sub-topics recur most often and deserve proportionally more study time.
• Engineering Mathematics (Section 1) and Farm Machinery (Section 2) are the next-heaviest technical sections (9 questions / 15–16 marks each) — Vector Calculus, Differential Equations, and farm-machinery performance/cost calculations are consistently tested.
• General Aptitude (10 questions, 15 marks) is scored the same as a mid-sized technical section — verbal reasoning and quick numerical-ability tricks (working backward through multi-step word problems, ratio/percentage traps) are worth dedicated practice since they carry no negative-marking asymmetry advantage over technical questions.
• Numerically heavy sections (Irrigation & Drainage, Agricultural Process Engineering, Dairy & Food Engineering) each appear with fewer but uniformly 2-mark questions — mastering the standard formulas (Thiem equation, LMTD, psychrometric humidity, decimal reduction time, drying-rate mass balance) yields high marks-per-question efficiency.
• Several questions in this paper (Q.26, Q.29, Q.22, Q.55) show a discrepancy between an independently re-derived value and the officially released key; these are flagged explicitly in their explanations above so a student can verify the examiner's intended method rather than silently trusting either source.
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