GATE — Agricultural Engineering (AG) · India · IITs and IISc on rotation (2026: IIT Guwahati)
Original practice questions written to the published syllabus · not past papersUnit operations of food and milk plants: balances, fluid flow, heat exchange, thermal processing, evaporation, separation, freezing and hygienic design.
In the exams: GATE AG Section 7 (~9 marks a year 2019–26; thermal processing in all 8 papers); ICAR AICE subject 38; ICAR AIEEA-PG Unit VI; NCEES PE processing
GATE — Agricultural Engineering (AG)
Section 7 (Dairy and Food Engineering) averages about 9 technical marks, mostly numerical. Thermal processing (D, z, F, log reductions, spoilage probability) has appeared in every paper counted from 2019 to 2026, and heat-exchanger LMTD or effectiveness in nearly every paper. Evaporator mass balance and steam economy, rheology (power-law), centrifuge and cream separator balances and Plank's freezing time recur, the last two partly under Section 6. Expect numerical-answer questions where the traps are units (per hour against per second), wet against dry basis and the sign of the lethal-rate exponent.
How our questions for GATE spread across the units of Food & Dairy Engineering. Difficulty: 2 easy, 31 medium, 11 hard.
| Thermal processing kinetics: D, z and F values | 8 | |
| Freezing and Plank's equation | 5 | |
| Material and energy balances | 4 | |
| Fluid flow in food plants: pumps, pipes and rheology | 4 | |
| Heat exchangers: plate, tubular and scraped-surface | 4 | |
| Evaporation: single and multiple effect | 4 | |
| Spray and drum drying | 3 | |
| Centrifugation and cream separation | 3 | |
| Filtration and membrane processes: MF, UF, NF and RO | 3 | |
| Pasteurisation: HTST and LTLT | 2 | |
| UHT processing and sterilisation | 2 | |
| Homogenisation | 2 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
In a milk-to-milk regenerator the two heat-capacity rates are equal. If NTU = 4 in counter-current flow, the effectiveness is
D. 0.80
For counter-current flow with C_r = 1, ε = NTU/(1 + NTU) = 4/5 = 0.80. 0.98 is 1 − e^(−4), valid only when C_r = 0 (condensing or boiling on one side). 0.50 is the co-current value, (1 − e^(−8))/2; 0.20 is 1/(1 + NTU), the ineffectiveness.
Further reading: Singh and Heldman, Introduction to Food Engineering
D is 5 min at 110 °C and 0.5 min at 120 °C. The z value is
D. 10 °C
z = (T₂ − T₁)/log(D₁/D₂) = 10/log(10) = 10 °C. 4.3 °C uses the natural log (10/2.303); 1 °C divides by the ratio 10 instead of its log; 2.2 °C divides by the difference in D (10/4.5).
Further reading: Singh and Heldman, Introduction to Food Engineering
Cans contain 1000 spores each of a spoilage organism with D₁₂₁.₁ = 1.0 min. After a process of F₀ = 6 min, the expected number of spoiled cans in a batch of 50,000 is
D. 50
N = N₀ × 10^(−F/D) = 10³ × 10⁻⁶ = 10⁻³ survivors per can, i.e. one spoiled can per thousand; 50,000 × 10⁻³ = 50. 0.05 forgets the initial 1000 spores (10^−6 per can x 50,000); 500 and 5 miscount the log reductions by two either way (10^−2 and 10^−4 per can).
Further reading: Singh and Heldman, Introduction to Food Engineering
A first-order death rate constant k = 0.0921 s⁻¹ corresponds to a D value of
A. 25 s
D = 2.303/k = 2.303/0.0921 = 25.0 s. 1/k = 10.9 s is the time for a 1/e reduction, not a decimal reduction. 2.5 s uses 0.2303 instead of 2.303.
Further reading: Singh and Heldman, Introduction to Food Engineering
Milk at 75 °C is heated to 140 °C by direct steam injection. If steam gives up 2200 kJ/kg and milk cp = 3.9 kJ/kg K, the steam condensed per kg of milk is about
B. 0.115 kg
m_s = cp ΔT/λ = 3.9 × 65/2200 = 0.115 kg/kg, about 10 % dilution, removed later by flash cooling. Using a 140 K rise (from 0 °C) instead of 65 K gives 0.248 kg. 1.15 kg is a factor-of-ten slip; 0.124 kg uses water's cp of 4.18 instead of milk's 3.9.
Further reading: Singh and Heldman, Introduction to Food Engineering
By Stokes' law, reducing the mean fat globule diameter from 3 µm to 0.75 µm reduces the creaming velocity by a factor of
D. 16
v ∝ d²; (3/0.75)² = 4² = 16. Proportionality to d gives 4; to d³ gives 64 and to d⁴ gives 256.
Further reading: Singh and Heldman, Introduction to Food Engineering
A fat globule rises at 0.64 µm/s under gravity. In a bowl giving 8000 g, the Stokes-law velocity is about
A. 5.1 mm/s
v scales with the acceleration: 0.64 × 10⁻⁶ × 8000 = 5.1 × 10⁻³ m/s = 5.1 mm/s. 0.64 µm/s ignores the centrifugal field; 0.51 mm/s and 51 mm/s are factor-of-ten slips.
Further reading: Singh and Heldman, Introduction to Food Engineering
The van 't Hoff osmotic pressure of a 0.1 mol/L non-electrolyte solution at 25 °C is about
D. 248 kPa
π = C R T = 100 mol/m³ × 8.314 × 298.15 = 247,900 Pa = 248 kPa. Using 0.1 mol/L without converting to mol/m³ gives 248 Pa (0.248 kPa), so 2.48 and 24.8 kPa are decimal slips; 496 kPa assumes i = 2, which applies to a salt such as NaCl.
Further reading: Singh and Heldman, Introduction to Food Engineering
Using Plank's equation, a 5 cm thick slab (ρ = 1000 kg/m³, λ = 266.6 kJ/kg, T_f = −1.5 °C, frozen k = 1.5 W/m K) frozen in air at −30 °C takes 2.7 h with h = 30 W/m² K. Doubling the surface heat transfer coefficient from 30 to 60 W/m² K (all else equal) gives a freezing time of about
D. 1.6 h
Surface term halves: 0.5 × 0.05/60 = 4.17e-4; internal term unchanged at 2.08e-4; sum = 6.25e-4; t = 9,354,000 × 6.25e-4 = 5850 s = 1.6 h. It does not halve (1.35 h) because the internal conduction term is unchanged. 2.7 h assumes h has no effect; 0.54 h keeps only the internal conduction term.
Further reading: Singh and Heldman, Introduction to Food Engineering
Using Plank's equation, a 5 cm thick slab (ρ = 1000 kg/m³, λ = 266.6 kJ/kg, T_f = −1.5 °C, frozen k = 1.5 W/m K) frozen in air at −30 °C with h = 30 W/m² K takes 2.7 h. Doubling the thickness to 10 cm, all else unchanged, gives a freezing time of about
B. 6.5 h
Pa/h doubles (1.67e-3) and Ra²/k quadruples (8.33e-4); sum 2.5e-3; t = 9,354,000 × 2.5e-3 = 23,390 s = 6.5 h. It lies between doubling (5.4 h) and quadrupling (10.8 h). 4.9 h doubles only the surface term and leaves the internal term unchanged.
Further reading: Singh and Heldman, Introduction to Food Engineering
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