GATE — Life Sciences (XL), Section U Food Technology · India · IITs and IISc on rotation (2026: IIT Guwahati)
Original practice questions written to the published syllabus · not past papersMass and energy balances and the three transfer processes — momentum, heat and mass — applied to foods: pumping viscous products, heating cans and pouches, and moisture and gas movement through foods and packaging films.
In the exams: GATE XE-G Section 4 (mass and energy balance, momentum transfer, heat transfer, mass transfer); ICAR AIEEA-PG Code 18 Unit II
GATE — Engineering Sciences (XE), Section G Food Technology
Section 4 names mass and energy balance; momentum transfer (flow rate and pressure drop relationships for Newtonian fluids in pipes, Reynolds number); heat transfer by conduction, convection and radiation and heat exchangers; and mass transfer (Fick's law, convective mass transfer, permeability through single and multilayer films). Expect numerical-answer questions on Reynolds number, Hagen–Poiseuille and Fanning pressure drop, composite-wall conduction, LMTD, Dittus–Boelter, Biot and lumped cooling, and film permeation; units and the Fanning/Darcy distinction are the common traps.
GATE — Life Sciences (XL), Section U Food Technology
Same Section 4 syllabus as XE-G; life-science candidates should practise the core balances, pipe-flow and heat-exchanger calculations, which are less familiar to them than the chemistry and microbiology sections.
How our questions for GATE spread across the units of Transport Phenomena in Food Processing: Fluid Flow, Heat & Mass Transfer. Difficulty: 25 easy, 38 medium, 25 hard.
| Flow in pipes: Reynolds number, friction factor and pressure drop for Newtonian fluids | 8 | |
| Heat exchangers: plate, tubular and scraped-surface; LMTD and effectiveness | 8 | |
| Units, dimensions and dimensional analysis | 7 | |
| Unsteady-state heat transfer: lumped systems and Heisler charts | 7 | |
| Material balances on mixing, concentration and drying | 6 | |
| Energy balances and steam tables | 6 | |
| Pumps for foods: centrifugal and positive-displacement | 6 | |
| Steady-state conduction through slabs, cylinders and composite walls | 6 | |
| Convection: film coefficients and dimensionless numbers | 6 | |
| Fluid statics and the Bernoulli equation | 5 | |
| Radiation heat transfer | 5 | |
| Convective mass transfer | 5 | |
| Permeability through single and multilayer films | 5 | |
| Flow of non-Newtonian foods in pipes | 4 | |
| Molecular diffusion and Fick's law | 4 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
5,000 kg/h of milk at 12 % total solids is concentrated to 45 % total solids. The water evaporated is closest to
D. 3,667 kg/h
W = F(1 − xF/xP) = 5,000 × (1 − 12/45) = 3,667 kg/h. 1,333 kg/h is the concentrate; 1,650 kg/h multiplies by 0.33 (45 − 12 as a fraction); 3,350 kg/h subtracts percentages (5,000 × 0.67).
Further reading: Singh and Heldman, Introduction to Food Engineering
Steam is the preferred heating medium in food plants mainly because it
B. releases a large latent heat at a constant, pressure-controlled temperature
Condensing steam delivers about 2,200 kJ/kg at a temperature fixed by its pressure, with very high film coefficients; condensate must be removed by traps.
Further reading: Singh and Heldman, Introduction to Food Engineering
The Bernoulli equation for an ideal fluid states that along a streamline the sum of
B. pressure, kinetic and potential energy per unit mass is constant
P/ρ + v²/2 + gz = constant for frictionless, incompressible, steady flow; real flows add pump work and friction terms.
Further reading: Geankoplis, Transport Processes and Separation Process Principles
Milk (ρ = 1,030 kg/m³, μ = 2.0 mPa s) flows at 1.5 m/s in a 38 mm tube. The Reynolds number is closest to
A. 29,355
Re = ρvD/μ = 1,030 × 1.5 × 0.038/0.002 = 29,355, turbulent. 29 and 29,355,000 come from unit slips in μ (Pa s vs mPa s); 2,936 uses D = 3.8 mm.
Further reading: Singh and Heldman, Introduction to Food Engineering
In a heat exchanger with a thin stainless-steel wall, the wall usually contributes little to the overall resistance because
A. metal conductivity is high compared with the fluid films
A 1–2 mm steel wall has resistance far below the convective films and fouling layers.
Further reading: Incropera and DeWitt, Fundamentals of Heat and Mass Transfer
Milk with Re = 29,000 and Pr = 13 is heated in a 38 mm tube (k = 0.55 W/m K). Using Dittus–Boelter for heating, Nu = 0.023 Re^0.8 Pr^0.4, the film coefficient is closest to
B. 3,450 W/m² K
Nu = 0.023 × 29,000^0.8 × 13^0.4 = 238; h = Nu k/D = 238 × 0.55/0.038 = 3,450 W/m² K. The 0.3 exponent is for cooling; 238 is Nu itself; 16.5 inverts k/D.
Further reading: Incropera and DeWitt, Fundamentals of Heat and Mass Transfer
Natural (free) convection is driven by
C. density differences caused by temperature differences
Warm fluid is less dense and rises; forced convection uses pumps or fans. The Grashof number characterises it.
Further reading: Incropera and DeWitt, Fundamentals of Heat and Mass Transfer
A food slab (k = 0.5 W/m K, ρ = 1,000 kg/m³, cp = 3,600 J/kg K) of half-thickness 20 mm is heated for 30 min. The Fourier number is about
C. 0.62
α = k/ρcp = 1.39e-07 m²/s; Fo = αt/L² = 1.39e-07 × 1,800/0.02² = 0.62. 0.156 uses the full thickness 40 mm; 0.01042 uses 30 s; 2.5 uses 10 mm.
Further reading: Incropera and DeWitt, Fundamentals of Heat and Mass Transfer
A counter-current heat exchanger has equal heat capacity rates (Cr = 1) and NTU = 2.0. Its effectiveness is
B. 0.667
For Cr = 1, ε = NTU/(1 + NTU) = 2/3 = 0.667. 0.865 is 1 − e^−2, valid only when Cr = 0; 0.500 is the co-current limit; 1.000 would need infinite NTU.
Further reading: Incropera and DeWitt, Fundamentals of Heat and Mass Transfer
Water evaporates from a 0.5 m² wet surface with km = 0.02 m/s; vapour concentrations are 0.017 kg/m³ at the surface and 0.008 kg/m³ in the air. The evaporation rate is
D. 0.09 g/s
N = kmA(Cs − C∞) = 0.02 × 0.5 × 0.009 = 9.00e-05 kg/s = 0.09 g/s. 0.17 g/s uses Cs only; 0.08 g/s uses C∞ only; 9.0 g/s is a unit slip.
Further reading: Geankoplis, Transport Processes and Separation Process Principles
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