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 papersThe mechanical operations that clean, sort, reduce, mix, homogenise and separate food materials, including filtration, centrifugation and membrane processes, with the design equations used to size them.
In the exams: GATE XE-G Section 4 (mechanical operations); ICAR AIEEA-PG Code 18 Unit II (cleaning, sorting, size reduction, mixing, separation, filtration, membranes)
GATE — Engineering Sciences (XE), Section G Food Technology
Section 4 names size reduction of solids, high-pressure homogenisation, filtration, centrifugation, settling, sieving, and mixing and agitation of liquids. Expect numerical-answer questions on Rittinger, Kick and Bond energies, Stokes settling and centrifugal settling velocities, g-number, constant-pressure filtration time, membrane flux, rejection and concentration factor, screen effectiveness and agitator power from the power number, with equipment-selection reasoning.
GATE — Life Sciences (XL), Section U Food Technology
Same Section 4 syllabus as XE-G; science graduates should practise the size-reduction, Stokes and filtration calculations and learn the equipment principles.
How our questions for GATE spread across the units of Mechanical Unit Operations: Size Reduction, Mixing & Separation. Difficulty: 17 easy, 47 medium, 19 hard.
| Size reduction: energy laws of Rittinger, Kick and Bond | 8 | |
| Membrane processes: microfiltration, ultrafiltration, nanofiltration and reverse osmosis | 8 | |
| Filtration: cake filtration theory and filter aids | 7 | |
| Sieving and particle-size analysis | 6 | |
| Mixing of solids, pastes and liquids; agitation and power number | 6 | |
| Centrifugation and centrifugal separators | 6 | |
| Expression and solid-liquid extraction | 6 | |
| Cleaning, sorting and grading of raw materials | 5 | |
| High-pressure homogenisation | 5 | |
| Settling and sedimentation | 5 | |
| Crystallisation | 5 | |
| Distillation and supercritical fluid extraction | 5 | |
| Peeling methods | 4 | |
| Size-reduction equipment for dry and wet foods | 4 | |
| Material handling: conveyors and elevators for foods | 3 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Aspiration (winnowing) separates chaff and dust from grain mainly by differences in
A. terminal velocity in an air stream
Light chaff has a lower terminal velocity and is carried away by air while grain falls. Colour sorters and magnets use other properties.
Further reading: Fellows, Food Processing Technology
Sorting differs from grading in that sorting
D. separates by one measurable property
Sorting uses a single property such as size, weight or colour; grading assesses several quality attributes together against standards.
Further reading: Fellows, Food Processing Technology
Wheat flour milling relies mainly on
B. roller mills combining compression and shear
Pairs of fluted break rolls and smooth reduction rolls, running at different speeds, separate endosperm from bran gradually.
Further reading: Fellows, Food Processing Technology
In the turbulent regime in a baffled tank, doubling impeller speed increases power by a factor of
D. 8
P ∝ N³ when Np is constant; 32 is the factor for doubling diameter (D⁵).
Further reading: McCabe, Smith and Harriott, Unit Operations of Chemical Engineering
By Stokes' law, the settling velocity of a small particle is proportional to
B. the square of its diameter
v = d²Δρg/18μ, so velocity varies with the square of diameter; doubling d gives four times the settling velocity.
Further reading: Geankoplis, Transport Processes and Separation Process Principles
The capacity of a gravity settling tank to remove particles of a given size depends mainly on its
D. surface area
A particle is removed if its settling velocity exceeds flow/surface area (overflow rate), so area, not depth, governs capacity.
Further reading: Geankoplis, Transport Processes and Separation Process Principles
In constant-pressure filtration with negligible medium resistance, collecting 1 m³ of filtrate takes 10 min. Collecting 2 m³ takes
A. 40 min
t ∝ V², so t = 10 × 2² = 40 min. 20 min assumes t ∝ V; 14 min uses √2; 80 min uses V³.
Further reading: Geankoplis, Transport Processes and Separation Process Principles
Concentration polarisation in membrane filtration is reduced mainly by
C. high cross-flow velocity over the membrane
Cross-flow sweeps the boundary layer of retained solutes from the surface; dead-end operation builds a cake.
Further reading: Cheryan, Ultrafiltration and Microfiltration Handbook
Reverse osmosis cannot concentrate fruit juice much beyond about 25–30 % solids mainly because
D. osmotic pressure rises with concentration and opposes the applied pressure
π = icRT grows with solute concentration, so very high pressures would be needed; RO is used to pre-concentrate.
Further reading: Cheryan, Ultrafiltration and Microfiltration Handbook
High supersaturation during crystallisation tends to produce
D. many small crystals
High supersaturation favours nucleation over growth.
Further reading: McCabe, Smith and Harriott, Unit Operations of Chemical Engineering
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