GATE — Engineering Sciences (XE), Section G Food Technology · India · IITs and IISc on rotation (2026: IIT Guwahati)
Original practice questions written to the published syllabus · not past papersLow-temperature preservation from the food's side: chilling and controlled atmospheres, freezing-point depression and ice crystallisation, freezing-time calculation, freezer types, frozen storage changes and thawing. The refrigeration cycle and cold-store design are in Agricultural Engineering's refrigeration-cold-chain.
In the exams: GATE XE-G Section 3 (chilling, freezing) and Section 4 (freezing and crystallisation); ICAR AICE 35.a Unit 2 (IQF products)
GATE — Engineering Sciences (XE), Section G Food Technology
Section 3 names chilling and freezing as processing principles and controlled atmosphere storage; Section 4 names freezing and crystallisation among thermal operations, alongside heat transfer. Expect numericals on freezing-point depression, heat removed in freezing (Siebel-type estimates), Plank's freezing time for slabs, cylinders and spheres, and half-cooling times, plus reasoning on slow versus fast freezing, drip and recrystallisation.
GATE — Life Sciences (XL), Section U Food Technology
Same syllabus as XE-G. Questions often connect low temperature to microbiology (psychrotrophs, Listeria, non-proteolytic C. botulinum) and to chemistry (unfrozen water, oxidation in frozen storage, blanching enzymes).
How our questions for GATE spread across the units of Chilling, Freezing & Frozen Food Technology. Difficulty: 15 easy, 32 medium, 12 hard.
| Chilling and chilled storage of foods | 8 | |
| Freezing-point depression and unfrozen water | 8 | |
| Freezing-time prediction: Plank's equation and its refinements | 7 | |
| Ice nucleation, crystal growth and recrystallisation | 6 | |
| Air-blast, plate, cryogenic and IQF freezers | 6 | |
| Freezing of fruits, vegetables, meat, fish and ready meals | 6 | |
| Cold-chain requirements for chilled foods | 4 | |
| Changes during frozen storage: freezer burn, drip loss and oxidation | 4 | |
| Time-temperature tolerance and shelf life of frozen foods | 4 | |
| Thawing and tempering | 4 | |
| Supercooling, isochoric and other emerging freezing methods | 2 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Non-proteolytic Clostridium botulinum is a particular hazard in long-life, reduced-oxygen chilled foods because it
D. can grow and form toxin down to about 3 °C
Non-proteolytic strains grow at about 3 °C and above, often without obvious spoilage signs, so sous-vide and vacuum-packed chilled foods need limits on shelf life, temperature or other hurdles.
Further reading: Fellows, Food Processing Technology
Bananas stored at 5 °C develop dark skin and fail to ripen. This is
A. chilling injury
Tropical fruits suffer chilling injury below a critical temperature, about 10–13 °C for bananas, though no ice forms. Freezer burn and cold shortening concern frozen foods and meat.
Further reading: Fellows, Food Processing Technology
For the same food with initial freezing point −1.5 °C, the fraction of freezable water frozen at −3 °C is about
D. 50 %
1 − 1.5/3 = 0.50, so half the freezable water is ice just 1.5 K below the initial freezing point. 92 % is the value at −18 °C; 0 % would mean no freezing below Tf.
Further reading: Singh and Heldman, Introduction to Food Engineering
Large frozen items such as whole tuna can crack when frozen very rapidly in liquid nitrogen because
C. water expands on freezing and the rigid outer shell cannot accommodate the core
The outer layer freezes first and becomes rigid; when the interior freezes and expands, stress cracks the shell.
Further reading: Sun (ed.), Handbook of Frozen Food Processing and Packaging
Plank's equation, infinite slab (P = 1/2, R = 1/8): fish 5 cm thick, ρ = 1,000 kg/m³, latent heat 250 kJ/kg, Tf = −1.5 °C, air at −30 °C, h = 20 W/m² K, frozen k = 1.5 W/m K. The freezing time is closest to
B. 3.6 h
t = (1,000 × 250,000/28.5)(0.5 × 0.05/20 + 0.125 × 0.05²/1.5) = 8.77 × 10⁶ × 1.458 × 10⁻³ = 12,792 s = 3.6 h. 1.6 h uses the half-thickness; 1.2 h uses sphere factors; 4.6 h uses the unfrozen conductivity 0.5 W/m K.
Further reading: Singh and Heldman, Introduction to Food Engineering
The same 5 cm fish slab (ρ = 1,000 kg/m³, λ = 250 kJ/kg, Tf = −1.5 °C, k = 1.5 W/m K) is frozen in a plate freezer with medium at −30 °C and h = 200 W/m² K. By Plank's equation the freezing time is closest to
C. 49 min
t = 8.77 × 10⁶ × (1.25 × 10⁻⁴ + 2.08 × 10⁻⁴) = 2,924 s ≈ 49 min. 487 min is ten times too long; 18 min keeps only the surface term and ignores internal conduction; 0.8 min confuses hours with minutes.
Further reading: Singh and Heldman, Introduction to Food Engineering
In Plank's equation, the shape factors for an infinite slab are
D. P = 1/2 and R = 1/8
Slab 1/2 and 1/8; cylinder 1/4 and 1/16; sphere 1/6 and 1/24.
Further reading: Singh and Heldman, Introduction to Food Engineering
For a thin product frozen in still air (very low h), doubling its thickness changes the freezing time by a factor of about
B. 2
With small h the external term Pa/h dominates, so time is roughly proportional to thickness: doubling a doubles t.
Further reading: Singh and Heldman, Introduction to Food Engineering
A frozen vegetable has practical storage lives of 360 days at −24 °C, 270 days at −18 °C and 90 days at −12 °C. It spends 100 days at −24 °C, 40 days at −18 °C and 20 days at −12 °C. Its remaining life at −18 °C is about
B. 95 days
Fraction used = 100/360 + 40/270 + 20/90 = 0.278 + 0.148 + 0.222 = 0.648; remaining 0.352 × 270 = 95 days. 110 days subtracts total days and ignores the different rates; 155 days leaves out the retail period at −12 °C; 195 days counts only the −24 °C period.
Further reading: Kennedy (ed.), Managing Frozen Foods
High quality life (HQL) of a frozen food is the storage time until
A. a trained panel first detects a significant difference from a reference sample
HQL is a 'just noticeable difference' (often 70 % correct in triangle tests); PSL is the longer limit of acceptability.
Further reading: Kennedy (ed.), Managing Frozen Foods
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