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 papersDesigning and validating heat processes that make food safe and shelf-stable: microbial death kinetics, heat penetration, process calculation by the general and formula methods, retorts and aseptic systems, and the regulatory framework for low-acid canned foods. Agricultural Engineering's food-dairy-engineering introduces D, z and F; this subject teaches the full process-establishment practice.
In the exams: GATE XE-G Section 3 (thermal processing) and Section 4 (thermal sterilisation); ICAR AIEEA-PG Code 18 Unit II (thermobacteriology, sterilisation); ICAR AICE 35.a Unit 2 (process time evaluation for canned products)
GATE — Engineering Sciences (XE), Section G Food Technology
Section 3 lists thermal processing, aseptic packaging and food standards; Section 4 lists thermal sterilisation alongside heat transfer. Expect numerical-answer questions on D from survivor counts, z from two D values, lethal rate and F0 for a constant hold, log reductions and spoilage probability, and short reasoning items on acid versus low-acid foods, cold spot and HTST/UHT quality advantage. Traps are the sign of the lethal-rate exponent, natural versus common logarithms, and minutes versus seconds.
GATE — Life Sciences (XL), Section U Food Technology
Same syllabus as XE-G Sections 3–4. Science graduates meet the same D, z, F and spoilage-probability calculations, usually framed through microbiology: target organisms, spore resistance, and the pH 4.6 boundary.
How our questions for GATE spread across the units of Thermal Processing & Canning Technology. Difficulty: 15 easy, 30 medium, 7 hard.
| Thermal death kinetics: D value, z value and the thermal death time curve | 9 | |
| Lethality, F0 and the 12D concept | 8 | |
| Acid and low-acid foods; pH 4.6 and the target organism | 6 | |
| Heat penetration tests and cold-spot determination | 5 | |
| Commercial sterility and spoilage probability | 5 | |
| Process calculation: the general method | 4 | |
| Aseptic processing and packaging | 4 | |
| Quality optimisation: cook value and nutrient retention | 4 | |
| Process calculation: Ball's formula method | 2 | |
| Retort types: still, rotary, hydrostatic and continuous | 2 | |
| Pasteurisation of acid and acidified foods | 2 | |
| Retort pouches and flexible containers | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
An organism has D₁₁₀ = 3.0 min and z = 10 °C. Its D at 121.1 °C is closest to
D. 0.23 min
D₁₂₁.₁ = D₁₁₀ × 10^((110 − 121.1)/10) = 3.0 × 10^(−1.11) = 3.0 × 0.0776 = 0.23 min. 38.6 min uses the wrong sign, giving a larger D at a higher temperature, which is impossible; 3.33 and 2.70 min multiply or divide by the exponent 1.11 instead of by 10 raised to it.
Further reading: Stumbo, Thermobacteriology in Food Processing
A process must deliver F₀ = 3.0 min, but the product can only be held at a constant 116 °C. The holding time needed (z = 10 °C, reference 121.1 °C, ignore come-up and cooling) is closest to
A. 9.7 min
L at 116 °C = 10^(−0.51) = 0.309; t = F/L = 3.0/0.309 = 9.7 min. 0.93 min multiplies instead of dividing; 15.3 and 5.9 min use the exponent 0.51 directly.
Further reading: Stumbo, Thermobacteriology in Food Processing
In a heat penetration test the retort is at 121.1 °C and the true initial product temperature is 70 °C. Extending the straight heating line back to zero time gives a pseudo-initial temperature of 30 °C. The lag factor jh is closest to
C. 1.78
jh = (TR − T_pih)/(TR − T_ih) = (121.1 − 30)/(121.1 − 70) = 91.1/51.1 = 1.78. 0.56 inverts the ratio; 0.42 divides by the retort temperature; 2.33 compares the two temperatures directly.
Further reading: Holdsworth and Simpson, Thermal Processing of Packaged Foods
Cold-spot readings taken every 2 minutes are 110, 116, 119, 120, 118 and 112 °C. With z = 10 °C, reference 121.1 °C and the trapezoidal rule, F₀ is closest to
A. 4.6 min
Lethal rates: 0.078, 0.309, 0.617, 0.776, 0.490, 0.123. F = Δt × (L₁/2 + L₂ + L₃ + L₄ + L₅ + L₆/2) = 2 × 2.292 = 4.58 min. 2.3 min forgets the 2-minute interval; 4.8 min gives the end points full weight; 9.2 min doubles the interval twice.
Further reading: Stumbo, Thermobacteriology in Food Processing
A can with both ends bulged that can be pushed in by thumb pressure but return when released is described as a
C. soft swell
A soft swell has both ends bulged but yielding to pressure. A flipper looks normal until struck; a springer has one end bulged that can be pushed in, making the other bulge; a hard swell cannot be pushed in at all.
Further reading: Fellows, Food Processing Technology
Swollen cans of an acid fruit contain hydrogen gas and no viable microorganisms. The most likely cause is
B. acid corrosion of the tinplate
Hydrogen swell results from acid attacking the metal; it is chemical, not microbial. C. botulinum does not grow in acid fruit; air entry does not produce hydrogen; flat-sour organisms produce acid without gas. The corroding tinplate releases the hydrogen.
Further reading: Fellows, Food Processing Technology
Compared with a cylindrical can of the same volume, a retort pouch generally needs a shorter process time because
C. its thin profile shortens the heat path
The slab-like shape gives a short path to the centre, so the cold spot heats faster. Laminates do not conduct better than metal; the F₀ target is the same; contents may still heat by conduction. The heat path meant is the distance heat must travel to the cold spot.
Further reading: Holdsworth and Simpson, Thermal Processing of Packaged Foods
In aseptic processing and packaging,
C. food and package are sterilised separately and combined in a sterile zone
Aseptic systems sterilise the product by rapid heating and the package by chemical or thermal means, then fill and seal in sterile conditions. In-container retorting is conventional canning.
Further reading: Fellows, Food Processing Technology
Packaging material for aseptic cartons is commonly sterilised with
D. hydrogen peroxide followed by hot air to remove residues
Hydrogen peroxide with heat is the most widely used sterilant for aseptic carton material and bottles. Sterilisation must happen before filling and sealing, not after.
Further reading: Fellows, Food Processing Technology
The cook value C is usually expressed with
D. reference 100 °C and z = 33 °C
The cook value uses a quality z (commonly 33 °C) at 100 °C. 121.1 °C with z = 10 °C defines F₀; the others are not standard cook-value conventions.
Further reading: Holdsworth and Simpson, Thermal Processing of Packaged Foods
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