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 papersMicroorganisms and enzymes put to work: starter cultures, the microbiology of fermented foods, industrial fermenter design and operation, downstream recovery, and the modern biotechnology (recombinant enzymes, precision fermentation, single-cell protein) that is reshaping ingredients.
In the exams: GATE XE-G and XL-U Section 2 (fermented foods and beverages); ICAR AICE 35.a Unit 6 (food microbiology and biotechnology)
How our questions for GATE spread across the units of Fermentation Technology & Food Biotechnology. Difficulty: 13 easy, 13 medium, 1 hard.
| Batch, fed-batch and continuous fermentation kinetics | 6 | |
| Fermented dairy foods: curd, yoghurt and cheese microbiology | 5 | |
| Lactic acid bacteria, yeasts and moulds as starter cultures | 4 | |
| Fermented vegetables: sauerkraut, kimchi and pickles | 3 | |
| Vinegar fermentation | 3 | |
| Starter culture production, preservation and bacteriophage control | 2 | |
| Fermented cereal and legume foods: idli, dosa, tempeh, miso and soy sauce | 2 | |
| Fermenter design, aeration and agitation | 1 | |
| Production of food enzymes, organic acids, amino acids and flavours by fermentation | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Which set of properties describes lactic acid bacteria?
B. Gram-positive, catalase-negative, non-spore-forming, acid-tolerant
Lactic acid bacteria are Gram-positive, catalase-negative, non-sporing and aerotolerant, and tolerate the acid they make. Spore-formers (Bacillus, Clostridium) and Gram-negative aerobes (Pseudomonas, acetic acid bacteria) are different groups.
Further reading: Hutkins, Microbiology and Technology of Fermented Foods
Which organism forms koji for soy sauce and miso?
D. Aspergillus oryzae
Koji is A. oryzae (or A. sojae) grown on a solid substrate, supplying amylases and proteases. R. oligosporus makes tempeh, P. roqueforti blue cheese, and S. cerevisiae bread and alcoholic beverages.
Further reading: Hutkins, Microbiology and Technology of Fermented Foods
What is the main advantage of direct vat set cultures over in-plant bulk starter propagation?
D. They avoid the propagation steps where phage entered
Concentrated frozen or freeze-dried cultures go straight into the vat, avoiding the mother-intermediate-bulk chain in which phage and contaminants were picked up, and giving consistent activity. They cost more per tonne, need frozen or dry storage, and still require rotation and hygiene.
Further reading: Hutkins, Microbiology and Technology of Fermented Foods
In yoghurt, what does Lactobacillus delbrueckii subsp. bulgaricus supply to Streptococcus thermophilus?
C. Peptides and amino acids from proteolysis
L. bulgaricus is more proteolytic and releases peptides and amino acids that stimulate S. thermophilus; S. thermophilus in turn supplies formic acid, CO₂ and pyruvate to L. bulgaricus. Diacetyl from citrate is made by some mesophilic cultures; lactose is not part of milk fat.
Further reading: Hutkins, Microbiology and Technology of Fermented Foods
At which peptide bond does chymosin cleave κ-casein?
B. Phe105-Met106
Chymosin specifically hydrolyses the Phe105-Met106 bond of κ-casein, releasing the caseinomacropeptide and destabilising the micelles. Phe23-Phe24 is in αs1-casein and is cleaved by chymosin during ripening, not coagulation; the others are not the coagulation site.
Further reading: Hutkins, Microbiology and Technology of Fermented Foods
Which organism binds the soybeans of tempeh into a firm cake?
D. Rhizopus oligosporus
The mycelium of R. oligosporus grows through and between the cooked beans in one to two days at about 30 °C. A. oryzae makes koji; B. subtilis makes natto; S. cerevisiae is a bread and beverage yeast.
Further reading: Hutkins, Microbiology and Technology of Fermented Foods
In the Monod equation, what is Ks?
C. The substrate concentration at which μ = μmax/2
μ = μmax·S/(Ks + S); when S = Ks, μ = μmax/2. Ks is analogous to Km in enzyme kinetics. μmax is the maximum rate, Y the yield, and feed rate is an operating variable.
Further reading: Stanbury, Whitaker and Hall, Principles of Fermentation Technology; Doran, Bioprocess Engineering Principles
A chemostat has μmax = 0.5 h⁻¹, Ks = 0.2 g/L, feed substrate 10 g/L, yield 0.5 g/g and dilution rate 0.25 h⁻¹. What is the steady-state biomass?
C. 4.9 g/L
S = Ks·D/(μmax − D) = 0.2 × 0.25/0.25 = 0.2 g/L. X = Y(S_R − S) = 0.5 × (10 − 0.2) = 4.9 g/L. 5.0 ignores residual substrate; 2.5 multiplies by D; 9.8 forgets the yield.
Further reading: Stanbury, Whitaker and Hall, Principles of Fermentation Technology; Doran, Bioprocess Engineering Principles
A chemostat is run with dilution rate D. What happens if D exceeds the maximum growth rate attainable at the feed concentration?
D. The cells wash out of the vessel
At steady state μ = D; if D is greater than the highest μ the organism can reach, cells leave faster than they are produced and wash out, while substrate rises to the feed level. Biomass cannot rise without limit in a continuous flow.
Further reading: Stanbury, Whitaker and Hall, Principles of Fermentation Technology; Doran, Bioprocess Engineering Principles
Which organism produces nearly all the world's citric acid?
A. Aspergillus niger
Submerged fermentation by A. niger on molasses or glucose is the standard process. S. cerevisiae makes ethanol, L. lactis lactic acid, C. glutamicum glutamate and lysine.
Further reading: Hutkins, Microbiology and Technology of Fermented Foods
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