ASRB NET and ARS: dairy disciplines · India · Agricultural Scientists Recruitment Board (ICAR)
Original practice questions written to the published syllabus · not past papersThe lactic acid bacteria and other organisms used as starters: their metabolism, genetics, production, failure and control.
In the exams: ICAR AICE subject 36 Unit 5; ICAR AIEEA-PG group 16 (classification, genetic aspects and carbohydrate metabolism of LAB; propagation and inhibition of starters)
ASRB NET and ARS — Dairy Microbiology
Recall at Master's depth of organisms, pathways, enzymes, culture types, phage groups and control measures, with simple calculations of inoculum and phage titre.
How our questions for ASRB NET and ARS spread across the units of Starter Cultures & Lactic Acid Bacteria. Difficulty: 19 easy, 11 medium, 2 hard.
| Lactic acid bacteria: genera and current taxonomy | 4 | |
| Homofermentative and heterofermentative metabolism | 3 | |
| Lactose, galactose and citrate metabolism; diacetyl formation | 3 | |
| Mesophilic and thermophilic starters | 3 | |
| Starter propagation, bulk starter systems and direct-vat-set cultures | 3 | |
| Bacteriophage in dairy plants and its control | 3 | |
| Inhibition by antibiotics, sanitisers and bacteriocins | 3 | |
| Proteolytic systems of starters | 2 | |
| Single, multiple and mixed-strain starters; defined and undefined cultures | 2 | |
| Adjunct and secondary cultures: propionibacteria, moulds, smear bacteria | 2 | |
| Preservation of cultures: freezing and freeze-drying | 2 | |
| Genetics and genetic improvement of starters | 2 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Homofermentative LAB ferment glucose by:
B. The Embden-Meyerhof-Parnas pathway
Glycolysis with aldolase gives two lactate per glucose.
Further reading: Fox et al., Fundamentals of Cheese Science
From one molecule of glucose, a heterofermentative LAB produces:
D. Lactate, CO2 and ethanol or acetate
One lactate, one CO2 and one ethanol (or acetate).
Further reading: Fox et al., Fundamentals of Cheese Science
Which organism ferments citrate to produce diacetyl?
C. Lactococcus lactis biovar diacetylactis
Biovar diacetylactis has the citrate permease CitP; Leuconostoc also uses citrate.
Further reading: Cogan and Accolas (eds), Dairy Starter Cultures
Mesophilic starters have an optimum growth temperature of about:
A. 25 to 30 degrees Celsius
Thermophilic starters grow best at about 37-45 C.
Further reading: Cogan and Accolas (eds), Dairy Starter Cultures
Which product is made with a thermophilic starter?
B. Yoghurt
Yoghurt uses S. thermophilus and L. bulgaricus; the others use mesophiles.
Further reading: Fox et al., Fundamentals of Cheese Science
Which organism forms the eyes in Emmental cheese?
B. Propionibacterium freudenreichii
Propionic fermentation of lactate releases CO2.
Further reading: Fox et al., Fundamentals of Cheese Science
Direct-vat-set cultures contain roughly:
C. 10^10 to 10^11 cfu per gram
Highly concentrated frozen or freeze-dried cultures.
Further reading: Cogan and Accolas (eds), Dairy Starter Cultures
At what growth phase are starter cells usually harvested for freezing?
D. Late exponential or early stationary
Cells survive freezing best at this stage.
Further reading: Fox et al., Fundamentals of Cheese Science
45 plaques form from 0.1 millilitre of a 1 in a million dilution of whey. What is the phage titre?
D. 4.5 x 10^8 pfu per millilitre
45 / (0.1 x 10^-6) = 4.5 x 10^8 pfu/mL. 4.5 x 10^7 forgets the 0.1 mL volume.
Further reading: Fox et al., Fundamentals of Cheese Science
Which sanitiser residue is particularly inhibitory and persistent on surfaces?
A. Quaternary ammonium compounds
QACs adsorb to surfaces and inhibit starters.
Further reading: Cogan and Accolas (eds), Dairy Starter Cultures
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