ASRB NET and ARS: dairy disciplines · India · Agricultural Scientists Recruitment Board (ICAR)
Original practice questions written to the published syllabus · not past papersWhere milk bacteria come from, how they grow and spoil milk, and how processing changes the count.
In the exams: ICAR AICE subject 36 Unit 3; ICAR AIEEA-PG group 16 (classification by growth temperature, spoilage, grading of raw milk, bacteriological aspects of processing)
ASRB NET and ARS — Dairy Microbiology
Recall of organisms, groups and tests at Master's depth, with calculations of plate counts, generation times and heat kinetics.
How our questions for ASRB NET and ARS spread across the units of Microbiology of Raw & Market Milk. Difficulty: 18 easy, 8 medium, 3 hard.
| Bacteriology of thermisation, pasteurisation, boiling, sterilisation and UHT | 4 | |
| Sources of contamination: udder, animal, milker, equipment, water, air | 3 | |
| Psychrotrophs, mesophiles, thermodurics and thermophiles | 3 | |
| Microbiological grading of raw milk: SPC, MBRT and resazurin tests | 3 | |
| Psychrotroph enzymes and their survival of heat treatment | 2 | |
| Microbial spoilage: souring, curdling, gassiness, ropiness, proteolysis, lipolysis | 2 | |
| Natural antimicrobial systems in raw milk; the lactoperoxidase system | 2 | |
| Microflora of mastitic milk | 2 | |
| Effect of chilling and storage on raw milk flora | 2 | |
| Bactofugation and microfiltration for bacteria removal | 2 | |
| Biofilms in milking and processing equipment | 2 | |
| Post-pasteurisation contamination | 2 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Psychrotrophs are defined as bacteria that:
A. Grow at 7 degrees Celsius or below, whatever their optimum
The definition is ability to grow at 7 C or below; psychrophiles have optima at 15 C or below; thermodurics survive pasteurisation.
Further reading: Robinson (ed.), Dairy Microbiology Handbook
Which genus usually dominates the flora of raw milk after several days of refrigerated storage?
B. Pseudomonas
Gram-negative psychrotrophs, mainly Pseudomonas, outgrow mesophiles in the cold.
Further reading: Walstra, Wouters and Geurts, Dairy Science and Technology
Psychrotroph enzymes are a problem in UHT milk mainly because they:
C. Partly survive UHT treatment
Enzymes produced in raw milk survive heat and act in storage; the bacteria are killed.
Further reading: Robinson (ed.), Dairy Microbiology Handbook
Sweet curdling of pasteurised milk is caused by:
D. Rennet-like proteases of Bacillus species
Proteases coagulate casein without acid development.
Further reading: Walstra, Wouters and Geurts, Dairy Science and Technology
Activation of the lactoperoxidase system in raw milk is appropriate when:
B. Cooling facilities are not available
The Codex guideline restricts it to situations without refrigeration; it delays growth for hours, not weeks.
Further reading: Walstra, Wouters and Geurts, Dairy Science and Technology
Which mastitis pathogen is classed as contagious?
A. Staphylococcus aureus
S. aureus spreads cow to cow at milking; the others are environmental.
Further reading: Robinson (ed.), Dairy Microbiology Handbook
In the methylene blue reduction test, a longer reduction time indicates:
A. Better quality milk
Fewer active bacteria lower the redox potential more slowly.
Further reading: Robinson (ed.), Dairy Microbiology Handbook
The 72 degrees Celsius for 15 seconds pasteurisation standard was confirmed to inactivate which more heat-resistant pathogen?
A. Coxiella burnetii
Coxiella, the Q-fever agent, is slightly more heat-resistant than M. tuberculosis; spores survive pasteurisation.
Further reading: Robinson (ed.), Dairy Microbiology Handbook
Bactofugation of cheese milk mainly prevents:
B. Late blowing by Clostridium tyrobutyricum
Removing clostridial spores reduces or replaces the need for nitrate or lysozyme.
Further reading: Walstra, Wouters and Geurts, Dairy Science and Technology
Why are biofilm bacteria a persistent contamination source?
B. They resist sanitisers far more than free cells
The matrix and altered physiology protect them.
Further reading: Walstra, Wouters and Geurts, Dairy Science and Technology
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