ICAR AICE-JRF/SRF (PhD): Dairy Chemistry, Dairy Microbiology, Dairy Technology, Dairy Engineering · India · NTA for ICAR
Original practice questions written to the published syllabus · not past papersFlow and deformation of milk, cream, yoghurt, cheese, butter and ice cream, and how instruments measure texture.
In the exams: ICAR AICE subject 37 Unit 5 and subject 38 Unit 1
How our questions for ICAR AICE-JRF/SRF spread across the units of Rheology & Texture of Dairy Foods. Difficulty: 31 easy, 50 medium, 28 hard.
| Newtonian and non-Newtonian fluids; power-law and Herschel-Bulkley models | 12 | |
| Stress, strain and rheological classification | 10 | |
| Viscometers and rheometers | 10 | |
| Mechanical models: Maxwell, Kelvin-Voigt and Burgers | 10 | |
| Dynamic oscillatory measurements | 10 | |
| Texture profile analysis | 10 | |
| Rheology of cheese, butter, yoghurt and ice cream | 10 | |
| Effect of processing and stabilisers on rheology | 10 | |
| Viscoelasticity: creep and stress relaxation | 9 | |
| Large deformation and fracture | 9 | |
| Relating instrumental and sensory texture | 9 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
In an up-and-down shear-rate loop on a stirred yoghurt, what indicates thixotropy?
B. The down-curve lies below the up-curve
Structure broken on the way up gives lower stress on the way down, so the down-curve lies below and encloses a hysteresis area. A down-curve above the up-curve indicates rheopexy; coincident curves mean no time dependence; passing through the origin only shows there is no yield stress.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
For a power-law fluid with n = 0.5 in a tube viscometer, the Rabinowitsch-Mooney correction multiplies the apparent (Newtonian) wall shear rate by (3n + 1)/(4n). What is the factor?
A. 1.25
(3 × 0.5 + 1)/(4 × 0.5) = 2.5/2 = 1.25: the true wall shear rate is higher than 4Q/(πR³) for a shear-thinning fluid. 0.80 inverts the factor; 1.00 applies only when n = 1; 0.625 omits n in the denominator.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
A plant's QC laboratory reports yoghurt viscosity with a single-spindle rotational viscometer. What must accompany each reading for it to be meaningful?
C. Spindle, speed, temperature, time and sample handling
A single-point spindle reading is an apparent viscosity whose value depends on spindle geometry, speed, temperature, reading time and how the sample was stirred. Temperature alone is not enough; composition does not define the test; cone angle and gap apply to cone-and-plate instruments.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
Why are creep and relaxation tests on dairy gels run at small deformation?
B. To stay within the linear viscoelastic region
Inside the linear region the response is proportional to load and describes the intact material; larger deformations break bonds so results reflect damage. Small deformation does not shorten the test, does not affect fat melting, and does not turn a gel Newtonian.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
During a heating sweep, a cheese's tan δ rises from 0.3 at 20 °C to 1.2 at 70 °C. What has happened?
B. It has changed from solid-like to liquid-like as it melted
tan δ above 1 means viscous response dominates, so the cheese has softened and begun to flow as fat melted and the protein network loosened. More elasticity would lower tan δ; shear-thickening is a steady-shear property; drying would stiffen, not liquefy, the cheese.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
Butter softens noticeably when worked and partly regains firmness on standing. What is this called?
A. Work softening
Working breaks bonds in the fat crystal network, and some re-form at rest: work softening. Rheopexy is thickening under shear; syneresis is whey expulsion; cold agglutination is clustering of fat globules in cold raw milk.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
Why is high-methoxyl pectin used in acidified milk drinks?
A. It adsorbs to casein and prevents aggregation at low pH
At acid pH the negatively charged pectin adsorbs onto positively charged casein and keeps particles apart by steric and charge repulsion, preventing sedimentation. Calcium gelling is a low-methoxyl pectin property; pectin raises rather than lowers viscosity; it does not change pH.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
How does homogenisation of yoghurt milk change gel firmness?
C. It increases firmness, as protein-coated globules join the network
Homogenised globules are coated with casein and whey protein and behave as active network particles. Casein is redistributed, not removed; fat does take part once coated; homogenisation does not change gelation pH.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
Perceived firmness of cheese on the first bite is most closely related to which instrumental measure?
A. Compression hardness or fracture stress
First-bite firmness relates to the force needed to deform and fracture the sample. Whey viscosity and melt tan δ describe other states; springiness measures recovery, not firmness.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
In an oscillatory test, the stress of a purely viscous liquid is out of phase with the strain by:
A. 90°
Stress in a viscous liquid follows strain rate, which leads strain by 90°. 0° is a pure elastic solid; 45° is a material with equal G′ and G″; 180° does not occur in a passive material.
Further reading: Steffe, Rheological Methods in Food Process Engineering; Rao, Rheology of Fluid, Semisolid, and Solid Foods
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