ICAR AICE-JRF/SRF Dairy Colloids: Emulsions, Foams & Gels: practice questions

1 · Colloidal state of milk and colloid stability · medium

Why does skim milk look slightly bluish?

1. Skim milk contains copper ions from the separator that tint it blue
2. Riboflavin in skim milk absorbs yellow light and so leaves the milk looking blue
3. Whey proteins in solution form a blue pigment complex with calcium phosphate
4. Casein micelles are small and scatter short blue wavelengths more strongly

2 · Emulsifiers and the HLB concept · easy

What is the approximate HLB of distilled monoglycerides?

1. About 3–4
2. About 10–12
3. About 15
4. About 18–20

3 · Emulsion formation, homogenisation and stability · medium

What mainly covers the new fat surface created by homogenising milk?

1. Fresh milk fat globule membrane synthesised by enzymes during homogenisation
2. Adsorbed casein micelles and fragments, plus whey protein
3. Lactose crystals that deposit from the serum onto every globule surface
4. Nothing; the new surface stays bare until the milk is pasteurised

4 · Creaming, flocculation, coalescence and partial coalescence · hard

Why does milk heated well above minimum pasteurisation cream less than raw milk?

1. Heat inactivates the cold agglutinins that cluster the globules
2. Heat converts the globules into casein micelles
3. Heat strips all the fat globule membranes off the globules entirely
4. Heat melts the fat so it becomes denser than the serum

5 · Protein and polysaccharide gels · hard

Agar gels set at about 35–40 °C but melt only near 85 °C. What is this behaviour called?

1. Ostwald ripening
2. Thermal hysteresis
3. Thixotropy
4. Syneresis

6 · Hydrocolloid stabilisers and protein-polysaccharide interactions · medium

How does high-methoxyl pectin stabilise acidified milk drinks at pH about 4?

1. It hydrolyses casein into soluble peptides
2. It raises the pH back to 6.7 so that casein is negatively charged again
3. It adsorbs on positive casein and repels by steric and charge effects
4. It forms a calcium egg-box gel that traps every particle

7 · Techniques to study structure: microscopy and particle sizing · hard

An emulsion has 4% dispersed phase by volume and a Sauter mean diameter d32 of 0.5 µm. What is its interfacial area per mL of emulsion?

1. 0.12 m²
2. 0.048 m²
3. 0.48 m²
4. 4.8 m²

8 · Techniques to study structure: microscopy and particle sizing · medium

Zeta potential is usually derived from which measurement?

1. Conductivity of the dispersion measured with no field applied
2. Electrophoretic mobility in an applied electric field
3. pH change on adding acid
4. Angle of light scattering at rest

9 · Organogels, hydrogels and nanoemulsions · easy

What is an oleogel?

1. A fat crystallised fully into the beta polymorph
2. Liquid oil given solid-like structure by a gelator network
3. A water-swollen network of hydrophilic polysaccharide chains
4. An oil-in-water emulsion stabilised only by casein

10 · Organogels, hydrogels and nanoemulsions · easy

Nanoemulsions are commonly defined as emulsions with mean droplet radius below about:

1. 10 µm
2. 1 nm
3. 1 mm
4. 100 nm