ICAR AICE-JRF/SRF Biochemistry for Dairy Science: practice questions

1 · Water, pH and buffers · hard

Milk triacylglycerols separate as globules instead of dissolving in the serum mainly because of:

1. Strong covalent links between fat molecules that form during milk secretion
2. Electrostatic attraction between positively charged fat molecules
3. The hydrophobic effect
4. The high density of milk fat compared with the aqueous phase

2 · Water, pH and buffers · hard

Why does the measured pH of milk differ when it is measured at 20 °C and at 60 °C?

1. Lactose hydrolyses to glucose and galactose above 50 °C, releasing acid into the milk serum
2. Glass pH electrodes stop responding to hydrogen ions once milk exceeds 40 °C
3. Heat alters the ionisation of water and buffer salts and precipitates calcium phosphate
4. Heating destroys the hydrogen ions present in milk serum, so pH always rises

3 · Amino acids, peptides and protein structure · hard

When milk is heated above about 70 °C, β-lactoglobulin forms complexes with κ-casein. The key chemical reaction is:

1. Thiol–disulfide exchange involving β-lactoglobulin's exposed free thiol
2. Maillard condensation between lactose and the N-terminal amino group of κ-casein
3. Hydrolysis of the Phe105–Met106 bond of κ-casein
4. Phosphorylation of β-lactoglobulin by casein kinase in the serum

4 · Amino acids, peptides and protein structure · hard

Which amino acid residue in the casein chains carries the phosphate groups that bind calcium phosphate in the micelle?

1. Serine
2. Lysine
3. Methionine
4. Proline

5 · Lipid classes and fatty acids · hard

The main conjugated linoleic acid isomer in milk fat, rumenic acid, is:

1. trans-10, cis-12 18:2
2. cis-9, cis-12 18:2
3. trans-9, trans-12 18:2
4. cis-9, trans-11 18:2

6 · Lipid classes and fatty acids · hard

Hydrolytic rancidity in raw milk is promoted by agitation, foaming or homogenisation before pasteurisation because these:

1. Introduce oxygen that oxidises unsaturated fatty acids by a free-radical chain
2. Raise the temperature enough to activate copper-catalysed oxidation
3. Damage the globule membrane, exposing the fat to lipoprotein lipase
4. Destroy the lipase so that free fatty acids cannot be esterified again

7 · Enzymes: classification and Michaelis-Menten kinetics · hard

Why does stored UHT milk sometimes gel or turn bitter although the UHT process killed all vegetative bacteria?

1. Chymosin from the cow's udder is reactivated by cooling after the UHT treatment
2. Lactic acid bacteria regrow from heat-resistant spores formed during UHT treatment
3. Heat-stable psychrotroph proteases survive UHT and act during storage
4. Alkaline phosphatase hydrolyses casein when the pack is stored warm for weeks

8 · Glycolysis, TCA cycle and oxidative phosphorylation · hard

Using P/O ratios of 2.5 for NADH and 1.5 for FADH₂, how many ATP are obtained from the complete oxidation of one acetyl-CoA in the TCA cycle?

1. 12
2. 10
3. 15
4. 8

9 · Nucleic acids and protein synthesis · hard

In the mammary epithelial cell, the caseins are phosphorylated and assembled into micelles in the:

1. Golgi apparatus and secretory vesicles
2. Nucleus
3. Mitochondria
4. Cytosol, while translation on free ribosomes is still under way

10 · Vitamins as coenzymes and minerals · hard

Cobalamin (vitamin B12) is the coenzyme of methylmalonyl-CoA mutase. This enzyme is needed in the metabolism of:

1. Glucose to pyruvate in glycolysis
2. Propionyl-CoA from odd-chain fatty acids
3. Acetyl-CoA to malonyl-CoA for fatty acid synthesis
4. Ammonia to urea