Practice Exam
1. The main difference in solubility between a rennet casein and a caseinate is
Rennet casein is insoluble at any pH, while caseinate is only insoluble at pH = pI
2. You have 3 different protein samples: (1) milk protein concentrate, (2) a rennet casein and a (3) whey
protein concentrate. Your colleague has messed up and has not labelled the containers correctly. You
perform an SDS-PAGE analysis to determine the protein composition. With SDS-PAGE proteins are
separated from one another by gel electrophoresis based on size.
What can you say about the results for SDS-page for these three samples? The bank pattern of
all three samples are different from each other on SDS-page
(Rennet casein only contains casein, while milk protein contains casein + whey protein and whey
protein only contains whey proteins. Native whey proteins tend to be much smaller in size then
casein so this will be a clear distinction. The milk protein concentrate will show bands for both
proteins on the gel)
3. For 32 whey protein concentrates, the viscosity (mPa*s) of 10 g (powder)/L suspensions was
measured at pH 7, see graph below. From these results it is clear that the viscosity for some samples
is significantly higher than that for other samples. This is most likely due to:
Differences in amount of aggregates
(When the powder is being produced it is often pasteurised followed by a (spray)drying step. Both
processing can have an effect on the denaturation and consequent aggregation of the protein
depending on the conditions chosen by the supplier. The aggregates cause an increase in the
viscosity, so the more aggregated samples will have a higher viscosity. Lactose is soluble and will not
have a large effect at these concentrations. Neither do the salts have a major influence at these
concentrations. The proteins are the same for all samples, they all contain only whey proteins)
4. You measure the foam properties of a whey protein concentrate in two ways. In the first way (1) you
just disperse the powder (10 g/L) in the buffer (pH 7) and you measure the foam properties.
In the second way (2) you also disperse the powder (10 g/L) in the buffer (pH 7), after which you
centrifuge the sample and measure the foam properties of the supernatant.
For sample 2 (the supernatant) the foam stability as well as the overrun increased significantly (by a
factor 5 -10) compared to sample 1.
Which of the used analyses of the dispersion (sample 1) and supernatant (sample 2) will most likely
show a large difference:
None of the above (ash content, protein composition, protein content)
, (Depending on the powder properties not all proteins dissolve completely and some particles are left
behind that can destabilise the foam. Removing these particles, for instance by centrifuging, can
decrease the amount of insoluble particles and thus improve the foamability)
5. When proteins adsorb to the interface, the surface pressure (II) increases. You are measuring the
increase of surface pressure in time (dII/dt), while solubilizing a protein for 3 different protein
concentrations. What is the most likely outcome for the initial increase of surface pressure (first
minutes)?
The initial increase of surface pressure (dII/dt) increases linearly with the protein concentration
6. You prepare a soluble micellar casein solution, which is then split into two fractions. One fraction you
keep as is, the other is subjected to a heat treatment (90 °C, 30 minutes). What happens to the
proteins during this treatment?
The casein proteins have dissociated (micelle has fallen apart)
7. The graph below shows the solubility curve of a protein sample as function of pH.
Which statement about this graph is true?
The sample contains whey proteins
8. Gelatin gels show an increase in melting temperature for systems with higher gelatin concentration
(see figure).
This effect is due to ... at higher gelatin concentrations:
An increased denaturation temperature
(The gel strength of gelatin gels is determined by the concentration op triple helices. A larger
amount of triple helices means they are more resilient to melting since it requires more energy to
unravel them which results in a higher melting temperature)
1. The main difference in solubility between a rennet casein and a caseinate is
Rennet casein is insoluble at any pH, while caseinate is only insoluble at pH = pI
2. You have 3 different protein samples: (1) milk protein concentrate, (2) a rennet casein and a (3) whey
protein concentrate. Your colleague has messed up and has not labelled the containers correctly. You
perform an SDS-PAGE analysis to determine the protein composition. With SDS-PAGE proteins are
separated from one another by gel electrophoresis based on size.
What can you say about the results for SDS-page for these three samples? The bank pattern of
all three samples are different from each other on SDS-page
(Rennet casein only contains casein, while milk protein contains casein + whey protein and whey
protein only contains whey proteins. Native whey proteins tend to be much smaller in size then
casein so this will be a clear distinction. The milk protein concentrate will show bands for both
proteins on the gel)
3. For 32 whey protein concentrates, the viscosity (mPa*s) of 10 g (powder)/L suspensions was
measured at pH 7, see graph below. From these results it is clear that the viscosity for some samples
is significantly higher than that for other samples. This is most likely due to:
Differences in amount of aggregates
(When the powder is being produced it is often pasteurised followed by a (spray)drying step. Both
processing can have an effect on the denaturation and consequent aggregation of the protein
depending on the conditions chosen by the supplier. The aggregates cause an increase in the
viscosity, so the more aggregated samples will have a higher viscosity. Lactose is soluble and will not
have a large effect at these concentrations. Neither do the salts have a major influence at these
concentrations. The proteins are the same for all samples, they all contain only whey proteins)
4. You measure the foam properties of a whey protein concentrate in two ways. In the first way (1) you
just disperse the powder (10 g/L) in the buffer (pH 7) and you measure the foam properties.
In the second way (2) you also disperse the powder (10 g/L) in the buffer (pH 7), after which you
centrifuge the sample and measure the foam properties of the supernatant.
For sample 2 (the supernatant) the foam stability as well as the overrun increased significantly (by a
factor 5 -10) compared to sample 1.
Which of the used analyses of the dispersion (sample 1) and supernatant (sample 2) will most likely
show a large difference:
None of the above (ash content, protein composition, protein content)
, (Depending on the powder properties not all proteins dissolve completely and some particles are left
behind that can destabilise the foam. Removing these particles, for instance by centrifuging, can
decrease the amount of insoluble particles and thus improve the foamability)
5. When proteins adsorb to the interface, the surface pressure (II) increases. You are measuring the
increase of surface pressure in time (dII/dt), while solubilizing a protein for 3 different protein
concentrations. What is the most likely outcome for the initial increase of surface pressure (first
minutes)?
The initial increase of surface pressure (dII/dt) increases linearly with the protein concentration
6. You prepare a soluble micellar casein solution, which is then split into two fractions. One fraction you
keep as is, the other is subjected to a heat treatment (90 °C, 30 minutes). What happens to the
proteins during this treatment?
The casein proteins have dissociated (micelle has fallen apart)
7. The graph below shows the solubility curve of a protein sample as function of pH.
Which statement about this graph is true?
The sample contains whey proteins
8. Gelatin gels show an increase in melting temperature for systems with higher gelatin concentration
(see figure).
This effect is due to ... at higher gelatin concentrations:
An increased denaturation temperature
(The gel strength of gelatin gels is determined by the concentration op triple helices. A larger
amount of triple helices means they are more resilient to melting since it requires more energy to
unravel them which results in a higher melting temperature)
