Physical aspects - Exercises

18 important questions on Physical aspects - Exercises

You have a maximum packing density of 74%. Why can you not calculate the viscosity of a 80% oil emulsion using this equation? And how would this have an effect on the structure of the oil droplets and the assumptions made in the equation?

You cannot include 80%, as the maximum packing is 74%. To have more than 74%, the droplets need to deform, and then you cannot consider the emulsion droplets as hard sphere anymore. The equation does not hold anymore, the factor 2.5 used for hard spheres will then change.

What are the structures of Na-caseinate and whey proteins?

  • Na-caseinate is a flexible protein and does not have a tertiary structure. It is a linear flexible chain with one end that is mainly hydrophilic, and the other end that is mainly hydrophobic.
  • Whey proteins are globular proteins and do have a tertiary structure. Their hydrophobic sites are mostly present in the core of the proteins and they mostly preserve this globular shape at the interface.

Why is the foam stability of whey proteins the highest in the iso-electric point?

In the case of the more globular whey proteins, the packing of the proteins at the interface will determine the properties of the film. At low and high pH values (far from the pI), the positive or negative charges will provide electrostatic repulsion between the proteins, and therefore it is difficult to approach each other. At pH values close to the iso-electric point, the electrostatic repulsion is diminished, and the proteins can approach each other much easier. More protein is able to adsorb to the interface, which increases the foamability.

Would you expect a higher foam stability for WPI close to the iso-electric point, or further away from the iso-electric point?

Closer to pI, as there will be more protein present so a higher stability against Ostwald ripening and coalescence.

How can salt addition lead to a faster adsorption rate ?

At a pH far from the pI, the proteins will repel each other. Upon the addition of salt, the salt will screen the charges of the proteins (decrease in the double layer), thereby decreasing the electrostatic repulsion between the proteins. This enable the proteins to approach the interface, and therefore there is a faster adsorption.

Why is skimmed milk to a good approximation comparable to the casein dispersion?

In skimmed milk, the fat has been removed, and what is left is dispersion of casein micelles and minor components, such as whey proteins and lactose. We assume that these minor components only have a small effect on the behaviour of the mixture, and that the behaviour is determined by the large casein micelles.

How does the degree of esterification affect the charge of pectin?

During esterification, the carboxyl group is replaced with a methyl group. This will affect the amount of charges on the pectin, as the carboxyl group can become negatively charged (depending on the pH), and a methyl group can not become charged. A high degree of esterification therefore leads to less charges, and a lower charge density.

A pectin with a large degree of esterification is referred to as high methoxy pectin (HMP)
A pectin with a small degree of esterification is referred to as low methoxy pectin (LMP)

Which proteins are present in a skim milk powder made using only microfiltration?

Whey proteins + Caseins (k-casein, alpha-lactalbumin, alpha-S1-casein)

To stabilize the emulsion, different polysaccharides can be used. One of the polysaccharides used is xanthan. Xanthan is often used for dressings with oil and without oil, why?

The reason why this polysaccharide is used for dressings without oil, is that xanthan can easily form a network and has a high degree of shear thinning behaviour. Upon the application of stress, the solid-like character transformed easily into a more liquid like behaviour and the product starts to flow.

What is the yield stress?

The maximum stress for which no flow (shear) is observed.

A stable emulsion can be caused by 2 networks

  • Polysaccharide network with oil droplets --> polymer network
  • Network formation of emulsion droplets with polysaccharides --> particle network

How does the concentration of xanthan influence the depletion interactions in an emulsion?

  • Low concentration (0.5 g/L) --> attractive interactions between oil droplets, small flocs will cream to the top of the sample, phase separation
  • Medium concentration (1 g/L) --> more interaction between oil droplets, larger flocs are formed. Weak network formation collapsed very soon. Also phase separation
  • High concentration (2 g/L) --> aggregation of oil droplets, network is formed. Gel formation. However, rearrangements of oil droplets in time, the network collapses after a certain delay time and the droplets start creaming.

Which interactions are responsible for the gelation of gelatin?

Gelatin gels are formed at low temperatures. At lower temperatures, gelatin chains form triple helices which are held together by hydrogen bonds. As hydrogen bonds become weaker at higher temperatures, gelatin gels melt at higher temperatures.

In which way can alginate gels be prepared?

Alginate gels can be prepared by the addition of calcium ions. As the alginate has negative charges at higher pH values, the calcium acts as a bridge to keep the pectin chains together. The relevant interactions here are salt bridges. The structure of the alginate is such that a certain “egg- box” structure can be formed.

What does glucono-lacto-daltone (GDL) do?

It lowers the pH very slowly.

Why is the slow change in pH and slow release of calcium important in alginate gels?

A very fast change in pH of calcium release may give some inhomogeneities in the systems. The pH may be different at different locations within the gels, and the calcium concentration may also depend on where the calcium is added. Due to these inhomogeneities, the gelation will only take place very locally. A slow change will assure that a more homogeneous gel will be formed.

Is the gel formation of gelatin reversible? And of alginate?

Gelatin --> yes
Alginate --> no

At a pH of 7.0, describe the network and stability of an alginate/gelatin gel.

Gelatin is positively charged (below pI) and alginate is negatively charged. There is a competition between the positive gelatin and the positive calcium ions to interact with the negative alginate.

Higher concentrations of alginate --> probability gelatin + alginate increases, interactions with calcium is decreased. The lower amount of calcium bridges decreases the network formation of the alginate, and therefore a collapse of the gel.

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