Physical Aspects

Question 1

What does intrinsic viscosity depend on?

Answer 1

Mark howink relation: n=kM^a where k is conformation and a is stiffness.

Question 2

What is meant by low and high volume fractions?

Answer 2

Low volume <10%
High volume >10%

Question 3

When a particle is stabilized by a polysaccharide and we want to calculate the viscosity of the solution, what assumptions do we make?

Answer 3

1. Diluted system
2. Polymer layer behaves as impenetrable layer that cannot be deformed

Question 4

When does bridging flocculation occur, at low or high polymer concentrations?

Answer 4

Low as attraction between negative polymer and positive protein are possible.

Question 5

When does depletion interactions occur, at low or high polymer/protein concentrations?

Answer 5

At high of polymer, protein or both. At low conc. of both it is stable. At high concentration if polymer does not fit between particle it will move out causing osmotic pressure difference. Water will travel outside to balance and thus making particles move closer together which may lead to coalescence in case of oil droplets.

Question 6

What are the 4 techniques to reduce depletion?

Answer 6

1. Dilute
2. Change pH
3. Add salt
4. Lower Mw

Depletion interactions occur more for rigid polysaccharide e.g. xanthan gum

Question 7

What is the definition of a gel?

Answer 7

A continuous space filling network of solid matter that encapsulates a dispersed liquid gel.
If water is the dispersed phase then the gel is called a hydrogel.

Question 8

What is an example of enzymatic induce gelation?

Answer 8

Chymosin which cleaves k-casein leading to casein aggregation. Another example is transglutaminase which induces covalent bonds between lysine and glutamine.

Question 9

Does casein make a particle or polymer gel? and globular proteins? and alginate?

Answer 9

Particle, particle and polymer respectively.

Question 10

What is the difference between particle and polymer gels?

Answer 10

Particle gels are aggregation of compact particles while polymer gels are a 3d network held together by junction zone. More junction zone more firm and brittle. Less, more elastic and cohesive.

Question 11

What is Type I gel?

Answer 11

No interaction between component A and B. Only A form a gel. Increasing conc. of B increase effective conc. of component A and so gel strength increases. At higher conc of B phase separation may occur leading to decrease of gel strength.

Question 12

What are the three Type II gels?

Answer 12

1. Coupled network -> interaction between AA, BB and AB , e.g. gelatin-alginate
2. Interpenetrating network -> No interaction, only AA and BB. Gel form through mutual entaglements.
3. Phase separate network -> at high concentration, depletion interaction can occur. A form gel and B form gel within hole in network of A.

Question 13

What are the difference in production of cheese and yoghurt?

Answer 13

First step in cheese is addition of chymosin to remove k-casein and induce aggregation. In yoghurt first step is homogenization to reduce fat droplets. Second step is addition of lactobacillus which breaks down protein to smaller peptides and uses the latter to produce flavour and reduce pH. Reduction in pH induce casein aggregation and network formation. The small droplet upon homogenization strengthen the network and increase gel strength. If no homogenization is performed, oil droplets are too big and would weaken the gel strength.

Question 14

What is fracture stress, fracture strain and young modulus related to in terms of gel properties?

Answer 14

Hardness, brittles, and stiffness respectively.

Question 15

What are the two (in)stability mechanisms in gels?

Answer 15

1. Amount of junction zones. More leads to higher gel strength.
2. Flexibility of the chains. More flexible weaker gel. More rigid stronger gel.

Question 16

How can a gummy bear be strong gel but still with low brittleness?

Answer 16

Because it contains helixes and its flexible parts within the polymer decrease brittlness.

Question 17

Does increasing electrostatic repulsion lead to higher gel strength?

Answer 17

Yes but only if the juntion zone are maintained otherwise the strength of the gel will decrease.

Question 18

What are the two effect of adding salt to gel strength?

Answer 18

1. Screen charges and leads to less repulsion and enhance helix formation leading to stronger gel.
2. Less repulsion lead also to increase flexibility which decreases gel strength

Outcome will depend on biopolymer used.

Question 19

are emulsions thermodynamically stable

Answer 19

no

Question 20

what type of polymer could stabilize emulsions?

Answer 20

modified starches which have amphiphilic character.

Question 21

Is flocculation a predecessor of coalescence?

Answer 21

Yes

Question 21

What are the 4 main instability mechanisms of emulsions?

Answer 21

Flocculation, Coalescence, creaming or sedimentation, and Ostwald ripening.

Question 21

Can flocculation lead to gel formation?

Answer 21

Yes, if a network is formed a gel can orginate.

Question 21

Do smaller or larger droplets have higher surface pressure?

Answer 21

Smaller

Question 22

What is pickering emulsions?

Answer 22

Small particles, like fat crystals that can adsorb at water oil interface and stabilize oil droplets.

Question 23

What are the three forces that must be considered when dealing with creaming and or sedimentation?

Answer 23

1. Gravity force
2. Buoyancy force
3. Friction

Question 24

Is there friction force in particles suspended in a gel?

Answer 24

No, the forces in this case are gravitational force, buoyancy force and yield force exterted by the network. The density of tha particle must be below a certain value to provide limited stress to the network, otherwise the network will break and sedimenation or creaming will occur.

Question 25

How can one reducing creaming/sedimentation?

Answer 25

1. Increasing viscosity of continous phase or providing yield stress (gel)
2. Decreasing droplet size -> coalescence can still occur
3. Matching densities -> not practical

Question 26

Are flocculation or coalescence more difficult to avoid?

Answer 26

Coalescence. For flocculation steric repulsion and ionic repulsion are sufficient with also low Mw surface active component while coalescence require thicker films usually protein and also polysaccharide addition.

Question 27

How can one reduce ostwald ripening?

Answer 27

1. Using oil with lower solubility in continous phase
2. Emulsion with a narrow size distribution -> small droplet cannot diffuse into larger as large droplets are not present.

Question 28

Why are commercial plant proteins not good emulsifiers?

Answer 28

Because they are usually presented as larger aggregate which diffuse too slowly to the interface in order to stabilize it.

Question 29

What are two strategies to increase film thickness around oil droplet in emulsion?

Answer 29

1. Layer-by-layer adsorption -> oppositely charged polysaccharide is added to protein which stabilizes the oil droplet. Protein and polysaccharide attract and polysaccharide increase thickness of film. Another layer of protein can be added and so also another polysaccharide layer. If conc. of polysaccharide too low bridging flocculation can occur and if too high depletion interactions may occur.
2. Pre mixed mixture for emulsion-> protein and polysaccharide are pre mixed together to form complexes and or coacervates. These will then diffuse to oil droplet interface. If too large may sediment or diffuse too slowly to interface.

Question 30

What happens when protein and polysaccharide have the same charges? What happens at low and high polysaccharide concentrations?

Answer 30

At low concentration polysaccharide will tend to increase viscosity of continous phase, flocculation is limited by electrostatic repulsion between protein stabilized oil droplets and with polysaccharide.
At high concentration, polysaccharide will tend to give depletion interactions which can result in flocculation, creaming and eventually phase separation.

Question 31

What happens when protein and polysaccharide have opposite charges at different conc?

Answer 31

At low bridging flocculation can occur as polysaccharide attached to more than one droplet.
At intermediate full coverage of droplet. IMPORTANT that charge is not 0. Polysaccharide are negative and protein are positive. If these charges balance out then total charge is 0. Therefore only steric repulsion can separate oil droplets but flocculation can still occur.
At high concentration depletion interaction can occur.

Question 32

How can one avoid bridging flocculation when adding polysaccharide to increase film thickness?

Answer 32

By pre mixing protein and polysaccharide so complex is formed and polysaccharide will not connect to more than one droplet.

Question 33

How can one measure average size of oil droplet?

Answer 33

With a mastersizer. However this cannot differentiate between flocculates and coalescence and normal droplets and therefore only light microscopy can give a valid indication.

Question 34

What is the size of oil droplet for which the emulsion is not stable or will not be stable?

Answer 34

> 10um

Question 35

Why can oil droplets change the maximum packaging density?

Answer 35

Because they are deformable and can acquire a hexagonal shape.

Question 36

What does the numerical factor depend on 5/2 in einstein equation?

Answer 36

Is the ratio between the viscosity of the droplet of the viscosity of the continuous phase. If soft particles are present constant will approach 1. If hard particles are present value will approach 5/2.

Question 37

What happens to the material when the volume fraction exceeds the maximum packaging density?

Answer 37

It will exhibit plastic flow behavior.

Question 38

What are the four main difference between foams and emulsions?

Answer 38

1. Gas density is much lower than oil
2. Gas is compressible -> higher packaging density
3. Gas is more soluble in water than oil
4. Emulsion droplets are much smaller than gas droplets.

Question 39

Are foam more stable when proteins has a charge or when no charge is present?

Answer 39

No charge so bubbles can receive full coverage. If protein are flexible then charge can help to increase rigidity and increase steric repulsion.

Question 40

It is known that max packaging density is 0.74. Can u calculate the viscosity at 0.8 volume fraction?

Answer 40

No, because the equation assumes hard spheres which would chage the value 2.5 to lower as we would be working with softer particle which are deformable.