Gel properties

Question 1

Gels in food can be formed by:

Answer 1

Proteins (gelatin or casein) or by polysaccharides (alginate or pectin).

Question 2

Differences in gel structure: between proteins and polysaccharides

Answer 2

1. Proteins: stabilize gels only after extensive heating and aggregation. Because they are small (5-50 kDa) they are very compactly folded and therefore they do not make gel (not increase viscosity). If you make aggregates they can cluster together an make a gel, using heating, change in pH, ionic strength or hydrolysis. Relatively high concentrations (7-15% w/w).
2. Polysaccharides (100-10000 kDa): are very large, linear, more efficient in stabilizing three dimensional network, low concentrations (0.5 - 4% w/w). So we need les polysaccharides than proteins to make gels.

Question 3

Applications of the proteins and the polysaccharides (tecno-functional properties).

Answer 3

Proteins: gelling and emulsifying and foam.
Polysaccharides: stabilisation, thickening and gelling.

Question 4

Native globular proteins will never make a gel, even at very high concentrations. Globular proteins will only form a gel after proteins first made aggregates. TRUE OR FALSE?

Answer 4

TRUE

Question 5

To make gel ,the proteins need to aggregate, this aggregation can be due to 1,2,3 and what happen:

Answer 5

1) Heat induced unfolding, leading to increased hydrophobic attractions.
2) Change in pH or ionic strength, leading to decreased electrostatic repulsion.
3) Hydrolysis which can result both in increased hydrophobic attractions as well as decrease electrostatic repulsion.

Question 6

Gels from gelatin: formation (drwa)

Answer 6

1. Native state, triple helix. –> heating up. 2. The triple helix dissociate you get single chains in solution –>cooling down. 3. Gel formation, connection between different chains.
   Last step is reversible

Question 7

There are differences in the gelling temperature but also in the gel strength. This last one depends on ….

Answer 7

Depends on the amount of helices formed.

Question 8

Gels from globular proteins (Draw):

Answer 8

1.Globular proteins in native state are folded–>heating. 2. Unfolded 3. Aggregate 4. Aggregates cluster together gel formation

Question 9

Are there differences in the globular proteins? Yes, effect of ionic strength.

Answer 9

Minimal gel concentration 4-15%, quite large amount of protein. But there are distinct differences between proteins. There is a minimum in the gel concentration at low ionic strength and the again increase. This is because the tupe of the structures that are formed.

Question 10

There are two methods to make cold set gels:

Answer 10

1. Making aggregates without making a gel then pH adjusted

2. Hydrolysis , not globular structure, more exposed hydrophobicity.

Question 11

Effect of pH and ionic strength on heat induced protein aggregation. Mention the appearance, the aggregate structure and how is going to be the pH and the ionic strength.

Answer 11

1) Low ionic strength, pH less than pI, pI less than pH high electrostatic repulsion, rigid rod, transparent.
2) Linear aggregates.
3) Branched aggregates.
4) High ionic strength , pH=pI , low electrostatic repulsion, Random aggregates, high turbidity.

Question 12

Gel strength: effect of ionic strength

Answer 12

At high ionic strength, i have a high gel strength and at low ionic strength i have a low gel strength. Small changes in the protein preparation can result in a large changes in the functionality.

• Gel strength
• Appearance
• Concentration

Question 13

Another factor that is important in gel strength is the formation of disulphide bridges.

Answer 13

• Stabilize the protein structure.
  If we heat the native protein we get disulphide bridge shuffling. By chemical reagent, we can block the suphydryl groups, the weaker becomes.
  Increased permeability, increased turbidity, syneresis, spontaneous gel rupture.

Question 14

Two whey protein concentrates are dissolved at the same concentration (8% w/w) an heated (80ºC, 30 min). In one sample you see a turbid gel, on the other sample stays transparent. Which properties of the samples are likely responsible for these differences ?

Answer 14

1- Differences in state of the protein. The proteins may already be unfolded and aggregated. If ionic strength/ pH is low, you should get transparent gel.
2. One sample may have been glycated due to Maillard reaction. This leads to change in pI, and therefore different aggregation / gelation behavior under similar conditions.
3- One sample may have had a higher ash content (salt), leading to higher ionic strength of the solution when dissolver.