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Polysaccharides - Exercises

Q: Why will a solution of lambda-carrageenan be more stable upon heating at pH<4, compared to kappa- and -iota carrageenan?

The anhydrogalactosyl residue is not stable at lower pH and will be totally open and be degraded. Consequently, the carrageenan molecule will become smaller and smaller, and the solution will decrease in viscosity. Obviously, this process will not occur in lambda-carrageenan, since no anhydrogalactose is present.

Q: Kappa-carrageenan and iota-carrageenan’s properties fully depends on the type and number of counter ions present. Which salt and which carrageenan would be chosen when a brittle gel is needed? Which combination of salt and carrageenan would result in a very elastic gel?

Brittle gel --> K+ and kappa-carrageenan Elastic gel --> Ca+ and iota-carrageenan

20 important questions on Polysaccharides - Exercises

Which pectin makes a good acid-sugar gel and why?

HM pectin. The relatively low number of free COO- groups can be neutralised by the pH allowing gel formation in the presence of sugar.

Why is alginate a suitable polysaccharide to form a strong and heat stable gel which can set at low temperatures?

Alginate gels have negative charges at high pH. Calcium acts as a bridge and a so-called egg-box structure is formed. The gelation is not temperature dependent and the interactions remain at higher temperatures, so it is thermostable.

The gel strength depends on the distribution of G and M units.

What kind of alginate gel will be the strongest?

More and longer G blocks

Why can kappa- and iota-carrageenan be considered as modified natural polysaccharides? What will be the modification?

Kappa- and iota carrageenan are produced upon alkaline extraction from mu- and nu-carrageenan, only composed of galactose and sulfate groups. During alkaline extraction of mu/nu carrageenan, sulfate falls off while an anhydrogalactosyl group is formed. As a consequence, different batches of both kappa- and iota-carrageenan might vary (slightly) in level of anhydro-galactose.

Why will a solution of lambda-carrageenan be more stable upon heating at pH<4, compared to kappa- and -iota carrageenan?

The anhydrogalactosyl residue is not stable at lower pH and will be totally open and be degraded. Consequently, the carrageenan molecule will become smaller and smaller, and the solution will decrease in viscosity. Obviously, this process will not occur in lambda-carrageenan, since no anhydrogalactose is present.

Kappa-carrageenan and iota-carrageenan’s properties fully depends on the type and number of counter ions present. Which salt and which carrageenan would be chosen when a brittle gel is needed? Which combination of salt and carrageenan would result in a very elastic gel?

Brittle gel --> K+ and kappa-carrageenan
Elastic gel --> Ca+ and iota-carrageenan

Kappa-carrageenan can interact with protein in several ways. Please explain (draw) the interaction at both low and high pH.

At pH values below the pKa of the protein, direct interactions of the positively charged protein may react directly with the negatively charged carrageenan. At higher pH values (pH>pKa), the negatively charged protein may interact with the negatively charged carrageenan through calcium bridges.

Please explain why both self-association and intermolecular interaction mechanisms play a role in the stabilisation of cholate milk with kappa-carrageenan.

The kappa carrageenan forms a weak gel in the aqueous phase through self-association with calcium and potassium in the form of alfa helix formed , while parts of the negatively charged carrageenan chain will also interact with the positively charged amino acids in the proteins at the surface of the casein micelles.

A food application lab received quite some complains about the performance of wheat starch in the application in a clear gel product which is sold in the supermarket from refrigerated shelves. The gel is not clear, shows some retrogradation and forms a water layer on top once the product is frozen and thawn again.
Explain why this wheat starch is not performing very well in this application.

Cereal starches in general and wheat starch in particular, contain some levels of lipids which are migrating into the alfa-helix of starch (amylopectin) causing some turbid gel. The amylose can also easily retrogradate forming insoluble starch ‘crystals’, which are not desired in a nice gel. Furthermore, this retrogradation and rearrangement of the gel network is enhanced upon freezing and thawing, at the same time encouraging syneresis due to water being excluded from the network.

A food application lab received quite some complains about the performance of wheat starch in the application in a clear gel product which is sold in the supermarket from refrigerated shelves. The gel is not clear, shows some retrogradation and forms a water layer on top once the product is frozen and thawn again.
How could these problems be solved?

First of all, a starch without lipids in its composition should be chosen for the recipe, like e.g. potato- or tapioca starch. Furthermore such starches could be chemically stabilized by adding substituents to the amylose and amylopectin molecules in order to prevent retrogradation: e.g. acetyl-starch or hydroxy propyl starch. Further improving stability and freeze-thaw stability can be done by crosslinking of the starch.

Please explain why the maximum substitution level for acetylated starch could be 3, but for hydroxypropyl starch much higher. Hint: think about the molecular structure of the two modified starches.

By adding hydroxypropyl substituent to starch, you are adding an additional hydroxyl group as well. Which can also be substituted, making rather long chains of hydroxypropyl units.

Two modified potato starches are synthesized: an acetylated starch and a hydroxypropyl starch.
Is there any diference in hydrophobicity between the starches? Please explain.

The long acyl (hydroxypropyl) chains are rather apolar and hydrophobic.

Two modified potato starches are synthesized: an acetylated starch and a hydroxypropyl starch. Which of the two starches will be more stable in gelling behaviour after being cooked in a milk product pH 6.5? Why?

HP starch will be more stable since the substituents are ether-linked which are quite stable. In contrast, the ester-linkages of the acetyl-substituents are sensitive to saponification at higher pH values.

Cellulose, xanthan and xyloglucan all consist of a backbone of beta 1-4 linked glucose residues. Please indicate the source of these three polymers.

Cellulose is mainly isolated from plant material, although also bacteria can produce cellulose. Xyloglucans are also isolated from plant material. E.g. tamarind xyloglucan is a seed -storage polysaccharide.
Xanthan is a bacterial polysaccharide.

Cellulose, xanthan and xyloglucan all consist of a backbone of beta 1-4 linked glucose residues. Which polymer in solution/suspension would be more susceptible for the salt level in the liquid product? Why?

The insoluble cellulose is again hardly effected by any salt addition. The same lack of any effect is expected for the xyloglucan, because it is a fully neutral molecule. In contrast, xanthan is known to act quite different upon the addition of different salts at different levels. The conformation of the molecule and chain association will differ for different salt concentrations.

What is the origin of gum Arabic?

Exudate of Acacia tree

Due to its unique solubility properties Gum Arabic is frequently used in confectionery. Please explain why.

Due to the presence of a highly branched arabinogalactan molecule and the presence of charges (e.g. galacturonic acids) in its structure, in addition to some protein, Gum Arabic is highly soluble. Even up to 30-40 % (w/w) solutions the viscosity is not significantly increased. It is a good filler for confectionary purposes because it creates a solution with a high dry matter with a relatively low viscosity.

Another important application of Gum Arabic is its use in aroma flavored beverages as well as oil- water emulsions. Please explain both applications on basis of its structural features.

When a solution of gum arabic in a high concentration is dried in the presence of flavour it leads to an easy encapsulation of the flavour, protecting the aroma release before consumption. The protein present in Gum Arabic (ca 3%) is present as a polypeptide backbone to which the arabinogalactan blocks are connected. In a oil-water mixture, the protein part will absorb onto the hydrophobic oil surface, while the hydrophilic carbohydrate parts are in the water phase. This behavour leads to a strong emulsifying property of Gum Arabic.

Which modification could you think of to make the highly insoluble and crystalline celluose into a soluble, viscosifying polymer?

Carboxymethyl cellulose. After swelling the cellulose, the carboxy methyl groups will be added to the backbone. The charges and consequently, their repulsion in solution will make cellulose quite soluble. The viscosity can be tailored by the level of substitution.

Which derivatisation step has to be performed to make Micro Crystalline Cellulose out of cellulose? And what will happen during that step?

Acid treatment at higher temperatures to cleave the amorphous parts of the cellulose crystals. Depending on the next step, often freeze drying or separation of the particles by high shear, you will obtain MCC having flavour carrying properties, foam stabilizer, emulsifier.

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Polysaccharides - Exercises

20 important questions on Polysaccharides - Exercises

Which pectin makes a good acid-sugar gel and why?

Why is alginate a suitable polysaccharide to form a strong and heat stable gel which can set at low temperatures?

What kind of alginate gel will be the strongest?

Why can kappa- and iota-carrageenan be considered as modified natural polysaccharides? What will be the modification?

Why will a solution of lambda-carrageenan be more stable upon heating at pH<4, compared to kappa- and -iota carrageenan?

Kappa-carrageenan and iota-carrageenan’s properties fully depends on the type and number of counter ions present. Which salt and which carrageenan would be chosen when a brittle gel is needed? Which combination of salt and carrageenan would result in a very elastic gel?

Kappa-carrageenan can interact with protein in several ways. Please explain (draw) the interaction at both low and high pH.

Please explain why both self-association and intermolecular interaction mechanisms play a role in the stabilisation of cholate milk with kappa-carrageenan.

Please explain why the maximum substitution level for acetylated starch could be 3, but for hydroxypropyl starch much higher. Hint: think about the molecular structure of the two modified starches.

Two modified potato starches are synthesized: an acetylated starch and a hydroxypropyl starch.Is there any diference in hydrophobicity between the starches? Please explain.

Two modified potato starches are synthesized: an acetylated starch and a hydroxypropyl starch. Which of the two starches will be more stable in gelling behaviour after being cooked in a milk product pH 6.5? Why?

Cellulose, xanthan and xyloglucan all consist of a backbone of beta 1-4 linked glucose residues. Please indicate the source of these three polymers.

Cellulose, xanthan and xyloglucan all consist of a backbone of beta 1-4 linked glucose residues. Which polymer in solution/suspension would be more susceptible for the salt level in the liquid product? Why?

What is the origin of gum Arabic?

Due to its unique solubility properties Gum Arabic is frequently used in confectionery. Please explain why.

Another important application of Gum Arabic is its use in aroma flavored beverages as well as oil- water emulsions. Please explain both applications on basis of its structural features.

Which modification could you think of to make the highly insoluble and crystalline celluose into a soluble, viscosifying polymer?

Which derivatisation step has to be performed to make Micro Crystalline Cellulose out of cellulose? And what will happen during that step?

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Two modified potato starches are synthesized: an acetylated starch and a hydroxypropyl starch.
Is there any diference in hydrophobicity between the starches? Please explain.