Gelling polysaccharides

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Gelling polysaccharides

• What is a gel• Look at

– Alginates– Pectin– Carrageenans

– Synergy• Xanthan• LBG

• Mechanisms for gelation

Notes can be found on ;

sbw5f/APPS/APPS/WINAPPS/Data/Slides and Lectures/SEHill/INDEX.HTM

http://webct.nottingham.ac.uk/webct/urw/lc4130001.tp0/cobaltMainFrame.dowebct ??????????????????????

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Gelation of proteins

• Said to occur when a small amount of solid is dispersed in a relatively large amount of solvent (usually water), by the property of mechanical rigidity.

• Defined as a protein aggregation phenomenon – attractive and repulsive forces are so balanced that a well ordered tertiary network or matrix is formed.

• Protein gels are composed of three dimensional matrices or networks of interwined, partially associated polypeptides in which water is entrapped.

• Is a continuous network of macroscopic dimensions immersed in a liquid medium and exhibiting no steady flow.

XPolysaccharides

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Gels

X

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Structure and Gels

Retorted gels 0.4% locust bean gum/0.4% carrageenan

Total 0.8% polysaccharide

Egg white ~12% protein

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Gel structures

Aggregates of spherical particles Framework of Rod-like particles

Physical gel with crystalline junctions Chemical gel -covalent junctions

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Structure of the polysaccharide

• Change temperature• Change solvent quality• Change ionic environment

It’s what happens to amylose

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Carrageenan (E407)

Red seaweed extract (Rhodophyceae)

iota carrageenanlambda carrageenankappa carrageenan

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1- 4-linked--D-galactopyranose

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kappa

lamda

1-3-linked-b-D-galactopyranose

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Thermoreversible gels Kappa better gel former than iota

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Agarose

seaweedgalactose residuessulfatedmore sulfate less well it gels

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Importance of ions

• General “salt” effect• Specific effects

For example:K+, Rb+, Cs+ favour gelation of both kappa and iota

Carrageenan

13Ion

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Gel FormationAssociation of chains (junction Zones) in order to produce a permanent network

Diverse models for gel formation:

Models proposed for carrageenan

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Image size 0.8 x 0.8 m

Atomic force microscopy

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Alginate

Mannuronic acid Guluronic acid

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Gelation of alginates

• High M-alginates form turbid gels low elastic modulus

• High G alginates: stiff, transparent, brittle gels• Gelation depends on cation• Ba2+ > Sr2+ > Ca2+ > Mg2+

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Pectin

a core chain of alpha (1,4)-linked D-galacturonic acid units interspersed with some L-rhamnose

R= rhamnoseU= galacturonic acid

About 40-100

Branched structureNeutral sugars alternate

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galacturonic acid forming cells for cations

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Pectin stable at low pH

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• Pectin with degree of esterification > 50% is referred to as high ester pectin.

• High ester pectins gel in the presence of high concentrations of cosolutes (e.g. 60% sugar) and at pH values < 3.4.

• Rapid set pectins have DE ~70% and slow set pectins have DE ~65%.

• Gelation is believed to occur through association of the pectin chains by hydrophobic bonding.

• Gels are thermally irreversible.

High ester pectin Low ester pectin

• Low ester pectins have DE < 50%.

• Low ester pectins gel in the presence of calcium ions. The reactivity increases as DE decreases.

• Gelation occurs as a consequence

of calcium ion crosslinking.

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Mixed gels

• Often more than one polymer exists• This can enhance to reduce gel quality

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Two component gel types

Swollen network

Interpenetrating network

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Coupled network

Phase separated network

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Gelation in Synergistic mixed polysaccharide gels

Locust bean gum gelling with carrageenan

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Xanthan galactomannan gels

?

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Firm, Brittle

Xanthan/LBG

GelatinHigh “M” Alginate

PectinHigh “G” Alginate

-CarrageenanAgar

High Acyl Gellan Gum

Low Acyl Gellan Gum

Gel Tex

ture

s

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Useful references

http://www.lsbu.ac.uk/water/

E-learning hydrocolloid program on Blackboard

Journals: Food Hydrocolloids and Carbohydrate Polymers Series of Books: Gums and Stabilisers for the Food Industry Book :Functional Properties of Food Macromolecules (Chapter by Morris on gelation)

Anything in the TX55-, QD4--, QP7-- section of the library