Gel Formation
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Transcript of Gel Formation
1
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
?
29Soft, Flexible
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