Gelling polysaccharides
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What is a gel
Look at
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Alginates
Pectin
Carrageenans
– Synergy
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Notes can be found on ;
sbw5f/APPS/APPS/WINAPPS/Data/
Slides and Lectures/SEHill/INDEX.HTM
http://webct.nottingham.ac.uk/webct/ur
w/lc4130001.tp0/cobaltMainFrame.do
webct ??????????????????????
Xanthan
LBG
Mechanisms for gelation
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X
Gelation of proteins
Polysaccharides
• 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.
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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
Physical gel with crystalline junctions
Framework of Rod-like particles
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 carrageenan
lambda carrageenan
kappa 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
seaweed
galactose residues
sulfated
more 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
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Ion
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Gel Formation
Association of chains (junction Zones) in order to produce
a permanent network
Diverse models for gel formation:
Models proposed for carrageenan
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Atomic force microscopy
Image size 0.8 x 0.8 m
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Alginate
Guluronic acid
Mannuronic 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
Branched structure
Neutral sugars
alternate
R= rhamnose
U= galacturonic acid
About 40-100
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galacturonic acid forming cells for cations
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Pectin stable at low pH
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Low ester pectin
High ester pectin
• Pectin with degree of esterification
> 50% is referred to as high ester
pectin.
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Low ester pectins have DE < 50%.
• High ester pectins gel in the
presence of high concentrations of
cosolutes (e.g. 60% sugar) and at
pH values < 3.4.
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Low ester pectins gel in the
presence of calcium ions. The
reactivity increases as DE
decreases.
• Rapid set pectins have DE ~70%
and slow set pectins have DE
~65%.
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Gelation occurs as a consequence
of calcium ion crosslinking.
• Gelation is believed to occur
through association of the pectin
chains by hydrophobic bonding.
• Gels are thermally irreversible.
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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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Phase separated network
Coupled 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
Low Acyl Gellan Gum
Agar
k-Carrageenan
High “G” Alginate
Pectin
High “M” Alginate
Gelatin
Xanthan/LBG High Acyl Gellan Gum
Soft, Flexible
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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
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