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/cob
altMainFrame.dowebct ??????????????????????
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Gelation of proteins
X
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
Framework of Rod-like particles
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Structure of the polysaccharide

• Change temperature
• Change solvent quality
• Change ionic environment

Its 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 m
ore 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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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
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 gt Sr2 gt Ca2 gt Mg2

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Pectin
a core chain of alpha (1,4)-linked D-galacturonic
acid units interspersed with some L-rhamnose
R rhamnose U galacturonic acid
Branched structure Neutral sugars
alternate
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 gt 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 lt 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.

• Low ester pectins have DE lt 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
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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
Gel Textures
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