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Gelling polysaccharides

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Presentation on theme: "Gelling polysaccharides"— Presentation transcript:

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 ??????????????????????

2 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.

3 Gels X

4 Structure and Gels Retorted gels 0.4% locust bean gum/0.4% carrageenan
Total 0.8% polysaccharide Egg white ~12% protein

5 Gel structures Aggregates of spherical particles
Framework of Rod-like particles Physical gel with crystalline junctions Chemical gel -covalent junctions

6 Structure of the polysaccharide
Change temperature Change solvent quality Change ionic environment It’s what happens to amylose

7 Carrageenan (E407) Red seaweed extract (Rhodophyceae) iota carrageenan lambda carrageenan kappa carrageenan

8 1- 4-linked- -D-galactopyranose

9 1-3-linked-b-D-galactopyranose
kappa lamda 1-3-linked-b-D-galactopyranose

10 Thermoreversible gels
Kappa better gel former than iota

11 Agarose seaweed galactose residues sulfated more sulfate less well it gels

12 Importance of ions General “salt” effect Specific effects For example:
K+, Rb+, Cs+ favour gelation of both kappa and iota Carrageenan

13 Ion

14 Gel Formation Association of chains (junction Zones) in order to produce a permanent network Diverse models for gel formation: Models proposed for carrageenan

15 Atomic force microscopy
Image size 0.8 x 0.8 m

16 Alginate Guluronic acid Mannuronic acid

17 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+


19 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


21 galacturonic acid forming cells for cations

22 Pectin stable at low pH

23 Low ester pectin High ester pectin
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. 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.

24 Mixed gels Often more than one polymer exists
This can enhance to reduce gel quality

25 Two component gel types
Swollen network Interpenetrating network

26 Phase separated network
Coupled network

27 Gelation in Synergistic mixed polysaccharide gels
Locust bean gum gelling with carrageenan

28 Xanthan galactomannan gels

29 Gel Textures 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

30 Useful references
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--, QP section of the library

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