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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/

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Presentation on theme: "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/"— Presentation transcript:

1 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/ur w/lc4130001.tp0/cobaltMainFrame.do webcthttp://webct.nottingham.ac.uk/webct/ur w/lc4130001.tp0/cobaltMainFrame.do webct ??????????????????????

2 2 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. X Polysaccharides

3 3 Gels X

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

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

6 6 Structure of the polysaccharide Change temperature Change solvent quality Change ionic environment Its what happens to amylose

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

8 8 1- 4-linked- -D-galactopyranose

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

10 10 Thermoreversible gels Kappa better gel former than iota

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

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

13 13 Ion

14 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 15 Image size 0.8 x 0.8 m Atomic force microscopy

16 16 Alginate Mannuronic acidGuluronic acid

17 17 Gelation of alginates High M-alginates form turbid gels low elastic modulus High G alginates: stiff, transparent, brittle gels Gelation depends on cation Ba 2+ > Sr 2+ > Ca 2+ > Mg 2+

18 18

19 19 Pectin a core chain of alpha (1,4)-linked D-galacturonic acid units interspersed with some L-rhamnose R= rhamnose U= galacturonic acid About 40-100 Branched structure Neutral sugars alternate

20 20

21 21 galacturonic acid forming cells for cations

22 22 Pectin stable at low pH

23 23 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.

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

25 25 Two component gel types Swollen networkInterpenetrating network

26 26 Coupled network Phase separated network

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

28 28 Xanthan galactomannan gels ?

29 29 Soft, Flexible Firm, Brittle Xanthan/LBG Gelatin High M Alginate Pectin High G Alginate -Carrageenan Agar High Acyl Gellan Gum Low Acyl Gellan Gum Gel Textures

30 30 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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