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

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.

Gels X

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

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

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

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

1- 4-linked- -D-galactopyranose

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

Thermoreversible gels Kappa better gel former than iota

Agarose seaweed galactose residues sulfated more sulfate less well it gels

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

Ion

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

Atomic force microscopy Image size 0.8 x 0.8 m

Alginate Guluronic acid Mannuronic acid

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+

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

galacturonic acid forming cells for cations

Pectin stable at low pH

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.

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

Two component gel types Swollen network Interpenetrating network

Phase separated network Coupled network

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

Xanthan galactomannan gels ?

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

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