N

Figure 4. Repeating unit of melanoidin. R-NH2 = amine, R' = H or CH2OH. Source: Ref. 18.

curs with the subsequent aggregation of these helices providing cross-linking junctions to build a continuous network (21,22). The number of charged sulfate groups along the polymer contributes to the degree of aggregation and the characteristics of the gel.

Carrageenan forms stable complexes with K-casein via the interaction between the sulfate anions and the highly positively charged region of the protein. The synergistic interaction between /c-carrageenan and locust bean gum has also been interpreted on the basis of junction cross-linking.

High methyoxy pectin gels at low pH and in the presence of a cosolute such as sucrose. At low pH, the carboxyl groups are protonated, causing a decrease in electrostatic repulsion. Addition of a cosolute lowers the water solvation of the polymer. Both factors increase hydrophobic interaction and association of the polymers into cross-linking junction zones (23).

Figure 5. The "egg box" model. Line = alginate polymer; dark circle = calcium ion. Source: Ref. 20.

Starch gelation represents a more complex system, which has received continuous attention from various disciplines. The structure of crystalline amylose (of B-form starch) as originally elucidated by X-ray diffraction consists of two parallel strands of right-handed sixfold helices packed in an antiparallel double helix (24). More recent work has suggested a parallel packing of left-handed, parallel-stranded double helices for crystalline amylose from both A- and B-form starch (25,26). In solution, the conformation of amylose assumes a random coil containing short segments of loose and irregular helical structure (27,28).

Various models have been proposed for the structure of amylopectin. Most investigations seem to support a cluster-type model in that amylopectin is composed of clusters of oriented chains with the branching points collected together toward the reducing end (29).

Gelation occurs when starch granules in suspension are heated above the gelatinization temperature and cooled. Heating causes the granules to swell irreversibly, accompanied by the solubilization of amylose while most of the amylopectin is retained. On cooling, the starch gel formed has a composite structure of swollen amylopectin granules distributed in a matrix of amylose gel (30). Rétrogradation involves crystallization of both the amylose in the gel phase and the amylopectin in the granules. The amylose molecules associate through hydrogen bonding into an insoluble precipitate. Amylopectin also exhibits interchain association. However, amylopectin molecules have an average chain length of 20 to 25 and a degree of polymerization of approximately 10s. Interchain association between the polymers can only extend for 15 to 20 glucose units before being interrupted by the branching points. Crystallization of the amylopectin fraction is slow and results in a gradual increase in the rigidity of the granules and hence the amylose gel matrix. Reversion of a starch gel to the granular crystalline state is termed rétrogradation, a process chiefly responsible for the staleness of bread.

It is important to note that in a gel the intermolecular association of polymers usually involves extensive segments of the polymeric chains held together by hydrogen bonding, electrostatic forces, hydrophobic or ionic interactions to form cross-linking junctions. Depending on the gel, a variety of rheological properties can exist. The entire field of polysaccharide gels has gone through a rapid ex pansion in the scope of basic research and intensified investigation in the relationship between the molecular arrangement and gelling characteristics.

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