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Agar polymers synthesized by species of the genus Gracilaria constitute a complex mixture of molecules, containing several extremes in structure. Sulphate hemi-esters, methyl ethers and pyruvic ketals can alter in a number of ways the structural regularity of agar based on strictly 3-O-linked /I-L-galactopyranose and 4-O-linked a-L-galactopyranose residues. In comparison with agars from Gelidium and Pterocladia, agars from Gracilaria can have higher degrees of sulphation, methoxylation and pyruvylation. The gelling ability of agars from most of Gracilaria species is considerably improved by adopting, before extraction, an alkali pretreatment which converts a-L-galactose 6 sulphate into 3,6-anhydro-a-L-galactose. Native agars obtained from Gracilaria cannot be classified, with few exceptions, as bacteriological grade agar as they have a high content of methoxyls and consequently high gelling temperatures. On the contrary, the genus Gracilaria is considered the most important source of food and sugar-reactive grade agars.Among techniques which can be used to study algal polysaccharides, combined H and 3 C nuclear magnetic resonance spectroscopy represent the most effective and powerful method for the investigation of the chemical structure of agarocolloids.

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Summary The encapsulation of chemicals and biological materials in natural carrier polymers has proven to be of great use in a variety of scientific and industrial applications. The natural polymers used as carrier materials in the encapsulation technology have the great advantage of being nontoxic, biocompatible and biodegradable. The most used natural polymers are algal polysaccharides such as alginate, carrageenan and agarose. Chitosan, an aminopolysaccharide derived from chitin, and gellan gum, an extracellular anionic polysaccharide secrete from microorganisms, have also been proposed for some applications. Chemical and physical properties, such as monomeric composition, degree of substitution, kinetic of gel formation, gel strength and shrinkage of matrices produced with such polysaccharides, can greatly affect both cell viability and rate of release of drugs, vaccines and other molecules entrapped in the gel matrix.