Immobilization in carrageenans is a method of gel entrapment. It is one of the most widely used methods for cell immobilization because it is cheap, simple, and reproducible with mild conditions during the immobilization. The major advantages of the entrapment method are high cell density, mild immobilization conditions, and low risk of loss of cells from the support (seeChapter 1 ). Immobilization in carrageenans is carried out in a similar way to that for alginate (seeChapter 7 ). The principle of gel entrapment is that biocatalysts are mixed with a pregel solution, and after gelification the biocatalysts are enclosed in the gel material. The carrageenan concentration depends on the type of carrageenan used, but has to be sufficient to produce a firm gel. The mixture of biocatalyst and carrageenan solution is extruded dropwise through an orifice (extrusion method) or hollow needle (dripping method), or is dispersed in liquid or air (dispersion method). Various shapes (cubes, beads, or membranes) of immobilized biocatalysts can be tailored for particular applications, e.g., spherical beds shown good flow properties in a packed-bed reactor. Gelification of carrageenan takes place when the solution is cooled down to the appropriate gelification temperature, but only in the presence of gel-inducing agents, usually cations, such as K+ . Generally, enzyme activities and yields of immobilized cells are relatively high. Gel formation is thermally reversible and gels may soften or disintegrate at elevated temperatures, and consequently are not well suited to applications involving higher temperature reactions, such as those employing thermophilic microorganisms (1 ).
