Calcium alginate gels formation

Calcium alginate is incompatible with alkalis and alkali salts. Propranolol hydrochloride has been shown to bind to alginate molecules, suggesting that propranolol and calcium ions share common binding sites in the alginate chains the formation of the calcium alginate gel structure was impeded in the presence of propranolol molecules. ... 

Small calcium alginate gel microspheres containing entrapped cells are formed. Like the general procedure, some cells protrude out of the surface of the smaller calcium alginate gel microspheres. These gel microspheres are resuspended in alginate solution to repeat the same droplet formation. 

Slightly soluble calcium phosphates can be used to control the availabiUty of Ca " in the formation of calcium pectinate or calcium alginate gels. In this way, optimum performance can be obtained in their use in jams, jellies, aqueous desserts, and so on. The beneficial effects from alginates, combined with phosphates, on the properties of ice cream and frozen desserts have been known for some time [104,105]. 

Fig. 4.18. Schematic representation of a calcium alginate gel (cross-linkage by egg box formation, cf. Formula 4.120 according to Franz, 1991)...

Polysaccharides can also be used to immobilise cells or enzymes, permitting the re-use of the catalyst and continuous flow systems. Alginates have the advantage that gel formation occurs under mild conditions, therefore cells remain viable and enzymes are not denatured but calcium gradually leaches out and the gel dissolves. Gellan or other combinations may prove superior for this application. 

The ability of alginates to form edible gels by reaction with calcium salts is an important property. Calcium sources are usually calcium carbonate, sulfate, chloride, phosphate, or tartrate (20). The rate of gel formation as well as the quality and texture of the resultant gel can be controlled by the solubility and availability of the calcium source. 

Substantial research efforts have been made to relate molecular properties to the structures of the junction zones and provide mechanisms for gel formation. Thus we have models for the formation and alignment of double helices for kappa-carrageenan, formation of triple helices for gelatin and egg box models for calcium induced gelation of alginate and pectin to mention a few examples (Morris 1986 Djabourov... 

Gel formation is via ion exchange of sodium in serum for calcium within the alginate dressing. A biodegradable gel is formed when the fiber is in contact with exudate, and the released calcium contributes to the clotting mechanism. The gel may be firm or soft depending upon the proportions of calcium and sodium in the fiber. It is removed with saline. 

Calcium is coordinated between certain uronic acid-containing polysaccharides (VIII), which can explain the tight binding of calcium and other multivalent ions in polysaccharide stmctures, and also how bivalent ions can induce gel formation in acidic polysaccharides such as alginic acid solutions. 

Although this article deals with complexes of neutral carbohydrates, brief mention should be made of the aldonate and alduronate anions (the undissociated acids do not form complexes), which show complexing behavior similar to those of the neutral polyols. Interest in these acids, and particularly in the poly(hexuronates), is due to the industrially important gel-formation of alginic acid and pectin with calcium ions, caused by complex-formation. There is an extensive literature on this subject,which is not fully discussed here. 

A number of hydroxylated water soluble polymers were examined as coreactants with polymer 52 in the absence of calcium alginate, and were judged on the basis of the rate of gel formation and the physical properties of the gel These polymers included sodium alginate, polyvinyl alcohol, and copolymers of HEMA with MAA. Of the polymers tested, best results were obtained with polymer 10a, a copolymer of HEMA with a mole fraction of about 10% MAA, which rapidly produced an elastic gel on exposure to polymer 52 in solution. Simple condensation of the carboxyls in polymer 10a with the epoxide functionality was ruled out as a competing reaction due to the measurable but slow reaction between polymer 52 and poly methacrylic acid. It is, therefore, likely... 

Hydrogel formation may be best rationalized, with reference to Fig. 33. The rapid formation of calcium alginate is accompanied by a slower reaction of polymer 10a (a) and 52 (b) at the surface of the gel (Fig. 331 II). The hydrogel coating eventually becomes a diffusional barrier to further reaction (Fig. 33 III) and can be recovered as an inverse of the alginate matrix by treatment with citrate ion (Fig. 33 IV). 

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