Esterification, alginate

Table III. Effect of buffer treatment and esterification of alginate on the mechanical stability of chitosan/alginate coacervate capsules3...

Pectin consists predominantly of sequences of galacturonic acid residues (which are quite similar to the G units in alginate), with occasional interruptions by rhamnose residues. At least some of the carboxyl groups are methyl esterified, the precise distribution depending upon the plant source and age, and an important aspect still not fully understood. Reasonably in view of their structural similarity, pectins of low degree of esterification behave like alginates, and gel with divalent ions. The more esterified materials gel under conditions of low pH and decreased water activity, Le. where intermolecular electrostatic repulsions are reduced in this case the junction zones are thermoreversible at, say, 40 C. 

In contrast, low-methoxy (LM) pectins gel in the presence of divalent cations, especially calcium, by the egg box mechanism proposed for alginates. Moreover, calcium pectinate gels prepared at neutral pH are heat stable, whereas acidic pH gels are thermo-reversible. Gel strength depends on the extent of esterification (levels from 30% to 50% are optimal), the distribution of ester groups on the chain, and the average molecular weight. LM pectins have been used traditionally in antidiarrheal formulations with kaolin. HM pectins were evaluated in controlled release matrix formulations. Pectin microspheres were reported to improve ophthalmic bioavailability of piroxicam in rabbits compared with commercial piroxicam eye drops. ... 

Carboxylic acid-containing polysaccharides can be easily functionalized by reactions such as esterification, amination, or amidation. Industrial esterification of polyuronides, including alginic acid (O Chap. 6.4), in aqueous medium can be easily accomplished by reaction with ethylene, propylene, butylene, andpentylene oxides in 50-80% yield [123]. 

Solubility soluble in dilute organic acids and water, forming stable, viscous, colloidal solutions at pH 3. Depending upon the degree of esterification, propylene glycol alginate is also soluble in aqueous ethanol/water mixtures containing up to 60% w/w of ethanol (95%). 

In the present work we have studied the dynamics of formation and structure of the air-water interface in the presence of (3-lg - - PS at 20°C and at pH 7 in a drop tensiometer. As PS with interfacial activity we have used propylene glycol alginates (PGA). To evaluate the effect of the degree of PGA esterification and viscosity, different commercial samples were studied. Xanthan gum (X) and X-carrageenan (X-c) were studied as nonsurface active polysaccharides. 

Degree of Esterification and Viscosity of Propylene Glycol Alginates... 

Baeza, R., Carrera, C., Pilosof, A.M.R., and Patino, J.M.R. Interfacial and foaming properties of propylenglycol alginates. Effect of degree of esterification and molecular weight. Colloids Surf. B Biointerfaces, 36, 139, 2004b. 

Scheme 7.10 Esterification of alginate with alkyl bromide...

Alginate is a polymer composed of uronic acid monomers. While this acidic polysaccharide can be recovered from bacteria, the commercial source is brown seaweed. Both propylene glycol esterification and acetylation of the polymer cause an increase in the thickening capabilities of the gum. The acetylation of alginate by pseudomonal species demonstrates alternative biochemical methods for polysaccharide modification. 

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