Algins (alginates) (2,4—6) are extracted from brown algae (Phaeophyceae). Alginates are salts (generally sodium) and the propylene glycol ester of alginic acid. Alginic acid is a generic term for polymers of d-mannuronic acid and l-guluronic acid (see the article Carbohydrates: classification, chemistry, labeling). In alginic acid molecules, at least three different types of polymer segments exist: poly(/?-d-mannopyranosyluronic acid) segments, poly(a-l-gulopyranosyluronic acid) segments, and segments with alternating sugar units. Ratios of the constituent monomers and ratios of chain segments vary with the source. Alginates are linear polymers. The degree of polymerization (molecular weight) is controlled and varied in commercial products.

Specific properties of algins depend on the percentage of each type of building block. An important and useful property of sodium, potassium, and ammonium alginates is their ability to form gels on reaction with calcium ions. Different types of gels are formed with alginates from different sources. Alginates with a higher percentage of poly(guluronic acid) segments form gels more readily and form the more rigid, more brittle gels that tend to undergo syneresis. Alginates with a higher percentage of poly(mannuronic acid) segments form the more elastic, more deformable gels that have a lesser tendency to undergo syneresis.

Algins are most often used as thickeners and are available in a range of viscosity grades. Algin solutions are pseudoplastic, that is, exhibit shear thinning. Solutions of propylene glycol alginate (PGA, a partial propylene glycol ester) are somewhat thixotropic and much less sensitive to pH and polyvalent cations. The specific properties exhibited by an algin solution depend on the ratio of monomeric units, the concentration and type of cations in solution, the temperature, and the degree of polymerization.

Primary products using sodium alginate are breakfast and cereal bars, fruit fillings, dry mixes for reconstitution with water or milk, and frozen products. Calcium alginate gels are found in structured foods, such as fruit pieces, onion rings, and pimiento strips for Spanish olives. Primary products in which PGA (which can be labeled algin derivative) is used are pourable salad dressings, buttered pancake syrups, sauces, and beverages. It is also used as a beer foam stabilizer. Alginic acid, which swells in water, is used as a tablet disintegrant.

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