Gellan

Gellan, known commercially as gellan gum, is an extracellular microbial polysaccharide produced by the organism Sphingomonas elodea (5). It is composed of a linear tetrasaccharide repeating unit of d-glucose-d-glucuronic acid-d-glucose-l-rhamnose. Two forms of gellan gum are available—the native acylated form and a deacylated form. Cooling of hot solutions of the low-acyl gellan (gum) forms firm and brittle gels of texture similar to that of agar and K-carrageenan gels.

Gellan gum requires cations for gelation; but unlike other ion-sensitive gelling polysaccharides, it gels with a wide variety of cations, including hydrogen ions. Divalent ions such as Ca2+ are, however, much more efficient at forming gels than are monovalent ions such as Na+ and K+. Since divalent cations decrease solubility, sequestering agents are required if such cations are present initially. Like agar gels, gellan gum gels exhibit marked setting and melting temperature hysteresis; and by selection of the ion and its concentration, both temperatures can be controlled.

Gellan gum is functional in milk, although a calcium ion-sequestering agent is normally required. It can be used in confectioneries; structured foods; pie and bakery fillings; bakery icings, frostings, and glazes; dairy products; water-based gels; and films and coatings. Generally, gellan gum is blended with other gums or ingredients to modify its performance.

GUAR AND LOCUST BEAN (CAROB) GUMS

Commercial guar gum (2,4-6) is the ground endosperm of seeds of the guar plant. The guar plant resembles the soy plant, and guar seeds are produced much as are soy beans. Commercial guar gum usually contains 80 to 85% polysaccharide, 10 to 14% moisture, 3 to 5% protein, 1 to 2% fiber, 0.5 to 1.0% ash, and 0.4 to 1.0% lipid.

Guaran is the purified polysaccharide from guar gum. It is composed of D-galactose and D-mannose and is, therefore, a galactomannan. It has a linear backbone chain of /?-D-mannopyranosyl units, approximately 56% of which (on average) are substituted with an a-D-galactopyranosyl unit, giving a comblike structure (Fig. 3). The mannan chain is rather evenly substituted with D-galactopyranosyl units, but in a somewhat random manner.

Like guaran and the endosperm polysaccharides of other legumes, locust bean (carob) gum (LBG) is also a galactomannan. Like guaran, it has a linear mannan backbone. However, in locust bean gum, on average only approximately 20% of the /?-D-mannopyranosyl units are substituted with an a-D-galactopyranosyl unit and the branched units are clustered. The locust bean gum molecule contains "smooth" regions that contain no a-D-galactopyranosyl side-chain units and "hairy" regions in which most main-chain units contain single-unit a-D-galactopyranosyl branches.

Commercial locust bean gum is the ground endosperm of the seeds of the locust (carob) tree. The tree, which grows primarily in the Mediterranean region, is slow to mature and does not begin to bear until it is about 15 years old. Hence, the supply of locust bean seeds is limited and is not expected to increase. Locust bean gum usually has about the same composition (polysaccharide, moisture, protein, fiber, ash, lipid content) as does guar gum.

Guar gum forms very high viscosity, slightly pseudoplastic solutions at low concentrations. Because commercial guar gum contains protein, fiber, and lipids, its solutions are cloudy. Particle size is important in the use of guar gums. Coarse granulations are used for rapid, easy dispersion; fine granulations give rapid hydration. All forms will develop some additional viscosity after their dispersions have been heated. Guar gum is nonionic, so the viscosity of its solutions is not greatly affected by pH. It hydrates most rapidly at pH 6 to 9.

Locust bean gum has low cold-water solubility and is generally used when delayed viscosity development is desired. Only when dispersions of locust bean gum are heated (eg, to 85°C [185°F]) and cooled is high viscosity obtained. The general properties of locust bean gum are similar to those of guar gum. A difference is its synergism with k-carrageenan, agar, and xanthan gum, with which it forms gels. Guar gum has only a weak synergistic effect on the viscosity of xanthan gum solutions and an even weaker effect on carrageenan and CMC solutions.

Guar gum is used in foods as a thickener and a binder of water. It is often used in combination with other gums, for example, with carrageenan and/or xanthan plus locust bean gum, particularly in dairy products such as ice creams, frozen novelties, whipped toppings, sour cream, cottage cheese, and low-fat yogurts. It is used with xanthan gum in pickle relish and sauces such as pizza and Sloppy Joe sauce to bind and control water. Both guar gum and locust bean (carob) gum are used (often in combination with carrageenan or xanthan gum) in stabilizers for ice cream. Other applications of these blends are in the manufacture of canned frostings, dips, cream cheese, cottage cheese dressings, and other processed cheese products. Guar gum is used in instant hot cereals and dry soup and other mixes, egg substitutes, dipping batters, pimiento strips for Spanish olives, sauces, condiments, and pet foods.

Two forms of enzymically modified guar gum are produced. One is a depolymerized guar gum designed to be a noncaloric bulking agent and a less viscous source of dietary fiber. The other is a guar gum that has had some of the D-galactosyl units removed to give it properties more like those of the more expensive locust bean gum.

Locust bean gum is almost always used in combination with one or more other gums in a variety of dairy products. In combination with xanthan gum and starch, it is used to make chewy fruit confections. Locust bean gum-carrageenan blends are used in pet food gels.

Tara gum is a galactomannan obtained from seeds of the tara shrub, which grows in northern regions of South America and Africa. It has a D-galactosyl unit:D-mannosyl unit ratio between those of guar gum and locust bean gum, aGalp aGalp

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