The term ‘gums’ refers to a range of natural polymers, mainly polysaccharides, that are widely used in the food industry to control the rheological and organoleptic properties of food products. They are employed to perform a number of functions including the thickening and gelling of products; the stabilization of foams, emulsions, and dispersions; the inhibition of ice and sugar crystal formation; and the controlled release of active compounds such as flavors and antioxidants.
An area of considerable current interest is in the application of gums as edible films, which can be used to coat fruit and vegetables and hence increase their shelf life. Their film-forming properties are also facilitating their application in biodegradable packaging. It is evident that the market is dominated by starch and gelatin.
The four main tree gum exudates are gum arabic, gum karaya, gum tragacanth, and gum ghatti, with gum arabic being, by far, the most important commercially. Gum tragacanth is obtained from shrubs of the Astragalus species notably A. gummifer and A. microcephalus, which are found in the dry mountainous areas in Iran and Turkey. It consists of a water-swellable fraction called tragacanthic acid (or bassorin) (60–70%) and a water-soluble fraction called tragacanthin. The former consists of a main chain of (1,4)-a-D-galactopyranosyl uronic acid residues with branches linked through (1,3)-bD-xylose units terminating in L fucopyranose. The latter is a highly branched arabinogalactan with a main chain of D-galactopyranosyl units either (1,6)- or (1,3)-linked with side chains consisting mainly of L-arabinofuranose but with a small proportion of D-galacturonic acid and L-rhamnose. Gum tragacanth gives rise to high-viscosity solutions even at 1% concentration. The viscosity decreases irreversibly on heating. The gum solution is stable under acid conditions and shows good emulsification characteristics, which has led to its use in salad dressings and sauces. Its cost and availability, however, have meant that it has been largely replaced in these products by other gums, notably xanthan gum. Gum tragacanth is still widely used in decorative icings for cakes.