GUM KARAYA (Sterculia Gum)

Identity & Origin
Gum Karaya, sometimes known as Sterculia gum, is the dried exudation of the Sterculia Urens tree and other species of Sterculia. The tree is native to India.

Physical Characteristics
The highest grade sorts of Gum Karaya are white, translucent and almost free of bark. The lower grades vary from light yellow to brown and may contain as much as 3% of insoluble impurities. Powdered Gum Karaya is white to greyish white.

Solubility
Gum Karaya, like Gum Tragacanth, does not dissolve in water to give a clear solution but rather forms a colloidal sol. Powdered gum karaya swells in cold water to an extent that a 3% to 4% sol will produce a heavy gel of uniform smoothness and texture. For higher concentrations it is necessary to cook the gum under steam pressure to make it soluble. A 20% to 25% solution may be prepared in this manner. It yields a thick, syrup-like liquid. Gum Karaya will form viscous sols in hydroalcoholic solutions ranging up to 60% to 35% alcohol concentration.

Viscosity
The viscosity of Gum Karaya is largely dependent on its freshness, that is, how recently it was gathered from the trees. The viscosity is affected by conditions of climate and growth. Viscosity is also affected by storage. Powdered Karaya will show a decrease in viscosity after storing over 6 months. Gum Karaya sols are sensitive to alkali and attain their maximum viscocity at pH 8.5. Above that pH the sols tend to become stringy.

Chemical Characteristics
Gum Karaya is a complex polysaccharide of high molecular weight. A molecular weight as high as 9.500.000 has been reported. On hydrolysis it yields galactose, rhamnose and galacturonic acid. Gum Karaya occurs as a partially acetylated derivative. The acid number has been found to vary from 13.4 to 22.7. The variation in acid number is influenced not only by the source of the sample but also by its age. The gum has a peculiar property of splitting off free acetic acid and this loss is loosely correlated with the particle size. Trimethylamine has also been identified in the hydrolysis products. Gum Karaya contains 12% to 14% moisture and less than 1% acid insoluble ash.

pH
The pH of a 1% Gum Karaya solution is 4.6. If small amounts of alkali are added to change the pH to 7 or 8, the gum tends to have a buffering action effect and will gradually reduce the pH again to the acid size.

Compatibility
Gum Karaya is compatible with other plant hydrocolloids as well as proteins and carbohydrates. There is apparently an incompatibility of Gum Karaya gels with pyrilamine maleate, a strong hydrotrope and antihistaminic. Electrolytes also cause a viscosity drop well as excessive acid. Alkalis cause the gels to become stringy.

Preservatives
Gum Karaya sols and jellies require preservatives since they are subject to bacterial attack. They are easily preserved with a mixture at a maximum of 0,17% methyl and 0,03% propyl o-hydroxybenzoate as well as with glycerin and propylene glycol. Benzoic acid as well as sodium benzoate at 0.1% concentration, will effectively preserve Karaya sols.

Uses Foods
The food industry consumes less than 50% of all the Gum Karaya. Its main function is to impart stability through binding and emulsifying properties. At 0.2 - 0.4%, Karaya prevents the bledding of free water and the formation of large ice crystals in ice pops, water ices and sherbets. Its excellent water-absorbing and water-holding properties together with its excellent acid compatibility make it suitable for this use. Karaya's acidic nature is not objectionable in cheese spreads and prevents water separation and promotes ease of spreading when used at up to 0.8%. Karaya has been used as a stabilizer in regular whipped cream and in imitation whipped cream. In addition to stabilizing meringue powders through its binding properties, Karaya enables a greater volume of meringue to be produced from a fixed amount of protein. In combination with Carrageenan, Karaya retards staling of bread and doughnut mixes and other bakery goods and improves the tolerance of the dough to overmixing. The preferred level of addition is 0.1-0.9% karaya with 0.02-0.1% Carrageenan. In meat processing, Karaya has been used at 0.25-1.0% as an emulsifier and binder. It functions by absorbing water resulting from the ice used in chopping and by emulsifying the protein, fat and moisture to give a cohesive, smooth appearance to the finished product.

Pharmaceutical Industry
A large part of the Karaya is used in two products. In the first product, bulk laxative, Karaya is usually processed 8-30mesh in size. In absorbing water the coarse particles swell enormously, forming a discontinuous type of mucilage that is very effective as a laxative.

The second important product is a denture adhesive in which the finely powdered gum is dusted on the dental plate and swells when it touches the moist surface of the gums. This gives a comfortable and tight fit of the plate.

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