Gum arabica

Description

Gum arabic, also known as acacia gum, is a natural gum consisting of the hardened sap of various species of the acacia tree. Originally, gum arabic was collected from Acacia nilotica which was called the « gum arabic tree »;[1] in the present day, gum arabic is predominantly collected from two related species, namely Acacia senegal[2] and Vachellia (Acacia) seyal. Producers harvest the gum commercially from wild trees, mostly in Sudan (80%) and throughout the Sahel, from Senegal to Somalia—though it is historically cultivated in Arabia and West Asia.

Gum arabic is a complex mixture of glycoproteins and polysaccharides. It is the original source of the sugars arabinose and ribose, both of which were first discovered and isolated from it, and are named after it.

Gum arabic is used primarily in the food industry as a stabilizer. It is edible and has E number E414. Gum arabic is a key ingredient in traditional lithography and is used in printing, paint production, glue, cosmetics and various industrial applications, including viscosity control in inks and in textile industries, though less expensive materials compete with it for many of these roles.

While gum arabic is now produced throughout the African Sahel, it is still harvested and used in the Middle East. For example, Arab populations use the natural gum to make a chilled, sweetened, and flavored gelato-like dessert. Gum arabic was defined by the 31st Codex Committee for Food Additives, held at The Hague, The Netherlands, from 19–23 March 1999, as the dried exudate from the trunks and branches of Acacia senegal or Vachellia (Acacia) seyal in the family Leguminosae (Fabaceae)”.
Gum arabic’s mixture of polysaccharides and glycoproteins gives it the properties of a glue and binder that is edible by humans. Other substances have replaced it where toxicity is not an issue, and as the proportions of the various chemicals in gum arabic vary widely and make it unpredictable. Still, it remains an important ingredient in soft drink syrup and « hard » gummy candies such as gumdrops, marshmallows, and M&M’s chocolate candies. For artists, it is the traditional binder in watercolor paint, in photography for gum printing, and it is used as a binder in pyrotechnic compositions. Pharmaceutical drugs and cosmetics also use the gum as a binder, emulsifying agent, and a suspending or viscosity increasing agent.[4] Wine makers have used gum arabic as a wine fining agent.[5]

It is an important ingredient in shoe polish, and can be used in making homemade incense cones. It is also used as a lickable adhesive, for example on postage stamps, envelopes, and cigarette papers. Lithographic printers employ it to keep the non-image areas of the plate receptive to water.[6] This treatment also helps to stop oxidation of aluminium printing plates in the interval between processing of the plate and its use on a printing press. Also called acacia after the original source, gum arabic is used as an emulsifier and a thickening agent in icing, fillings, chewing gum and other confectionery.[7]

Gum arabic is used as a binder for watercolor painting because it dissolves easily in water. Pigment of any color is suspended within the acacia gum in varying amounts, resulting in watercolor paint. Water acts as a vehicle or a diluent to thin the watercolor paint and helps to transfer the paint to a surface such as paper. When all moisture evaporates, the acacia gum typically does not bind the pigment to the paper surface, but is totally absorbed by deeper layers.[8]

If little water is used, after evaporation the acacia gum functions as a true binder in a paint film, increasing luminosity and helping prevent the colors from lightening. Gum arabic allows more subtle control over washes, because it facilitates the dispersion of the pigment particles. In addition, acacia gum slows evaporation of water, giving slightly longer working time.

The addition of a little gum arabic to watercolor pigment and water allows for easier lifting of pigment from paper and thus can be a useful tool when lifting out color when painting in watercolor.[8]

Gum arabic has a long history as additives to ceramic glazes. It acts as a binder, helping the glaze adhere to the clay before it is fired, thereby minimising damage by handling during the manufacture of the piece. As a secondary effect, it also acts as a deflocculant, increasing the fluidity of the glaze mixture but also making it more likely to sediment out into a hard cake if not used for a while.

The gum is normally made up into a solution in hot water (typically 10–25 g/litre), and then added to the glaze solution after any ball milling in concentrations from 0.02% to 3% of gum arabic to the dry weight of the glaze.[9] On firing, the gum burns out at a low temperature, leaving no residues in the glaze. More recently, particularly in commercial manufacturing, gum arabic is often replaced by more refined and consistent alternatives, such as carboxymethyl cellulose (CMC).