Karaya Gum or Kulu is a gum-yielding tree originating from India, where it occurs on dry rocky hills and in the dry deciduous forests in the north and centre of the subcontinent.
It is a medium-sized deciduous tree, typically 9 to 15 m (30 to 50 ft) tall with a short crooked trunk supporting a much branched, rounded crown made up of twisted branches. The bark peels off in transparent papery flakes, exposing smooth creamy-white underbark that contrasts strongly against the usually darker surroundings.
The leaves are large, 23 to 30 cm (9 to 12) long and just as wide with five pointed lobes, resembling a maple leaf. They are clustered at the ends of the branches and with the onset of the dry season change colour from green to yellow then fall to the ground, leaving the branches bare and exposed.
Flowering follows leaf-fall in the dry season with flower clusters arising at the ends of the branches bearing small yellowish flowers. The fertilised flowers are succeeded by star-shaped seedpods with a covering of fine hairs. When young they are yellow-green, with maturity becoming orange or bright red and have small black oval seed inside.
The bark on wounding yields 'Gum karaya' or 'Indian tragacanth', a commercial gum almost equal in importance to Gum arabic (from Senegalia senegal). It is an insoluble gum which, when placed in water swells to a jelly-like mass, though with enough water transforms into a thick, translucent paste. In India, it is mainly employed as a gelling agent in confectionery. Outside of India, it is more commonly used as a substitute for Gum arabic and Gum tragacanth, a gum obtained from several species of Astragalus, which originate in Asia Minor or modern-day Iran, Syria and Turkey.
Gum karaya has application as a thickening agent, gelling agent, stabiliser and emulsifier in a wide range of food, pharmaceutical and cosmetic products.
In food, it is used in bottled sauces and salad dressings, dairy products such as whipped cream and cheese spreads, as well as in sausage meats to improve adhesion between the different ingredients. Its moisture retention and moisture absorption properties also act to slow staling in baked goods and retards ice crystal formation in frozen desserts such as ice-cream, mousse and popsicles.
In cosmetics, it is used in hair sprays, beauty masks, shaving creams, toothpaste and is a common ingredient in denture adhesive powders.
Gum karaya also has application as a dye thickener in the textile industry and is used as a deflocculant and binder in paper manufacture.
Gum collectors in India drill one or two holes in the tree trunk then later collect the exude, which can weigh up to 1 kg (2.2 lbs). A single tree may yield 2 to 5 kgs (4.4 to 11 lbs) of gum in a season but is not tapped every season so that it can have a rest period to recover. The highest yields and best quality gum is reportedly obtained near the end of the dry season, just before the start of the monsoon in its native range.
The wood is medium-weight, averaging about 670 kgs per cubic meter (42 lbs per cubic ft) and is resistant to insect attack, but is not usually harvested except for firewood and for making charcoal.
Gum karaya has laxative activity, due to its ability to absorb large amounts of water and expand up to one hundred times its initial volume.
Grows naturally in sub-humid to moderately humid subtropical and tropical climates with a long dry season, generally in areas with annual lows of 17 to 21 °C, annual highs of 24 to 35 °C, annual rainfall of 300 to 1900 mm and a dry season of 6 to 10 months. However, trees yielding the most gum are found in areas with annual rainfall of 700 to 1300 mm.
New plants are usually started from seed which germinate readily and without the need for any pre-treatment. Seedlings germinate within three weeks and performs best on free-draining loam and sand soils of a moderately acid to slightly alkaline nature, generally with a pH of 5.5 to 7.5 and on sites with full to partial sun exposure. It has poor tolerance to slow-draining or waterlogged soils and to shade conditions.
There does not appear to be any record of it anywhere as a weed or invasive species.
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