Acacia (/?? ke??? / or /?? ke? si? /), also known as a thorntree, whistling thorn or wattle, is a genus of shrubs and trees belonging to the subfamily Mimosoideae of the family Fabaceae, described by the Swedish botanist Carl Linnaeus in 1773 based on the African speciesAcacia nilotica. Many non-Australian species tend to be thorny, whereas the majority ofAustralian acacias are not. All species are pod-bearing, with sap and leaves often bearing large amounts of tannins and condensed tannins that historically found use as pharmaceuticals and preservatives.
Other uses: Acacias were purposely introduced and planted in Southeast Asia and Oceania as a source of firewood and good quality charcoal (does not smoke), as well as timber for furniture and pulp for making paper (acacia produces high yields of pulp and produces strong paper). The tannin produced from the tree is of a good quality, but tends to redden with exposure to sunlight. The tree was also introduced as an ornamental shade tree, but in Singapore, it is no longer grown as a wayside tree due to the large amount of litter of "leaves", flowers and fruits that the tree produces.
In India, the tree was cultivated to feed the lac insect, which produces a resinous secretion that is harvested to produce lacquer. Acacia has the potential to protect poor soils from erosion and revive their mineral content. Acacia can grow on poor soils including clay, limestone and unstable sand dunes, even soil tainted with uranium wastes. It is also able to survive fire, dry spells and seasonally waterlogged soil. (In fact, the seeds germinate better when placed in hot ashes! ). The tree also contains nitrogen fixing bacteria which can help rejuvenate these poor soils.
The tree prevents soil erosion because of their extensive and dense roots and heavy leaf litter. But the seedlings don't grow well in the shade and in competition with weeds, so for deliberate planting, the seedlings have to raised elsewhere first. Ripening agents speed up the ripening process. They allow many fruits to be picked prior to full ripening, which is useful, since ripened fruits do not ship well. For example, bananas are picked when green and artificially ripened after shipment by being gassed with ethylene.  Calcium carbide is also used for ripening fruit artificially in some countries.
When calcium carbide comes in contact with moisture, it produces acetylene gas, which is quite similar in reaction to the natural ripening agent ethylene. Acetylene acts like ethylene and accelerates the ripening process, but is inadvisable because calcium carbide has carcinogenic properties.  Industrial-grade calcium carbide may also contain traces of arsenic andphosphorus which makes it a human health concern.  The use of this chemical for this purpose is illegal in most countries.  Catalytic generators are used to produce ethylene gas simply and safely.
Ethylene sensors can be used to precisely control the amount of gas. Covered fruit ripening bowls are commercially available. The manufacturers claim the bowls increase the amount of ethylene and carbon dioxide gases around the fruit, which promotes ripening. Climacteric fruits are able to continue ripening after being picked, a process accelerated byethylene gas. Non-climacteric fruits can ripen only on the plant and thus have a short shelf life if harvested when they are ripe. Chemistry of ripening During ripening, the starch in the fruit breaks down to form sugar.
The fruit skin changes colour, going from green to a bright red (apples & tomatoes) or yellow (mangoes, bananas and lemons). These changes attract birds and animals, who love to eat the sweet flesh. They throw away the seeds, which fall on the ground and germinate. The ripening of a fruit depends on the season. For example, we get mangoes only in summer and apples only in winter. The plant can detect changes in season by changes in temperature and humidity. It then produces ethylene which spreads across the plant. When ethylene reaches the fruit, it sends a signal to all the cells in the fruit.
The cells then make enzymes that break starch into sugar. The cells in the skin start making pigments which give the fruit its colour. When a fruit ripens, it still has a lot of acid. But there is so much sugar in it, that it masks the sour taste of the acid! Artificial ripening Ripe fruits cannot be stored and transported for a long time. Hence farmers in fruit orchards pluck the fruits raw. The natural ethylene in the fruits makes them ripen, so that they are ready to eat by the time you buy them. Sometimes the fruits are not yet ripe when they are ready to be sold. Hence they have to be artificially ripened.
Fruits are kept in hay-lined wooden boxes called crates. These crates are stacked on shelves and a wood fire is lit below them. The smoke contains ethylene and acetylenes gases, which induce ripening. Sometimes, fruits are placed in a room in which ethylene gas or acetylene gas is introduced. In another method, calcium carbide (CaC2) is applied over fruits. It reacts with moisture to form acetylene. While artificial ripening is fast, it doesn't quite give the fruit the flavour it gets when naturally ripened. So why not buy organic fruit when you can, and enjoy its taste in full!