Thaumatin

The number for this sweetener: E 957.  It is a naturally sweet protein approximately 2000 times sweeter than sugar which is used at very low levels, typically 0.5-3 ppm.

Thaumatins are a group of intensity sweet basic proteins isolate from the fruit of Thaumatococcus danielli Bennett, which grows in West Africa. Fruits are harvested and part processed to remove the section known to contain thaumatin.

Thaumatococcus danielli 

In practical terms, it is perhaps one of the least important of the permitted sweeteners in terms of use in soft drinks in that its taste quality makes it unsuitable for use as a sweetener except in products where a lingering licorice aftertaste can be tolerate.

It was recognized early on that if thaumatin contributed more than 50% of the sweetness, then the aftertaste became markedly noticeable.

Thaumatin was first permitted as a natural food in Japan in June 1979. In the UK it has been permitted as a sweetener for use in foods since 1983. It is an appropriate component in soft drinks when used at low levels in combination with sweeteners with rapid sweetness development.
Thaumatin