Thaumatin

"Natural" high-potency sweetener: Used as a flavor modifier in a variety of foods and beverages.
This sweetener, which has the brand name Talin, consists of a couple of proteins extracted from the katemfe fruit grown in West Africa. It is about 2,000 times sweeter than sugar. It has been designated "generally recognized as safe" (GRAS) by the FDA both as a sweetener and a flavor modifier. Like rebiana and mogrosides (other natural, high-potency sweeteners), it has a "licorice-like" aftertaste, which consumers may find distasteful and might be the reason it is not used more often. As a protein it is presumably safe, but it might cause occasional allergic reactions.