Thaumatin is a natural non-nutritive sweetener, but does not currently have use in the United States and is not listed as one of the six non-nutritive sweeteners approved by the FDA. The name thaumatin is derived from the katemfe plant – having the scientific name of Thaumatococcus danielli – which is found in western Africa. The Japanese have extracted and purified the sweet proteins from the plants fruit (katemfe fruit), called thaumatins, since its approval there in 1979.  In the United States, thaumatin has generally recognized as safe, or GRAS, status.  It is also approved for use in Europe, where it is known as E957. 
It is said that thaumatin has been used by the West Africans for hundreds of years to sweeten corn breads, sour fruits and also to make palm wine palatable. In the 1840’s, William Freeman (W F) Daniell (1818 – 1865), a British army surgeon and botanist, made note of how thaumatin was used and later reported the findings in the pharmaceutical journal.  The company Naturex produced a commercial product from thaumatin and markets it under the brand name Talin®.
The thaumatins of the fruit are isolated and result in an “odorless cream coloured powder that is highly water-soluble, stable to heat and acidic conditions.”  There are two main forms of proteins in the plant that are used: thaumatin I and thaumatin II.  Thaumatin is listed as being 2000-3000 times sweeter than sucrose (sugar) [5 – PDF] and does not contribute any carbohydrates to the diet. Since it is a protein it does contain 4 calories per gram, but with such a high potency relative to sucrose the amount used in products provides insignificant calories. “It has a licorice-like taste with delayed onset and then lingers therefore is not used in many products and always in synergy with other artificial sweeteners such as saccharin (masks its aftertaste at low levels), acesulfame K, and stevioside.”  One of the problems with the other natural non-nutritive sweetener available commercially – Stevia – is that it has a bitter taste. Thaumatin has been found to reduce the bitterness when added in extremely low levels to Stevia extract, including Rebaudioside A. There is currently no listed ADI for thaumatin, which means it can be used according to GMP (Good Manufacturing Practice). [5 – PDF]
One study has concluded after studying thaumatin in animals and humans that, “the results indicate that thaumatin when used as a flavour modifier and extender, and partial sweetener, is unlikely to be hazardous at the anticipated level of consumption.” Furthermore, another study by the IPCS states that “[t]haumatin showed no mutagenic or teratogenic effects and no allergenic effects were noted.” Also, “[t]he lack of toxicity, combined with its ready digestion to normal food components, indicate that thaumatin’s only dietary effect is to make an insignificant contribution to the normal protein intake.” Lastly, the article states another reassuring line stating that “the long history of human use of the fruit as a sweetener, now largely displaced by sugar in urban areas, and the absence of unusual or toxic effects following its ingestion, is attested by numerous affidavits from village elders in Ghana and the Ivory Coast. Thaumatin has been approved by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) (1985) and by the Scientific Committee on Food (SCF) of the European Commission (1988) – now the European Food Safety Authority (EFSA). With studies such as these, the historical value of natives using the plant, the GRAS status by the FDA – meaning there is no proof of any harm – and the approval by European organizations, it seems logical to view thaumatin as a safe product.
“Used in baked and savoury foods, desserts, dairy products, milk powders, ice-cream, iced milk, beverages, soft drinks, chewing gum, salt substitutes, coffee-ﬂavoured products, and fruit juices etc. Also used as a flavour enhancer for products such as beverages, jams and jellies, condiments, milk products, yogurt, cheese, instant coffee and tea, chewing gum, and can be used to replace MSG.”  Here is an extensive list that lists all the foods thaumatin can be added to. Essentially a lot of products can contain thaumatin and in such low doses it does not needed to be listed on the ingredients – as any ingredient under 1% of the total product is considered insignificant.
Like stevia extract, thaumatin has been used for centuries and shows no signs of danger to human health. Unlike stevia extract, thaumatin is tremendously sweeter than sucrose and only trace amounts are needed to sweeten food. This means, unlike some that worry stevia may become overused by consumers, there is not much fear of overusing thaumatin even if it becomes a commercial success. Thaumatin also seems to be most beneficial as a flavor enhancer, instead of an isolated sweetener. This means thaumatin will probably not be used alone in products, but in combination with other sweeteners. If thaumatin becomes an active ingredient in the United States it seems to have a place in stevia extracts to remove any lingering bitter taste. However, thaumatin may find its way into products containing sucrose, HFCS, or artificial sweeteners. Assuming this happens, consumers need to be aware of what else thaumatin is used with before purchasing a product that contains it.