Any good sources--either online *OR* offline--on those African plants?
Are you saying that there's been absolutely NO attempt at commercialization of these African plants?
It looks like there has been at least some research concering the Seredipity Berry:
The natural proteins [of the Seredipity Berry] as a group are the sweetest compounds ever discovered. The sweet taste - which depends on nearly 100 different sensory receptors on the tongue - can be detected in the presence of thaumatin at concentrations well below one part protein molecule per 100 million parts of water. On a scale in which 0 refers to no sweetness, 1 refers to table sugar or sucrose, then thaumatin is nearly off the scale at 3,000, more than 10 times sweeter than other sugar substitutes like saccharin or aspartame.
Because these kinds of complex sensory-stimulating proteins typically require binding to specific taste receptors, much of their biology remains to be worked out in the kind of studies done on the space shuttle and using modern tools of biological crystallography. Already within the bulk commercialization by biotechnology companies, Tate & Lyle's product, Talin, is marketed from thaumatin. Also, at the Unilever Research Laboratory in The Netherlands, the gene for this sweetener has been cloned into biological production using the microorganisms E. coli and yeast to substitute for the original African shrub.
As a non-caloric sweetener, thaumatin has attracted attention as a candidate for control of obesity, oral health and diabetic management. Thaumatin already is being marketed as a nutritional supplement in blood sugar stabilizers for childhood behavioral problems and the more than 3.5 million sufferers from attention deficit disorder. Among soft drink consumers alone, nearly 20.6 million tons of chemicals are used around the world - nearly 4 kilograms per capita, with a growth of about 20% towards the end of the decade.
Here is one reference for the African plants:
http://www.dsharma.org/biotech/bitter.htm