Stevia is actually ~four sweetening compounds, not one [1]... the after taste you're discussing comes from stevia extract with the four compounds in their natural ratios.
I believe Coke is starting to use stevia because biosynthesis is bringing the cost down, and allowing you select which of the four compounds you'd like[2]... although I haven't managed to taste yet it myself, I believe this solves the after taste problem.
I believe Coke is starting to use stevia because biosynthesis is bringing the cost down, and allowing you select which of the four compounds you'd like[2]... although I haven't managed to taste yet it myself, I believe this solves the after taste problem.
[1] https://en.wikipedia.org/wiki/Steviol_glycoside [2] https://www.google.com/patents/EP2645847A1?cl=en -- can't find the news link where I read about the application of it though :(