Sugar in soft drinks and other processed foods may be bad for us, but cheating our taste buds has proved difficult — and the substitutes we do have (like the coal derivative found in Sweetn Low) can sound even worse. A more natural solution is sweetener derived from plants like stevia, which has made its way into the recently launched Coca-Cola Life and other beverages. But as Daniel Engber writes inThe New York Times, stevias bitter aftertaste can easily turn a diet soda sour. …