This is a question that perplexed me for years, especially after several disappointing attempts to substitute powdered ginger tablespoon-for-tablespoon for the fresh stuff in stir-frys. The flavor difference was surprising, though not half as surprising as the “heat” blast that practically gave me brain damage. Indeed, a little goes a long way where powdered ginger is concerned, possibly the reason for its historic popularity among poor Europeans and Americans.
Those who’ve had similar experiences with ginger’s pungency won’t be surprised to learn that ginger’s “heat”-causing compounds (known as gingerols) are close chemical cousins to the capsaicin found in chilies and the piperine in pepper. But then why is dried ginger so much more powerful? It’s because as gingerol molecules dry, they transform into compounds called shogaols which are several times more pungent than the gingerols they came from.
Apply a little heat and still another chemical transformation occurs as the shogaols become zingerones (and no I’m not making these words up…and when you eat’em they turn into um, er…flibberdings!) and adopt a less pungent, sweeter disposition.