We must assume you mean the European radish (a small member of the Brassica family), not the Asian radish or DaiGon). All Brassica plants contain various natural concentrations of allyl iso-thiocyanates, which give them the strong taste, even cabbages and Brussels sprouts (mild), as well as real mustard (very strong). read more
But when you don’t bite into it, it isn’t spicy. And no, that’s not semantics. The radish itself doesn’t contain capsaicin, or any other spicy molecule. If left alone, it’s completely bland. The molecules that give us a peppery jolt when we bite into the radish are allyl isothiocyanate, which do not exist inside whole radishes. read more