The unbroken horseradish root has very little smell. When cut or grated, however, enzymes from the now-broken plant cells causes changes. The enzymes break down sinigrin to produce allyl isothiocyanate (mustard oil). This irritates the mucous membranes of the sinuses and eyes. Once exposed to air (by grating) or heat, it needs to be mixed with vinegar. Otherwise it becomes unpleasantly bitter-tasting.