Grated, fresh horseradish is a pungent source of mustard oil. The intact horseradish root has hardly any aroma. When cut or grated, however, enzymes from the now-broken plant cells help created the distinctive aroma of horseradish. Grated mash should be used immediately or preserved in vinegar for best flavor. Once exposed to air or heat it will begin to lose its pungency, darken in color, and become bitter tasting over time, though all medicinal properties are still present in dried material.