Horseradish (Armoracia rusticana, syn. Cochlearia armoracia) is a perennial plant of the Brassicaceae family, which also includes mustard, wasabi, broccoli, and cabbages. The plant is probably native to south eastern Europe and western Asia, but is popular around the world today. It grows up to 1.5 meters (5 feet) tall and is mainly cultivated for its large, white, tapered root.
The intact horseradish root has hardly any aroma. When cut or grated, however, enzymes from the damaged plant cells break down sinigrin (a glucosinolate) to produce allyl isothiocyanate (mustard oil), which irritates the sinuses and eyes. Once grated, if not used immediately or mixed in vinegar, the root darkens, loses its pungency, and becomes unpleasantly bitter when exposed to air and heat.
Cooks use the terms “horseradish” or “prepared horseradish” to refer to the grated root of the horseradish plant mixed with vinegar. Prepared horseradish is white to creamy-beige in color. It will keep for months refrigerated but eventually will start to darken, indicating it is losing flavour and should be replaced. The leaves of the plant, while edible, are not commonly eaten, and are referred to as “horseradish greens”.
In the U.S., the term “horseradish sauce” refers to grated horseradish combined with mayonnaise or salad dressing. Prepared horseradish is a common ingredient in Bloody Mary cocktails and in cocktail sauce, and is used as a sauce or spread on meat, chicken, fish and in sandwiches.