Any of several species of plant grown for its acrid seeds and leaves, which are called mustard greens. The mustard plant belongs to the same family as broccoli, Brussels sprouts, collards, kale and kohlrabi. Down through the centuries it has been used for culinary as well as medicinal purposes; the most notable example of the latter is mustard's purported efficacy as a curative for the common cold. The name is said to come from a Roman mixture of crushed mustard seed and must (unfermented grape juice), which was called mustum ardens ("burning wine"). Likewise, the French word moutarde ("mustard") comes from a contraction of their moust ("must") and a form of ardent ("hot" or "fiery"). There are two major types of mustard seed white (or yellow) and brown (or Asian). A third species, the black mustard seed, has been replaced for most purposes by the brown species because the latter can be grown and harvested more economically. White mustard seeds are much larger than the brown variety but a lot less pungent. They're the main ingredient in American-style mustards. White and brown seeds are blended to make English mustard. Brown mustard seeds are used for pickling and as a seasoning, and are the main ingredient in European and Chinese mustards. Mustard seeds are sold whole, ground into powder or processed further into prepared mustard. Powdered mustard is simply finely ground mustard seed. Mustard seeds can be stored for up to a year in a dry, dark place and powdered mustard for about six months. Whole seeds are used for pickling, flavoring cooked meats and vegetables and as a source for freshly ground mustard. Powdered mustards and freshly ground seeds are used in sauces, as a seasoning in main dishes and as an ingredient in salad dressings.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.