From Wikipedia: “Mustard is a condiment made from the seeds of a mustard plant (white/ yellow mustard, Sinapis alba; brown/ Indian mustard, Brassica juncea; or black mustard, B. nigra).
The whole, ground, cracked, or bruised mustard seeds are mixed with water, vinegar, lemon juice, wine, or other liquids, salt, and often other flavorings and spices, to create a paste or sauce ranging in color from bright yellow to dark brown. The taste of mustard ranges from sweet to spicy.
Commonly paired with meats and cheeses, mustard is added to sandwiches, salads, hamburgers, corn dogs, and hot dogs. It is also used as an ingredient in many dressings, glazes, sauces, soups, and marinades. As a cream or as individual seeds, mustard is used as a condiment in the cuisine of India and Bangladesh, the Mediterranean, northern and southeastern Europe, Asia, the Americas, and Africa, making it one of the most popular and widely used spices and condiments in the world.”
“Mustard is most often used at the table as a condiment on cold meats.It is also used as an ingredient in mayonnaise, vinaigrette, marinades, and barbecue sauce. Mustard is also a popular accompaniment to hot dogs, pretzels, and bratwurst. In the Netherlands and northern Belgium it is commonly used to make mustard soup; which includes mustard, cream, parsley, garlic and pieces of salted bacon. Mustard as an emulsifier can stabilize a mixture of two or more immiscible liquids, such as oil and water. Added to Hollandaise sauce, mustard can inhibit curdling.”
“The amounts of various nutrients in mustard seed are to be found in the USDA National Nutrient Database. As a condiment, mustard averages approximately 5 calories per teaspoon. Some of the many vitamins and nutrients found in mustard seeds are selenium and omega 3 fatty acid.”
“The many varieties of prepared mustards have a wide range of strengths and flavors, depending on the variety of mustard seed and the preparation method. The basic taste and "heat" of the mustard are determined largely by seed type, preparation and ingredients. Preparations from the white mustard plant (Sinapis alba) have a less pungent flavor than preparations of black mustard (Brassica nigra) or brown Indian mustard (Brassica juncea). The temperature of the water and concentration of acids such as vinegar also determine the strength of a prepared mustard; hotter liquids and stronger acids denature the enzymes that make the strength-producing compounds. Thus, "hot" mustard is made with cold water, whereas using hot water produces a milder condiment, all else being equal.
Mustard oil can be extracted from the chaff and meal of the seed.”
“Prepared mustard is sold in glass jars, plastic bottles, or metal squeeze tubes. Because of its antibacterial properties, mustard does not require refrigeration for safety; it will not grow mold, mildew, or harmful bacteria. Mustard can last indefinitely without becoming inedible or harmful, though it may dry out, lose flavor, or brown from oxidation. Mixing in a small amount of wine or vinegar may improve dried-out mustard. Some types of prepared mustard stored for a long time may separate, which can be corrected by stirring or shaking. If stored unrefrigerated for a long time, mustard can acquire a bitter taste.
When whole mustard seeds are wetted and crushed, an enzyme is activated that releases pungent sulphurous compounds; but they quickly evaporate. An acidic liquid, such as wine or vinegar, produces a longer-lasting paste. However, even then prepared mustard loses its pungency over time; the loss can be slowed by keeping a sealed container (opaque or in the dark) in a cool place or refrigerator.”
“Locations renowned for their mustard include Dijon (medium-strength) and Meaux in France; Norwich (very hot) and Tewkesbury, famed for its mustard, in the United Kingdom; and Düsseldorf (hot) and Bavaria in Germany. They vary in the subsidiary spices and in the preparation of the mustard seeds. The mustard husks may be ground with the seeds, or winnowed away after the initial crushing; "whole-grain mustard" retains some unground or partially ground mustard seeds. Bavarian sweet mustard contains very little acid, substituting copious amounts of sugar for preservation. The Tecuci mustard from Romania is a sweet variety very popular in Eastern Europe and is suitable for barbecued meats such as mititei. Sometimes prepared mustard is simmered to moderate its bite, and sometimes it is aged. Irish mustard is a whole-grain mustard blended with whiskey, stout (commonly Guinness), or honey.”