Mustard selection and storage
Seeds: Most markets carry mustard seeds in several forms, including whole, ground and powdered. Powdered mustard is a combination of brown and white seeds mixed with turmeric or saffron for added flavor and color. There are now a number of specialty dried mustards available, such as chili mustard powder, peppercorn, mint, and chive. You can make your own flavored mustard powders by adding dried flavorings to simple plain powdered mustard and avoid the gourmet cost. Store whole mustard seeds in airtight containers in a cool, dry place up to one year; ground and powdered mustard up to six months.
Mustard Oil: Intensely hot and spicy mustard oil is derived from pressing mustard seeds. It can be found in specialty, Indian and Oriental markets. The oil is a nice golden color and quite aromatic. A small bottle will go a very long way. It should be stored in the refrigerator because it will easily go rancid.
Prepared Mustard: Since it is prepared with an acid component and salt, store-bought prepared mustard will not spoil.
However, it will lose flavor and fire as it ages, even when unopened, and doubly so for flavored mustards. Use unopened mustard within one year. Unless you use a lot of mustard, it's recommended you purchase small jars and replenish often for the fullest, most robust flavor and heat. Store prepared mustard in the refrigerator and use it as quickly as you can, although it can be used safely up to a year. Opened mustard only keeps about a month at room temperature, so be sure to refrigerate it.
Many chain markets as well as Oriental markets will also carry the mustard greens or leaves from the brown mustard plant, which are cooked much like spinach or simply eaten raw. The flavor of the greens is reminiscent of prepared mustard, with a perfumy edge and a hint of radish.