Lentils are only available dried. They are not used fresh.
Select lentils that are dry, firm, clean, and unshriveled. The color of lentils you choose will depend on your usage, but in general, the color should be fairly uniform. Canned lentils are also available, but it is just as easy to cook your own.
If your recipe calls for a lentil that will retain its shape when done, common brown lentils are the usual choice.
Brown lentils still have their seed coat and have not been split. Most red, yellow, and orange lentils tend to disintegrate with long cooking because the hulls have been removed. Slightly sweet in flavor, these are best reserved for pureed soups or stew thickeners.
Other choices include French lentils which are olive-green and slate-colored. These will cook up the firmest. Persian green lentils will turn brown as they cook and become nice and tender while still retaining their shape. Considered the most flavorful (and most expensive) are the French Puy lentils, which also retain their shape.
You may be able to find lentil flour in some specialty markets. It is used in India to make a fermented dough for bread making.
Dried lentils have an indefinite shelf-life, yet another reason why our ancestors kept them as a staple food. With age, the color may fade a bit, but the flavor will not noticeably deteriorate.
Store lentils in a sealed package or airtight container in a cool, dry place. For best flavor and presentation, use dried lentils within one year.
Cooked lentils may be refrigerated up to one week in a sealed container. Cooked lentils may also be frozen up to six months. However, they may fall apart when reheated if not handled gently.