Mahlab is a spice made from the seeds of a variety of sour cherry known as the Mahlep or Mahleb tree. The spice has a distinctive flavor which tastes somewhat like cherries and slightly like almonds, with a hint of a sharp, sour taste, and it is very popular in baked goods and holiday foods throughout the Middle East and in neighboring regions like Greece. This spice can be somewhat costly and difficult to obtain outside of the Middle East; specialty markets sometimes carry mahlab, either ground, whole, or in spice blends, especially during the holiday season.
The mahlep tree, known to biologists as Prunus mahaleb, is a type of cherry tree which is native to much of Eurasia. It is also sometimes called the Saint Lucie Cherry, and it yields distinctive almost black and very sour cherries in the later summer. The fruits of this tree are used in jams, preserves, and desserts, and they can also be dried. To extract the mahlab, the pits are cracked to reveal the small seeds inside.
Incidentally, St. Lucie Cherries also smell quite delicious, and many people plant them as ornamental trees, since they have a beautiful weeping growth habit. They are also extremely hardy, making them suitable as a base for grafting, and they produce very useful, beautifully grained wood. The slight bitter almond scent of the tree in bloom also appears in mahlab.
Baked goods commonly call for mahlab, and it is also used in some cheeses and meat dishes. Several traditional holiday breads like Armentian choereg, Turkish ka-kat, and Greek tsoureki call for mahlab, and it is also used to make Ramadan treats in the Middle East. Cypriots use mahlab in flaounes, traditional cheesy Easter cakes. With rising interest in Middle Eastern cuisine around the world starting in the 1970s, these sour cherry pits began to produced by several companies to meet the demand.
Ideally, you should purchase mahlab as a whole spice, because the whole seeds will retain their flavor far longer than the ground spice. When you need mahlab for a recipe, you can grind the pits in a spice mill or with the use of a mortar and pestle. As with all spices, mahlab should be stored in a cool dry place, and you should try to use it within a year or so for the most powerful flavor.