sumac
The dried, ground berries of the tanner's sumac shrub, a coarse, deep red powder. It tastes fruity and sour, close to lemon but softer and slightly astringent.
Sumac is the dried fruit of the tanner's sumac shrub (Rhus coriaria), sold coarsely ground as a deep red, slightly moist powder. In Persian, Turkish and Levantine cooking it adds sourness without adding liquid, which is why it is often scattered over rice, kebabs, onions, salads or yoghurt at the end. The flavour is fruity and tart, a little astringent and noticeably rounder than lemon juice, and it tints food reddish. Some kinds are ground with salt, which is worth keeping in mind when seasoning. It is sold in Turkish and Middle Eastern grocers, in well-stocked spice sections and by mail order; kept airtight and away from light it holds its aroma for a few months.
Substitute:
There is no equivalent substitute. Finely grated lemon zest with a pinch of salt, perhaps a few drops of lemon juice, supplies the sourness but not the fruity astringency or the red colour; for sprinkling over finished dishes it is only a stopgap.