What is the difference between dried lime and sumac?

Dried lime vs Sumac

The short answer

Two souring agents from the Persian Gulf and the Levant, used differently. Dried lime — loomi, limu omani — is a whole lime boiled and sun-dried until hollow and dark, and it gives a fermented, musty sourness released slowly into stews, usually pierced and removed. Sumac is a ground drupe, sharp and lemony, and is used as a finishing seasoning. One works over an hour in a pot; the other works on the plate.

Side by side

Dried lime and Sumac compared side by side
	Dried lime	Sumac
Botanical identity	Citrus aurantiifolia (Christm.) Swingle	Rhus coriaria L.
Part used	fruit	fruit, berry
Origin	The Gulf, and Oman in particular, where the technique originated and from which the trade name loomi omani comes. The limes themselves are often grown elsewhere and processed regionally.	The Mediterranean basin and West Asia, with the plant growing wild across a broad arc from Sicily through Anatolia to Iran.
Flavour notes	sour, earthy, bitter, citrus	sour, fruity, tannic, astringent, bright
Heat	Scoville does not apply	Scoville does not apply
Flavour	Intensely sour with a fermented, earthy depth and a real bitterness from the pith and seeds. The sourness is cleaner than tamarind, the bitterness more pronounced than sumac, and the musty note is unique among the souring agents here.	Sharply sour with a fruity, almost cranberry-like character and a tannic, mouth-drying finish. The sourness is clean and lingering, without the sharp immediacy of vinegar, and unlike lemon it brings no bitterness from peel and no liquid at all — which is exactly why it is used where a wet acid would ruin the texture.
Aroma	Sour, musty and faintly fermented, with citrus oil somewhere underneath. It smells more like a cellar than a fruit bowl, which is disconcerting the first time and is exactly right.	Faint. A light fruity, slightly resinous smell with a tannic edge, but sumac is nearly silent in the jar compared with the aromatic spices, and its aroma is not what it is for.

Can you swap them?

Substitution is directional: the answer is often different in each direction, and both are recorded.

Sumac

Situational

Use: ~1× by volume — Roughly a teaspoon of sumac per dried lime, added later in the cooking than the whole fruit would have gone in.

Closer in behaviour than fresh citrus because it is dry and can be added to anything, and further away in flavour than the shared sourness suggests. Sumac is tannic and fruity where dried lime is musty and bitter, and it colours the dish red. It supplies acidity and none of the fermented depth.

How the replacement compares on each sensory axis
Flavour	Aroma	Colour	Heat
partial	partial	different	n/a

Works for

Rice and grain dishes
Marinades and rubs
Stews where the sour note is supporting rather than defining

Does not work for

Ghormeh sabzi and dishes named for the lime
Anything that should stay pale, since sumac stains
Cases where the bitterness of the whole fruit was doing structural work

Cuisine: Moves a Persian or Iraqi dish towards a Levantine one.

Established culinary practice

Both dry souring agents belong to neighbouring cuisines and a cook there would treat choosing between them as a decision about which dish they are making rather than as a substitution.

Which should you buy?

Dried lime

Whole, and buy more than you think you need because they keep for years and cost very little. Whole limes give a slow sourness that builds through a stew with the bitterness held in check, which is the behaviour Persian and Iraqi cooking is built around; ground powder delivers everything at once, bitterness included, and is worth having only for rubs and spice mixes. Grind your own when you want it. Middle Eastern grocers sell them by the bag for a fraction of what a small supermarket jar costs, and the weight test works through the packaging.

Sumac

This is the one common spice where the usual "buy whole" advice mostly does not apply. Whole sumac drupes are hard to source outside Middle Eastern grocers and awkward to grind well, because the aim is to mill the flesh while excluding the hard seed — which is a job for a mill and a sieve, not a domestic grinder. Buying ground is the normal and reasonable choice. Spend the effort instead on buying unsalted sumac from a supplier that states its origin, and on buying small quantities often, since colour is what fades.

Full detail on dried lime and sumac, including evidence grades and sources for every claim above.