Mắm tôm is the Vietnamese fermented shrimp paste often served with bún đậu mắm tôm.
The condiment is made by fermenting small shrimp or krill with salt until the mixture develops a dense texture, strong aroma and deep purple-gray color. It may be mixed with lime juice, sugar, chili and other seasonings before serving. Fermentation changes both the flavor and texture, producing the concentrated savory character associated with the condiment.
Mắm tôm is particularly familiar as the dipping sauce for bún đậu mắm tôm, a northern Vietnamese combination of rice vermicelli, fried tofu and assorted accompaniments. It is also used in other Vietnamese dishes, although many diners dilute or season it to moderate its powerful aroma. It should not be confused with mắm nêm, another fermented fish-based dipping sauce, or nước mắm, the clear fish sauce made from fermented fish and salt.
Because shrimp paste has a pungent smell, restaurants may offer a fish-sauce alternative. The condiment’s intensity is not a sign that it is spoiled; its strong aroma is a normal result of fermentation.