Most people think of the more well-known Nuoc Mam (Vietnamese Fish Sauce) when it comes to flavoring and dipping Vietnamese food. Mam ruoc (Vietnamese fermented shrimp paste) is its much more pungent cousin. It's more popular in North and Central Vietnam, while Mam Nem (Vietnamese Fermented Anchovy Sauce) is more popular in the South. And if you're South-Central like me, you grow up eating both!
According to Vietscape, ruoc are small shrimp caught during the rainy season. They are dried in the sun for three months, then mixed with salt, ground into a powder, and placed in a jar to pickle in the sun for another month-and-a-half. Sugar is then added to that mixture and left to ferment for another month. The mixture is then dried again in the sun for 10 days.Mam Ruoc Hue
These dried blocks are how I typically see belacan, the Malaysian version of shrimp paste sold in Asian grocery stores. Different versions of shrimp paste exist in many other Southeast Asian cuisines as well.
Vietnamese mam ruoc tends to be light pinkish-gray and is a thick paste rather than a block. All that drying and pickling and drying and pickling again results in a very thick, very concentrated, very salty, shrimp flavor. In some recipes such as Bun Bo Hue (Vietnamese Hue-Style Beef Noodle Soup), there really is no other substitute. It's also the "secret ingredient" in my recipe for baechu kimchi (Korean pickled napa cabbage).Mam Ruoc Hue
As a dipping sauce, its pungency can be diluted with lime juice. Add chili peppers, garlic, and sugar to round out the taste. You can dip raw or boiled vegetables, hot pot ingredients, rice paper rolls, or anything you wish really. My ba noi (Vietnamese paternal grandmother), the oldest '88, and I used to dip plain rice paper with mam ruoc.