Fish sauce: how to choose and use nước mắm

Vietnamese fish sauce, nước mắm, is a clear amber liquid made by fermenting anchovies with salt for many months. To choose a good bottle, look for a short ingredient list (ideally just anchovy and salt), a clean savory smell and a protein rating on the label, often written in degrees "N."

What nước mắm is

Nước means water or liquid, and mắm refers to fermented fish or seafood. Nước mắm (nuhk mum) is the essential seasoning of Vietnamese cooking, used the way salt or soy sauce is used elsewhere: in marinades, soups, stir-fries, braises and, most famously, dipping sauces.

On its own, it smells strong. In food, it adds deep, savory, rounded flavor rather than a fishy taste.

How it's made

The traditional method is simple but slow:

• Fresh anchovies are mixed with salt, often in a ratio of roughly three parts fish to one part salt.
• The mixture is packed into large wooden barrels or vats.
• It ferments for many months, often a year or more.
• Liquid slowly drips out and is collected.

The first liquid drawn off is the richest. In Vietnamese, it's called nước mắm nhỉ or nước mắm cốt, roughly the "first press" or premium extract. Later extractions are lighter and less flavorful.

Famous regions

Vietnam has several well-known fish sauce areas, especially along the southern and central coasts. Phú Quốc, an island in the Gulf of Thailand, and Phan Thiết, on the south-central coast, are two of the most famous names. You may see them on labels.

How to read a label

• Ingredients: the best bottles list anchovy (cá cơm) and salt. Some add sugar, which is fine for a milder taste. Long lists with flavor enhancers, colorings or preservatives usually indicate a cheaper, blended product.
• Protein degree (độ đạm): often printed as a number with "°N," like 30°N or 40°N. It's a measure of nitrogen, a rough guide to how concentrated and savory the sauce is. Higher numbers usually mean richer, stronger sauce, and a higher price.
• Color: good fish sauce is clear and reddish-brown or amber, not murky.

Some bottles are labeled nước mắm truyền thống (traditional fish sauce). Others are nước chấm, a pre-diluted dipping sauce, which isn't the same thing; check before buying.

Which one should you buy?

For everyday cooking, a mid-range bottle with anchovy and salt (and maybe sugar) works well. Some cooks keep two: a premium high-degree sauce for dipping and finishing, and a more affordable one for marinades and soups.

Smell it after opening. A good fish sauce smells savory and briny, without bitterness or a harsh, ammonia-like note.

How to use nước mắm

• Dipping sauce: mixed into nước chấm with lime, sugar, garlic and chili.
• Marinades: with garlic, shallots, sugar and pepper for grilled meats like the pork in cơm tấm.
• Soups: a splash at the end to season broth, including pho.
• Braises: with caramel for thịt kho and cá kho.
• Stir-fries: added near the end so it doesn't scorch.
• Dressings: for salads and noodle bowls.

Add it in small amounts, then taste. It's salty, and a little goes a long way.

Storing fish sauce

Keep the bottle tightly closed in a cool, dark cupboard. Fish sauce is very salty and keeps a long time. Over months, it may darken and its flavor may deepen, which many cooks consider fine. Salt crystals sometimes form at the bottom of high-quality sauces; that's normal. Refrigeration isn't required, but some people prefer it for very long storage.

A quick taste test

To compare brands, mix a teaspoon of each into a small bowl of warm water with a pinch of sugar. Taste them side by side. You'll notice differences in saltiness, sweetness, depth and aftertaste.

Allergens and swaps

Fish sauce contains fish. Some brands also contain shrimp or other ingredients, so read the label if you have a shellfish allergy. For vegetarian cooking, use a vegetarian fish sauce, usually made from soy, mushrooms or seaweed, or use light soy sauce with a little extra salt and lime. See vegetarian pho for ideas.

Pick one good bottle with anchovy and salt on the label, and every Vietnamese recipe you cook will taste more like it should.