Soy sauce and fish sauce: how the two differ

Soy sauce and fish sauce both bring salt and a deep savory note, which is why they are sometimes treated as interchangeable. They are not. Soy sauce is brewed from soybeans, usually with wheat, and it tastes rounded, brown and malty. Fish sauce is made from fermented fish, usually small anchovies, and it tastes sharper, more pungent and more intensely savory. Each belongs to a different family of dishes, and swapping one for the other changes the character of the food, not just its salt level.

Knowing the difference helps you read a recipe. A stir-fry built on soy sauce will taste of toasted grain and show a dark, glossy color, while a fish sauce dressing tastes sharp and savory under its lime, sugar, garlic and chili. When a recipe calls for one sauce and you only have the other, the change is a substitution that needs tasting and adjusting. Some dishes handle it well, and others lose the very flavor they were built around.

How each sauce is made

Soy sauce is made by fermenting soybeans, usually with roasted wheat, salt and water. A mold culture known as koji is added to break down the proteins and starches in the mixture. The mash is brewed for months before the liquid is pressed out, then filtered and pasteurized. Many factors shape the final flavor, including the grain ratio, the length of brewing and any additions such as sugar or caramel. Tamari is a related sauce that is traditionally made with little or no wheat, though labels should still be checked.

Fish sauce is made by layering small fish, typically anchovies, with salt and letting them ferment for many months in sealed containers. As the fish breaks down, the liquid that drains off becomes the sauce, which is then filtered and bottled. Fish sauce is therefore a product of salt and time, and its strength comes from the concentrated savory compounds released during that long fermentation. Both sauces are naturally salty, so both are added in measured amounts and tasted before any extra salt goes in.

Flavor, salt and color on the plate

Soy sauce brings salt, a rounded savory depth and a dark or amber color. Its flavor is malty, toasty and sometimes faintly sweet. The labels can be confusing. Light soy sauce in Chinese cooking is thinner and used mainly for seasoning, and despite its name it is not lower in salt. Dark soy sauce is thicker, less salty and often sweeter, and it is used mainly for color and gloss in braises and marinades. Japanese soy sauce, or shoyu, is typically balanced and widely used for dipping and everyday seasoning.

Fish sauce brings intense salt and a sharp, funky savoriness. Straight from the bottle it smells strong, but cooking softens that aroma into a deep background note. It rarely colors a dish, so food seasoned mainly with fish sauce stays pale, while food seasoned with soy sauce turns brown. Both sauces contain glutamates, natural compounds that enhance savory flavor, which is part of why both make food taste fuller.

A short tasting makes the contrast clear. Put a few drops of each sauce on separate spoons and taste them one after the other. Soy sauce tastes rounder, salty and malty, with a finish that fades quickly, while fish sauce tastes sharper and more pungent, with a savory finish that lingers. Once you have tasted the two side by side, the choice in a recipe becomes much easier to make.

Using them alone and together

Use soy sauce when you want a brown, savory finish: stir-fries, braises, glazes, dipping sauces for dumplings and marinades for meat and tofu. Use fish sauce when you want a sharp, bright savory note, as in Thai and Vietnamese dressings and dipping sauces built on lime, sugar, garlic and chili, in curry pastes and in seasoning for salads and grilled meat. When you substitute one for the other, start with less than the recipe calls for, then taste and adjust. Soy in place of fish sauce loses the pungent, briny depth of fish sauce, and fish sauce in place of soy loses color and some of its roundness. When a recipe names one sauce and you want the other, check whether the dish depends on a specific flavor or simply on salt, since the first kind rarely swaps well.

Some cooks combine the two in marinades and sauces, using fish sauce in a smaller amount for depth and soy sauce for color and body. This works well with grilled or braised meat and with stir-fry sauces, where the pair gives a fuller savory flavor than either one alone. Add fish sauce late in cooking or stir it into a dressing when you want its brightness, since long cooking mellows its sharp edge. Both sauces keep well once opened. Soy sauce is best kept in a cool, dark place and is often refrigerated to protect its flavor, while fish sauce keeps well in a cool cupboard when tightly capped. Read labels if you have a wheat, soy or fish allergy, or if you cook for vegetarians, since fish sauce is not vegetarian and many soy sauces contain wheat.