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Fermented tofu is firm bean curd that is dried in the air, sometimes until a fine mold forms on the surface, and then aged in a brine mixture for anywhere from 12 hours to several months or longer. Unlike the very light flavor of normal tofu, fermented tofu has a strong taste and odor that some people might find unpleasant. The texture and the sour odor are sometimes compared to aged cheeses, and the process used to ferment the tofu is actually very similar to that of making soft cheeses. There are different types of brines and other methods that are used to create fermented tofu, with the most pungent method producing a food called stinky tofu. Fermented tofu often is used as a side dish, as a spice, to thicken sauces, or eaten fried.
The first step in making many types of fermented tofu involves taking regular tofu that has been firmly pressed and completely air-drying it. It can be covered in straw or another type of loose organic material while drying. During the drying process, spores and fungi in the air will attached to the tofu and grow into a mold on the surface. This is one type of fermentation the tofu can experience. Not all fermented tofu is allowed to grow mold, however, and some is laced with specific types of bacteria so it will ferment in a certain way.
The second step that can be performed when making fermented tofu is to submerge and age the tofu in a brine solution. The most basic solution is rice wine and salt in nearly equal proportions and mixed with water. The tofu, which may have been fermented by mold during the air-drying process, is stored in the brine mixture and allowed to age to develop flavor and go through a second fermentation. This helps to develop the strong flavor associated the food. It also gives the tofu a nearly unlimited shelf life in a refrigerator.
The brine mixture can have several ingredients added to it that will affect the color and flavor of the finished tofu. One popular choice is a type of fermented red rice that turns the tofu a deep red color and adds a mild alcohol taste. Another version involves using leaves in the brine that turns the tofu a green color. Common additions to nearly any brine mixture include sesame oil and chili peppers.
When using fermented tofu in cooking, it can be heated and broken into pieces that will help to thicken sauces. It also can be added to stir-fry dishes and treated like a spice. Frying the tofu and serving it in slices with leafy green vegetable covered in sesame oil also is a popular preparation. Uncooked tofu is sometimes served with breakfast dishes or rice in a small separate bowl so it can be eaten like a side dish.