Tofu is a chameleon. It has little flavor of its own, but takes on the taste of the other ingredients in a dish, soaking up and showcasing seasonings. Aside from its versatility, tofu's virtues lie mainly in its texture, which holds up well in soups.
Types of Tofu in Soups
You can use either firm or soft tofu in soups, but you'll get a different result with each, so choose the one that's right for your recipe, or adjust your expectations and enjoy what you have. Soft or silken tofu has a high water content. It is spongier than firmer varieties of tofu so it soaks up flavors and holds up especially well in broths, absorbing their subtleties. Because it is delicate, though, soft or silken tofu is prone to breaking apart if you don't handle it carefully. If you're using it, avoid unnecessarily vigorous stirring.
Firm and extra firm tofu have lower water content than soft or silken tofu, and hold their shape more effectively. You can cut firm tofu in small cubes, larger cubes or long strips, and they're unlikely to break apart when you cook them in soup, even if you stir them forcefully. Because firm tofu has a relatively low water content, it's best not to cook it in soup for extended periods of time because the extra exposure to moisture will make it less firm. If firm tofu is right for your recipe, add it 10 to 20 minutes before your soup is finished. This will allow plenty of time for it to soak up flavor without compromising its texture.
Many Asian grocery stores sell deep-fried tofu in cubes and strips. Although it would seem that a deep-fried food wouldn't hold up well in soup, it actually retains its texture quite well. Commercially available deep-fried tofu can come in pieces that don't fit gracefully on a soup spoon, so cut them in halves or quarters before adding them to your soup.
Flavoring Tofu in Soup
Most traditional soup recipes that call for tofu are based on Asian flavors, because tofu is a traditional staple in many Asian cuisines. The Chinese classic hot and sour soup uses strips of silken tofu, which take on the broth's flavors of soy sauce, rice vinegar and sesame and chili oils. Japanese miso soups use cubes of silken tofu, which take on the flavor of the miso broth and the seaweed that seasons it. Korean soups with tofu often use kimchee as a seasoning. The chili and fermentation flavors make the tofu pungent and savory. Vietnamese noodle soups often use fried tofu, which takes on the flavors of mint, basil, cilantro and lime.
Nutrients in Tofu When Cooked in Soup
Tofu is rich in protein, calcium and magnesium, among other nutrients. The health benefits you receive from the tofu you use in soup depends on the type of tofu you use. Because firm tofu contains less water relative to its volume than the softer varieties, it contains more soy and therefore offers more of the characteristic nutritional benefits. Fried tofu may be fried in unhealthy oils so, despite its crispy appeal, it will be a less healthy choice than either soft or firm tofu that hasn't been deep-fried.