Chinese cooking emphasizes the control of heat, and there is a common saying, "A thousand boils for the tofu, ten thousand boils for the fish," which vividly points out that tofu and fish are ingredients that withstand long simmering. This slow-cooked process is what makes it a dish of "kung fu."
The word "boil" (滚) here refers to slow simmering. After the liquid in the pot comes to a boil, it is cooked over low to medium heat, keeping the surface of the soup gently bubbling without overflowing. Compared to leafy green vegetables, fish and tofu are indeed more resistant to prolonged boiling. From this perspective, the saying "A thousand boils for the tofu, ten thousand boils for the fish" has some merit.
Firstly, from a nutritional standpoint, as long as the heating does not exceed 120°C, the nutrients in fish and tofu—such as proteins, minerals, and the dietary fiber in tofu—will not be significantly lost. Secondly, after a long simmering time, the structure of soy protein changes from dense to loose, making it easier to digest and absorb. Fish is extremely rich in protein, and its muscle fibers are relatively soft and fine. When heated, they contract and do not absorb flavor easily, so slow cooking over low heat can enhance the delicious taste of the fish. Additionally, the umami in the food is not easily infused into the broth, but slow simmering allows it to penetrate more deeply. The broth can also fully absorb the delicious flavor and nutrients of the fish, making the soup more nutritious and tasty. After being heated over low heat for a long time, the protein in the fish does not contract severely, and the fat in the fish does not leach out too much, thus keeping the meat tender and fresh. Finally, both tofu and fish have their own distinct odors, which can be removed through stewing.
However, it should be noted that "A thousand boils for the tofu, ten thousand boils for the fish" is just a figure of speech, not an actual unit of measurement, and it does not mean that the longer the tofu and fish are cooked, the better. On one hand, some nutrients in fish and tofu are sensitive to prolonged "boiling." For example, B vitamins and soy isoflavones in tofu are heat-sensitive. If you want to obtain these substances from food, you can choose to simply cook the tofu or eat it raw in a salad. Similarly, the unsaturated fatty acids and fat-soluble vitamins in fish cannot withstand long stewing. Therefore, methods like steaming, quick-boiling, or blanching can be used to retain these nutrients to the greatest extent. On the other hand, overcooking can make the texture tougher and the dish hard and dry. Therefore, in terms of time, it is generally best to control it to about 15-20 minutes. For this type of dish, a clay pot that heats evenly is particularly suitable, and it is important not to use high heat.