Why does the coffee turn sour after it is cooled?

Sometimes we have this experience: we have a cup of coffee in a coffee shop or ourselves, and we love to drink black coffee, so we don't add sugar or milk, but before long, as the coffee gets cold, the originally mellow coffee will also become sour and difficult to swallow. Why on earth is this? Is there any way to avoid it? In fact, drinking coffee is like eating fish. Very often, a steaming fish is served fresh and delicious, but once it is cold, the smell of the fish will become very strong, and coffee will sometimes be the same phenomenon. Coffee turns sour after cooling, because coffee beans are a kind of fruit, so they are rich in tannins, which are released when the coffee temperature drops, and when the temperature is lower, the human taste is more sour-which is probably why dry white wine must be refrigerated to drink-so we can clearly feel that the coffee has obviously become sour after cooling. Of course, not all coffee will turn sour after cooling. Generally speaking, the lower the roasting degree of coffee is, the more sour it will be, while many deep-roasted coffee will not have this problem. In addition, it is said that the better the quality of coffee beans, the smaller the difference in flavor between hot and cold. If you want to keep the coffee from becoming sour, then the best way is to drink it hot. If it is already cold, try adding some light milk or milk to make the sour taste less obvious.