Hot and sour soup is an Asian dish that is most commonly associated with Chinese cuisine. Technically speaking, the phrase "hot and sour soup" simply refers to any soup that is both spicy and sour at the same time. Taking this technical definition of the term, the Thai dish tom yam soup can be also included in the classification. The most popular version of the dish is from the Chinese province of Szechuan and includes chicken stock, ginger, bamboo shoots and an egg. The egg is dropped into the soup and creates the characteristic streaks of white that can be found throughout the broth.
Ingredients that are common to Chinese cooking are also included in hot and sour soup, such as soy sauce, ginger and sesame oil. Other, more extravagant ingredients can also be included in the soup, such as tree ear mushrooms, black mushrooms and lily buds. The dish always includes vinegar, but this can be different types of vinegar, such as rice vinegar or even white wine vinegar. Mushrooms and some form of meat are also usually added, with chicken or pork being two common choices. White pepper is added in traditional Chinese versions for overall heat, but this can be supplemented with chili, if desired.
One particularly characteristic aspect of hot and sour soup is the egg that is dropped into the broth after thickening. This has led to the soup sometimes being referred to as egg-drop soup, and it causes the small, ribbon-like flecks of white that litter the finished soup. The chef waits until the soup has thickened, then uses a fork or pair of chopsticks to add the egg to the soup. He or she can also add the egg in a circular motion to better spread it out, and some cooks prefer to remove the soup from the heat and add the egg one spoonful at a time. The soup shouldn’t be cooked too long with the egg inside, because it might overcook.
Chefs use several techniques to create a hot and sour soup, and these are vital to achieve an authentic version of the dish. Cornstarch is used to thicken the soup, and this can be combined into a paste with water before being added to the dish. Generally, the ingredients should be chopped into strips of similar sizes, because this assures that they all cook at the same rate. Using natural vinegar or rice vinegar is common with this dish, and cooks also need a high-quality stock to make up the main flavor.