"Heat the wok first, then add cool oil, and stir-fry over high heat quickly" is the best stir-frying method. Related research shows that when cooking this way, the vitamin C loss in vegetables is about 17%, whereas in dishes slow-cooked over low heat, the vitamin C loss can be as high as 59%.
First, heat the wok, then pour in the oil, and immediately add the ingredients. At this point, the oil temperature will not be too high, staying below 200°C. However, if the oil temperature is too high, it can cause a series of decomposition and polymerization reactions in the oil, releasing harmful volatile components such as low-level aldehydes and ketones, producing large carcinogenic cyclic molecules, and also generating smoke.
When ingredients are added to warm oil, they experience a momentary buffer upon heating. The cook can use this moment to quickly separate or spread the ingredients. This allows the proteins on the surface of the ingredients to gradually denature, making them easier to unfold, ensuring more even and thorough heating, resulting in a better texture, shape, and color for the finished dish.
When the wok bottom is hot and the oil is relatively cool, adding the ingredients creates a pushing force as the oil temperature continuously rises. This force can make the ingredients float quickly, preventing them from sticking to the wok and keeping them from breaking.
How to judge the temperature of stir-frying oil?
To judge the oil temperature, it can be divided into four stages:
One or two "成热" (cold-warm oil): The oil temperature is 30°C-60°C. The oil surface is calm with almost no smoke or sound. It is suitable for making crispy peanuts, cashews, etc.
Three or four "成热" (low-temperature oil): The oil temperature is 90°C-120°C. The oil surface has white bubbles but no smoke, and it makes a slight sound when shaken. It is suitable for "溜肉" (quick-frying meat).
Five or six "成热" (medium-temperature oil): The oil temperature is 150°C-180°C. The oil surface is churning with a hint of blue smoke, and it makes a sound when shaken. It is suitable for "炒" (stir-frying), "炝" (quick-frying with scallions), and "炸" (deep-frying).
Seven or eight "成热" (high-temperature oil): The oil temperature is 200°C-240°C. The oil surface becomes calm again with straight blue smoke, and it makes a sound when shaken. It is suitable for "爆" (quick-frying) and "重油炸" (deep-frying).