"Hot pot, cool oil" and "high heat, quick stir-fry" are the best methods for stir-frying. Related research shows that when cooking this way, the vitamin C loss in vegetables is about 17%, while in dishes simmered over low heat, the vitamin C loss can be as high as 59%.
First, heat the pan, then pour in the oil, and immediately add the ingredients to the pan. 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 compounds, and also creating smoke.
When ingredients are added to warm oil, they experience a brief 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 expand and stretch. This ensures more even and thorough heating, resulting in a better texture and a more attractive appearance for the finished dish.
When the pan bottom is hot and the oil is relatively cool, adding ingredients to the oil allows them to be pushed up by the oil as its temperature continuously rises. This creates a lifting force that can quickly bring the ingredients to the surface, preventing them from sticking to the pan and breaking apart.
To judge the oil temperature, it can be divided into four stages:
One to two parts heat (cold-warm oil): The oil temperature is 30°C-60°C. The oil surface is calm with almost no smoke or sound. Suitable for making crispy peanuts, cashews, etc.
Three to four parts heat (low-temperature oil): The oil temperature is 90°C-120°C. The oil surface has white bubbles but no smoke, and there is a slight sound when shaken. Suitable for "liu" meat (a quick stir-fry with a sauce).
Five to six parts heat (medium-temperature oil): The oil temperature is 150°C-180°C. The oil surface is churning with light blue smoke, and there is a sound when shaken. Suitable for stir-frying, "qiang" (quick-frying with spices), and deep-frying.
Seven to eight parts heat (high-temperature oil): The oil temperature is 200°C-240°C. The oil surface appears calm, with straight columns of blue smoke, and there is a sound when shaken. Suitable for "bao" (quick-frying) and deep-frying.