Many people like to stir-fry at high temperature when cooking, and they are used to waiting for the oil in the pot to smoke before cooking. This is unscientific. High temperature oil will not only destroy the nutrients of food, but also produce some peroxides and carcinogens. It is suggested to heat the pot first, and then pour the oil. At this time, you don't have to wait for the oil to smoke. If there is no oil, it will cause the lack of vitamins and essential fatty acids in the body and affect human health. You can't just emphasize eating vegetable oil instead of animal oil. In a certain dose, animal oil (saturated fatty acid) is beneficial to human body. At present, it is difficult for ordinary families to fry with any oil, but we suggest that it is best to use several oils alternately, or use one oil for a while and another oil for a while, because few oils can solve all the oil demand problems. For people with dyslipidemia or special people with abnormal weight, we emphasize the choice of vegetable oil with high monounsaturated fatty acids. For people with normal blood lipids and weight, the total oil consumption should be controlled at no more than 25 grams per day, and polyunsaturated fatty acids and monounsaturated fatty acids basically account for half. Good quality soybean oil is dark yellow, generally light yellow; Rapeseed oil is yellow, with some green or golden yellow; Peanut oil is light yellow or orange, and cottonseed oil is light yellow. If pigments or impurities are added in the production process, the color and transparency will change. Touch a little oil with your fingers, rub it in your palm, and smell it after rubbing. Good quality oil should have its own oil flavor according to different varieties, and there should be no other peculiar smell. If there is a slight fishy smell, it may be "gutter oil", and normal cooking oil will only emit a faint fragrance. Heat oil. If it smells, it is more likely to be "gutter oil". Touch a little oil with your fingers, rub it in your palm, and smell it after rubbing. Good quality oil should have its own oil flavor according to different varieties, and there should be no other peculiar smell. If there is a slight fishy smell, it may be "gutter oil", and normal cooking oil will only emit a faint fragrance. Heat oil. If it smells, it is more likely to be "gutter oil". High transparency, less moisture and impurities, good quality. Good vegetable oil, after standing for 24 hours, should be clear and transparent, without turbidity, precipitation and suspended matter. Dip a little oil in your mouth with chopsticks, and don't have bitter, burnt or rancid smell. You can dip chopsticks in a drop of oil and taste it. If the taste is sour or smelly, it is probably "gutter oil". You can light the fuel and listen to its sound. Qualified products are products with normal combustion and no sound; Abnormal combustion and "squeaking" sound are due to excessive moisture; When burning, there was a "crackling" explosion, indicating that the water content of the oil was seriously exceeded, which may be an adulterated product.