Expert: Fan Zhihong, Associate Professor, Department of Nutrition and Food Safety, College of Food Science and Engineering, China Agricultural University, Ph.D. in Food Science.
During holidays, many people enjoy dining out, feeling that home-cooked meals just don't hit the spot in terms of color, aroma, and flavor, believing that only restaurants can make those special "grand dishes." But why are these dishes so shiny, so fragrant, and have such a great texture?
In fact, to a large extent, it's because restaurant dishes are "stir-fried after deep-frying," with some even being directly deep-fried. For example, when making "Di San Xian" (a dish of stir-fried potato, eggplant, and green pepper), the potatoes and eggplant are first blanched in hot oil before being scooped out and then stir-fried a second time. This prevents them from turning dark and makes them particularly bright and fragrant. Another example is what's called "Gan Bian Dou Jiao" (dry-fried green beans), which are actually fried in a wok until crispy—quick, fragrant, and visually appealing. Whether it's shredded pork or shrimp, they are all first tossed into the hot oil for a "bath" before being stir-fried with other ingredients.
However, this is where the problem arises. Stir-frying after deep-frying requires at least half a wok of oil, but you don't need that much for a single dish. So what happens to the leftover oil? Of course, it's poured back into the oil container to be used again next time. This way, the oil is heated time and time again, making it no different from the oil used for frying dough sticks, which is heated for long periods.
Diners don't seem to have a strong aversion to this kind of oil. Their only complaint is that it feels a bit sticky and greasy, not as smooth as fresh oil. But people don't realize that this repeatedly heated oil is extremely harmful to the body, its danger being no less than, and even surpassing, that of hydrogenated vegetable oil!
A study by Spanish scientists analyzed commonly used household frying oil. They evaluated the degree of oxidation and polymerization in 538 oil samples and investigated the health of 1,226 family members. The results found that the higher the polarity of the oil—meaning the more it degrades after being heated—the greater the risk of family members developing hypertension. In other words, repeatedly heated oil increases the risk of high blood pressure. This association was found even in households where the oil was heated relatively few times; imagine how much more dangerous it must be in restaurants, where the oil is heated an unknown number of times!
The researchers also discovered that using oils with a very high degree of unsaturation, such as sunflower oil, has a more pronounced effect than using olive oil for frying. There is a common misconception that olive oil cannot be heated while soybean oil can. This is incorrect. In reality, olive oil is primarily composed of monounsaturated fatty acids, whereas oils like soybean, sunflower, safflower, and corn oil are rich in polyunsaturated fatty acids. The higher the degree of unsaturation in a fatty acid, the poorer its heat resistance. It is more prone to oxidation, polymerization, hydrolysis, cracking, and cyclization when heated, making it more harmful to the body. This is why industrial frying often uses palm oil or tallow, which are high in saturated fatty acids and have greater thermal stability.
Using these oils rich in unsaturated fatty acids to cook meat and fish also poses another significant danger: the cholesterol in animal products may be oxidized into oxysterols. A growing body of research evidence shows that cholesterol in food itself does not necessarily raise blood lipid levels and is not directly linked to the risk of heart disease. However, oxysterols are the main culprits behind damaging vascular endothelium and causing atherosclerosis. When stir-frying, some of the cholesterol from meat, fish, and eggs leaches into the cooking oil. If this oil is repeatedly heated, the cholesterol will oxidize along with the unsaturated fatty acids, thus posing a danger to people.
At this point, it's easy to understand why there are so many people with chronic diseases like high blood pressure, coronary heart disease, and diabetes among those who dine out frequently. In addition to the unreasonableness of the diet itself, the reused cooking oil is a significant risk factor. When dining out, it's advisable to choose more braised, steamed, and cold dishes, and reduce the proportion of stir-fried and deep-fried dishes to minimize this risk.
On the other hand, when cooking at home, never follow the methods taught on cooking shows like "Tian Tian Yin Shi" (Daily Diet), which instruct you to deep-fry ingredients like a professional chef. When we eat at home, we shouldn't always strive to replicate the taste and texture of restaurant food. The point of home cooking is to eat light, safe, comfortable, and nutritious food. If we turn our home kitchens into restaurants, constantly consuming reused frying oil and having excessive amounts of meat and fat every day, aren't we harming our entire family?