When cooking at home, it's inevitable to have some leftovers after meals. Used oil from frying is the most troublesome. Throwing it away feels wasteful, but keeping it is a concern because it's already been heated once. Reheating it will produce carcinogenic smoke earlier, degrade the quality of your dishes, and increase its oxidation level, which is also unpleasant from a psychological standpoint.
The main issue with used oil is to avoid reheating it to high temperatures, which causes smoke, and to prevent further thermal oxidation and polymerization. Once you grasp this principle, the solutions are straightforward, as there are many cooking methods that don't require heat. However, it's already a bit late to deal with the oil after it's been used. It's best to plan its purpose in advance before you even put it in the pan.
Method 1: Use it as a flavored oil
Homemade cold salads and noodles often benefit from a drizzle of flavored oil, such as scallion oil, garlic oil, Sichuan peppercorn oil, or chili oil. Towards the end of your frying, you can add various seasonings to the oil while it's at the right temperature to create these fragrant oils.
For example, when you're about to finish frying eggplant boxes, prepare a large amount of Sichuan peppercorns. Reduce the heat to low and slowly fry the peppercorns until their fragrance infuses the oil. Then, turn off the heat, strain out the solids and peppercorns, and pour the peppercorn-infused oil into a ceramic jar or bottle. Seal it and store it in the refrigerator for later use as a seasoning. Another example is after frying meatballs, add crushed chili and minced garlic, fry for two more minutes, then pour the mixture over a bowl of crushed chili. After cooling and straining, you'll have chili oil that can be sealed and refrigerated for later use. These seasonings have some antioxidant properties, which objectively extends the oil's shelf life for up to half a month.
Method 2: Use it in baked goods
When making flatbreads, sesame cakes, scallion pancakes, or other pastries that require oil, you can use the strained, used oil. The internal temperature of the bread during baking doesn't get very high (below 100°C), and the starch in the dough provides some protection, so the oil won't undergo significant oxidation.
Method 3: Use it for blanching vegetables or in soups
Many vegetables are blanched in boiling water before being served cold. It's best to add a tablespoon of oil to the boiling water. This keeps the vegetable leaves bright in color and tender in texture, even if they are slightly older, preventing them from being tough. Since the temperature is only 100°C and the blanching time is short (about a minute), there's no risk of smoking, making used oil a perfectly fine choice. You can also add a little used oil at the end when making vegetable soup. In particular, oil left from frying meat or chicken can add a nice touch of flavor.