Using day-old oil for deep frying is generally not recommended, especially if the oil has been used to fry other foods previously. As oil is reused for frying, it undergoes chemical changes due to the high heat and exposure to food particles. These changes can lead to the degradation of the oil's quality, flavor, and nutritional value, and it can also result in the production of harmful compounds.
When you use oil for deep frying, it undergoes a process called thermal oxidation, which breaks down the oil and forms free radicals and other harmful substances. Over time and with repeated use, these harmful compounds can increase, leading to the potential production of toxic components.
Reusing oil that has already been used for frying can also introduce flavors and odors from previously fried foods, affecting the taste of the current dish and causing unwanted flavors.
If you want to deep fry foods safely and maintain the best quality, it's best to use fresh oil for each frying session. Dispose of used oil properly by allowing it to cool, then transferring it to a container and disposing of it according to your local regulations. Some municipalities have recycling programs for used cooking oil.
If you must reuse oil, try to limit it to the same day and strain it through a fine mesh sieve or cheesecloth to remove any food particles. However, even with straining, the quality of the oil will still degrade with each use, so it's still best to use fresh oil whenever possible for deep frying.