For safety and quality, use use fresh cooking oil each time you fry. However, if you deep-fry large amounts of food frequently, it is not always practical from an economic standpoint. If used oil is not properly strained and stored after it cools, bacteria feeds on food particles left in the oil. Unrefrigerated oil becomes anaerobic and leads to the growth of Clostridium botulinum, which causes botulism, a potentially fatal food poisoning. Refrigerating or freezing oil retards bacterial growth. Rancid — meaning old and stale — oil contains free radicals, molecules that can damage cells and lead to increased cancer risk, as well as affect the quality of your food. The good news is that your nose can easily identify rancid oil. However, if well-strained, properly stored and not overheated on first use, these oils should be safe to reuse. But, if you do detect any smoke from an oil, discard it.
If you wish to clean and reuse your saved cooking oil, try this hack:
1. After deep-frying, allow your cooking fat to cool to room temperature or slightly warmer.
2. Measure into a small pot half a cup of water for every quart of used oil. Sprinkle it with one teaspoon of powdered gelatine per half cup of water, and let the gelatine hydrate for a few minutes.
3. Bring the water to a simmer (you can do this on the stovetop or in the microwave), stirring, until the gelatine dissolves.
4. Stirring vigorously and constantly, pour the gelatine/water mixture into the dirty oil. It should look very cloudy and relatively homogeneous at this stage. Cover the pot and place it in the refrigerator (or transfer the mixture to a separate container before refrigerating), then allow it to rest overnight.
5. The next day, pour the oil from the top of the pot or container into a separate clean, dry pot. Discard the disk of gelatine that remains. The clarified oil is ready to use.
NOTE: The first time you use the clarified oil, you’ll find that as it heats up, it will start to bubble a little bit. This is okay. Swirl the pan gently as it bubbles to help release any remaining droplets of water. It will eventually settle down until it’s ready for frying.