When people talk about bad restaurants, they usually say that they keep on reusing the same oil to deep fry their food. I always wondered why it’s a bad thing. It turns out that it is not necessarily bad all the time.
Here’s what I found on Livestrong’s website:
For safety and quality, it is better to 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. By preparing food for minimal contamination of the oil and straining the oil to get out any food particles left over, you can reuse most oils as long as they are properly stored.
Bacteria and Free Radicals
If used oil is not properly strained and stored after it cools, bacteria can feed on food particles left in the oil. Unrefrigerated oil can even lead 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.
Storing Oil for Reuse
As soon as oil cools enough to handle, strain through layers of cheesecloth, paper towels or coffee filters to remove food particles. Store in a clean glass jar. Never mix it with unused oil. Seal the jar tightly, label it with the date, and refrigerate or freeze for no longer than a month. It may become cloudy in the refrigerator or freezer, but will clear at room temperature. Never reuse oil if it foamed or changed color during heating, or if it has an odd odor or smells like the food you cooked.