Rancid oil can ruin your dish. But what is rancid oil exactly? And how can you tell if your oil has gone bad?
To find out, we talked to Eric Decker, a professor in the Department of Food Science at the University of Massachusetts Amherst. Decker explained that culinary oils are mostly composed of triglycerides; over time, these triglycerides oxidize, or react with oxygen, and decompose into small molecules. The good news is that some of these small molecules smell, so in the case of refined oils such as vegetable or canola oil, which have relatively little odor or flavor, it’s easy to determine whether or not it’s rancid. Simply pour a small amount into a spoon and give it a quick sniff. If it has an “off” odor—perhaps like crayons, metal, or something sour—it’s past its prime.
Unrefined oils, including extra-virgin olive oil, are trickier. According to Decker, they have aromas and flavors that can mask rancidity, so it’s hard for nonexperts to know if they are rancid by smell alone. As such, Decker recommends tasting the oil. Pour some into a cup and, if needed, warm the cup in your hands to get the oil to room temperature. Take a small sip (about a teaspoon’s worth) and suck on it as if you were pulling liquid through a straw, without swallowing or exhaling. If it is rancid, it will have an off-flavor that results from the combination of the olive oil flavor and the rancidity aromas. Because this flavor can be hard to describe, Decker recommends tasting—and smelling—any oil the first time you open the container so that you have a baseline to work from. This technique is especially helpful in the case of olive oils.
Lastly, rancid oils can also take on a stickiness, so if the container feels tacky around the inside of the spout, it might be time to toss it.