“Rancid.” Even the word sounds gross. Or at the very least, like a super angry punk band. Either way, it’s definitely not how you’d want anyone to describe the food you’re eating. The first step to side-stepping rancidity? Know your enemy.
“From a chemical standpoint, any oil or fat can go rancid,” says Luke LaBorde, Ph.D., Associate Professor in the Department of Food Science at Penn State University. That’s why you’ve probably smelled those stale aromas in a bottle of forgotten vegetable oil. As Dr. LaBorde explains it, the rancidity reaction (or, as it’s technically called, “hydrolytic rancidity”) is all about breakdown: When oils or fats start breaking down due to enzymes, oxygen, or heat, they “oxidize” into smaller chemical compounds called fatty acids. And it’s those smaller compounds that create off-flavors and aromas.
Oils like vegetable or olive oil are especially susceptible to going rancid. And if you’re dealing with toasted versions like Asian sesame oil or toasted nut oils, the reaction happens especially quickly, since these oils were already exposed to heat when they were made. Fats that are solid at room temperature, like lard, coconut oil, and chicken fat, are less prone to rancidity, since these saturated fats are more chemically stable, says LaBorde.
But even fresh oil can go rancid in a matter of minutes. If you reuse and reheat the same oil or fat over and over (to make, say, several batches of the best fried chicken ever) the heat will trigger the rancidity reaction, creating fishy off-flavors in your frying oil that then affect the taste of your food.
Crackers and cookies can go rancid, too—as can basically any food that contains fat and tends to sit around for a few weeks in your pantry. (Consider this a rallying call to eat all the cookies you’ve got: the butter in those precious cookies is oxidizing!)
But not all rancidity is bad, Dr. LaBorde points out. “There are good rancidity reactions, too. Any time you have cheese, you have enzymes breaking down milk into fatty acids,” he says. Thankfully, that chemical process is all part of the cheesemaker’s plan, so the overall taste turns out delicious.
And that brings us to the best way to tell if your food has gone rancid: Give it a sniff. “The smell test covers just about everything,” says Dr. LaBorde. If your food has bitter, metallic, or soapy aromas, or just smells “off,” you’re probably dealing with rancidity. Another easy way to tell if there may be rancidity: If your bottle of oil feels sticky. That’s oil residue undergoing polymerization, says LaBorde—an advanced stage of the rancidity process. If your oil looks darker than usual, that’s another sign.
“When in doubt, throw it out. It’s not gonna kill you, but there is some [health] concern about all these complex reactions going on,” Dr. LaBorde says. And let’s not forget the most important factor here. Rancid food is just gross.