What to Smell to Check If the Oil Is Rancid
You are probably familiar with rancid oil, but less familiar with its smell. When checking oils in greasy foods for rancidity, smell metal, bitterness, or soap.
Still not sure? Touch the bottle. A sticky residue is a sign of increased rancidity and may indicate that the contents inside are beginning to irritate you. You can also pour some oil between your fingers and test for stickiness, if it seems sticky, toss it in.
Click the link below for a more complete explanation of this process, but enzymes, oxygen and heat play a role in causing rancidity, and technically any fatty food can go rancid. Vegetable and olive oils are especially susceptible to them, and you need to be even more careful with those who have already experienced some heat, such as toasted sesame seeds, so keep them in the refrigerator for extra protection. Fats that are solid at room temperature are more chemically stable and less likely to turn rancid, but I would sniff them from time to time to be sure.