When oil is reused for frying, one kind of breakdown product (erythro-dihydroxy) builds up more than another (threo-dihydroxy), because it comes from a different type of fat that forms more during heating.

When you fry potato chips over and over, a specific type of fat called trans-epoxy fatty acid builds up more than other similar fats.

As oil is reused for frying, a specific type of oxidized fat called E,E-hydroxy-LA becomes more common.

When oil is reused for frying, some unstable fat breakdown products disappear, but the stable ones they turn into don’t change much in amount.

Scientists think that looking at the balance between different types of broken-down fats might help tell how much oil has been used for frying.

When you fry potatoes over and over using avocado or olive oil, the oil gets more acidic, more oxidized, and loses some of its natural antioxidants.