Peroxides are formed naturally when oil is exposed to oxygen – oxidation happens, and the higher levels of peroxide, the more you’ll find defective flavours and odours in your olive oil.
The level of peroxide in an olive oil is the easiest way to measure rancidity – or whether or not the olive oil has gone bad. The lower the level of peroxide, the fresher and healthier the olive oil is.
This is a tricky one to measure, but in order for an olive oil to be labeled extra virgin, it must have a peroxide level equal to, or less than, 20 millequivalents/kg. To be labeled ultra premium, it must have a peroxide level of 9 millequivalents/kg.