Want to buy the best Extra Virgin Olive Oil for your money? There are a few numbers you have to check, and Peroxide Number (PV) is not well known, but tells you a lot about the quality of the oil. If you patronize a reputable Olive Oil Store you should have seen the PV number on the information tag for the oil (you will never see this in supermarket olive oil).
Unsaturated Free Fatty Acids in Olive Oil react with Oxygen in the air and form peroxides. Peroxides are responsible for the musty, rancidity found in poor quality or old oil. Heat and light also accelerate the formation of Peroxides. The lower the value the better. The International Olive Council allows a PV of 20 is a maximum value for oil to be classified as Extra Virgin, but you should be looking for a PV below 10 to make sure your oil is fresh and will remain so for some time.