According to International Olive Council rules, extra-virgin olive oil must have a maximum free fatty acid (FFA) level of 0.8 percent and a maximum peroxide value of 20.
The FFA level measures the amount of chemical breakdown of the oil. This tends to increase as the fruit ages - and also between harvest and process - but it's fairly stable once the oil is bottled. There's little connection between FFA levels and flavour.
The peroxide value (PV) measures the level of oxidation at any given time. It's affected by air or light coming into contact with the oil during processing and storage. The peroxide value can continue to rise in the bottle over time.
Beyond the technical ...
All the extra-virgin olive oils passed the chemical analysis, so why did our tasting panel pick up defects in 10 of them?
When an olive is picked, it has a PV of almost zero. A good olive oil pressed promptly under good conditions will end up in the bottle with a PV of 2 to 3. But there's still oxygen in the bottle - and so the oxidation process begins.
When oil reacts with oxygen it begins to deteriorate. The PV will increase in the bottle, with the peroxides breaking down to give nasty-smelling and nasty-tasting compounds.
Light also speeds up the oxidation process. "Off" flavours will be evident even at low PV values, if the oil has been exposed to light. But because most people don't sip and suck olive oil from a glass (as our tasters did), these faults are often difficult to pick.
Some in the scientific community are concerned that the International Olive Council's PV maximum of 20 is too high. Olives New Zealand (the New Zealand industry body) agrees.
To qualify for Olives New Zealand certification, New Zealand olive oils must have a PV value of less than 15. They must also be in dark bottles or packaged in a box - and they must show both the year of production and a best-before date of no more than two years after the oil was pressed. Olives New Zealand certification is indicated by a red sticker on the bottle.
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