How olive oil gets adulterated, and why it is hard to spot

Olive oil is one of the most frequently adulterated foods in the world. Many people picture that meaning strange chemicals or coloured water. It almost never is.

The three most common forms

1. The wrong grade

Oil that should be graded virgin, or refined, sold as extra virgin. It is still entirely olive oil — it is simply not the grade on the label, and it should cost less.

This is the most common form, and the hardest for a consumer to detect, because nothing is "wrong" with the oil in any obvious sense.

2. Cut with seed oil

Cheaper sunflower, canola or soy oil blended with a little olive oil and sold as extra virgin. At small proportions this is hard to taste, particularly if the olive oil used has a strong flavour.

3. The wrong origin

Oil from one country bottled in another and marketed as though it originated there. This is often legal if the label is accurate, but packaging can suggest something the label does not actually state.

Why it is hard to spot

You cannot see any of it. Colour does not help — it can be altered. Price helps a little, but expensive oil is faked too, and cheap oil may be honest about its grade.

Taste helps more. Oil cut with seed oil tends to taste flat — it loses the bitterness and pungency that come from polyphenols, because seed oil carries none.

What a laboratory looks for

• K270 — the ultraviolet absorbance that rises when refined oil is present
• Fatty acid profile — every oil has its own signature, and seed oils look different from olive
• Sterols — compounds whose profile differs between types of plant oil
• Free acidity and peroxide value — for freshness and handling

This is why a laboratory report matters and a home test does not. These require equipment that is in nobody's kitchen.

How to protect yourself

1. 1.Buy from a seller who can name the official importer
2. 2.Ask for a laboratory report for the current batch, with a laboratory named and dated
3. 3.Choose single origin rather than a "blend"
4. 4.Look for a harvest date on the label
5. 5.Taste the oil on its own before cooking with it
6. 6.Be wary of a price far below everything else on the shelf