What does the 'extra' mean on an olive oil bottle

A.� Virgin oils can be single estate or a blend of oils from sought-after producers - the most sought after are labelled 'First Cold-Pressing Extra Virgin'. First cold-pressing tells you this is the first squeeze of the olive and that no heat has been used in the extraction (a process that allows a second helping, but alters the oil's flavour).

The world 'extra' indicates the acidity is naturally less than one per cent - a measurement of oleic acid that makes all the difference when tasting.

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All the virgins oils are at their best when raw. They lose much of their character on heating, so they're largely wasted in a frying pan.

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If the label just says 'Olive Oil', the contents will have been heat treated or had chemical interference of some kind.

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Q.� What are the main types of olive oil

A.� The categories are virgin, rectified and a mixture of the two. Rectified oils are usually blended with a bit of virgin to give it character, but they make good mixers in a vinaigrgrette or mayonnaise.

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The newest category is called 'lite' which is a fully rectified, filtered oil, used in baking as a replacement fat.

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Q.� What about organic oil

A.� The quality of organic oils can vary. Producers of good virgin olive oils have no difficulty selling them, and have no incentive to convert to organics. Organic farmers trap pests instead of spraying them, avoid using fertiliser and keep the ground free of dead wood, which otherwise creates an ideal breeding ground for the olive fly.

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Q.� When are they pressed

A.� Olives ripen from green to violet to black and any variety can be pressed for oil, at any stage of the ripening process. This gives variations in flavour, viscosity and colour. As with all fruit juices, the quality is dictated by the freshness and ripeness of the raw material, the speed with which it reaches the miller and the care with which it's pressed and stored.

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Professional tasters never sample more than four oils a day, take a slice of apple between swallows and drink oil neat from opaque blue glasses, so their palates aren't confused by colour or clarity.

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Early picked olives deliver greener, leafier oils and a more obvious fondo - the foggy cloud that swirls around newly-pressed virgins - that clears as it settles, while late-harvested oils are sweeter, clearer and more golden. Single estate producers tend to blend early and late late harvests. As a rule of thumb, Greek oils are grassy and citrussy, Italian offerings are sweet and spicy, Spanish oils are peppery with a hint of tropical fruit, and French oils are pure and sweet.

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Q.� How good is olive oil for you

A.� It's low in saturated fats and high in unsaturated fats with lots of vitamin A, D and E. Olive oil is at its best in the first year and healthiest when taken uncooked. A grappa sized shot of extra virgin before a night at the bar is said to ensure a clear head the next day.

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�By Katharine MacColl