If there are fifteen olive oils on the supermarket shelf, the difference between them is not just the price. Looking at three things is usually enough.

Acidity: lower is better

Acidity is the proportion of free fatty acids in the oil. The later the olives are processed, and the longer they sit crushed, the higher that figure climbs. So acidity is not a measure of taste but of freshness and processing quality.

• Extra virgin - acidity under 0.8 per cent. Choose this for salads, meze and anything used cold.
• Virgin - under 2 per cent. It does the job for cooking.
• Riviera - blended with refined oil. Fine for frying, but if you are after the smell of olive oil you will not find it here.

Very low acidity (0.2 per cent, say) is not a virtue on its own; you have to like the taste as well.

The harvest date matters more than the use by date

Olive oil does not mature, it ages. Oil that has sat in the bottle for two years may still be within its use by date, but its aroma and its polyphenols have faded. If the label gives the harvest year, that producer is confident in their oil.

A rough rule: use it within the first year after harvest.

The colour of the glass does something

Light damages olive oil. Dark glass or a tin protects it noticeably better than clear glass. The same logic applies at home: keep the oil in a cool, dark cupboard rather than beside the hob. Beside the hob is the worst place there is.

Which oil for which dish

The smoke point of olive oil is not as low as people think, so using it in a pan is not a mistake. But spending an expensive extra virgin on deep frying is a waste: the heat drives off the very aromas you paid for.

• Raw - extra virgin. In dishes like shepherd's salad, hummus and haydari the taste of the oil itself is right there.
• Olive oil dishes - extra virgin or virgin. Leeks in olive oil is eaten cold, so the quality of the oil is felt directly.
• Frying - riviera or sunflower oil.

Bitterness is not a fault

A good extra virgin leaves a slight burn at the back of the throat as you swallow, and tastes faintly bitter. That is a sign it contains polyphenols. An oil that is entirely smooth, with no bitterness at all, is usually either old or cut with refined oil.