Buying

Acidity, and what the number really tells you

New olive oil running into a steel funnel at the mill

Not what it sounds like

The word misleads almost everyone. Free acidity has nothing to do with how sour or sharp an oil tastes; you cannot detect it on the palate at all. It is a laboratory measure of free fatty acids — mostly oleic acid — expressed as a percentage. Fatty acids are “free” when they have broken loose from the triglyceride molecules that make up the oil, and that happens when the fruit is damaged.

What raises it

Olives that have been bruised in picking, attacked by fly, left in heaps to warm and ferment, or simply kept waiting for days before milling all arrive at the press with more free fatty acids. So do olives picked from the ground. Acidity is, in effect, a record of how the fruit was treated between the tree and the mill. That is why it is used as the first line of the extra virgin standard: an oil must be below 0.8% to carry the name at all.

Acidity does not describe the taste of an oil. It describes the care taken with the fruit before pressing.

Makaron

What it cannot tell you

A low figure proves the fruit was sound and handled quickly; it does not, on its own, tell you the oil is aromatic, or that it is rich in polyphenols, or that it has been stored well since. For that there are other tests — peroxide value and ultraviolet absorbance for oxidation, total polyphenols for the antioxidants, and a trained tasting panel for defects and fruitiness. We run the laboratory tests on every lot and keep the results with the lot number, because one good number is a claim; the full sheet is a record.