Olives are unusual sources of oil, in that almost all other vegetable oils reside in the fruits’ seeds, whereas in olives the oil is in the flesh.
As olives ripen, their colors change from straw-colored to green, purple, and, finally, black. The transformation from green to black takes place over a period of about 3 to 4 months. Olives may be picked at any one of these stages (except when they are straw-colored), depending on their destiny, whether for oil or for eating at the table. (“May olives be eaten with the fingers?” “No, the fingers should be eaten separately.”—Henry Morgan.) Purplish olives generally produce better-quality oil than fully ripe black ones.
Some “black” or “ripe” California olives are picked at the purple stage and then blackened by treatment with alkali, air, or iron compounds (see below) to produce what are called “black-ripe” olives.
The olives on a tree don’t all ripen at the same time, so there is always a mixture of stages to be harvested. Perhaps the biggest problem faced by olive growers is deciding exactly when to harvest for the best yield of the best stage of ripeness for the olives’ intended purpose. Over the years, different countries and regions have developed and maintained their traditional harvesting practices, which contribute to the different flavor characteristics of, for example, Greek and Italian oils and even oils from different regions of Italy.
Historically, and by that I mean for thousands, not hundreds, of years, olives have been harvested by hand, either individually plucked or by means of a sort of comb known as a pettine (which is Italian for, well, “comb”) that is raked along the branches.
Alternatively, workers may simply beat the branches with poles to dislodge the fruit. Hand-harvesting is still widely used today, although in Spain, the world’s largest producer of olive oil (a lot of olive oil labeled “Italian” is shipped from Spain and bottled in Italy), I saw heavy, tractor-like machines clamp strangleholds on tree trunks and shake the bejeebers out of them, the ripest and less tenacious olives falling into nets placed on the ground around the tree.
For table use, all olives must be processed in some way; you can’t snack on them right off the tree because they contain a bitter phenolic compound called oleuropein. It must be removed either by microbial fermentation or by soaking in a strongly alkaline solution such as sodium hydroxide (lye).
In California, semi-ripe, greenish-purple olives are soaked in a series of lye solutions of diminishing concentrations, being rinsed and aerated after each soak. This treatment, aided in some cases by the addition of ferrous gluconate, an iron compound, turns the olives thoroughly black, after which they are canned.
This blackening process which, like many California customs, is not practiced anywhere else in the world. In Greece and Turkey, though, they do use a similar process to make fully ripe blackish olives dead black.