This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
>From On Food and Cooking by Harold Mcgee:
"Anyone who has bitten into a raw olive knows that olives must somehow
be processed befoe they are edible. Olives are usualy pickled, and they
contain a bitter glucoside called oleuropein ( from the olive's botanical
name, 'olea europa') which is usually removed first. This has been done
since Roman times by soaking the fruit in a lye solution and then washing
it thoroughly. The watery, oleuropein-rich residue left after raw olives
are pressed for oil - what the Romans called 'amurca' - was used, so
Cato and Virgil tell us, as a weed killer, insecticide, and a lubricant
for leather and axles. Today's Greek olives are as strong tasting as they
are because they have not been treated with lye to remove the oleuropein.
They are simply cured by packing dry in salt, or are pickled in a brine,
where they undergo a lactic fermentation. Green Spanish olives are picked
before they are ripe, treated with lye, and then brined. California ripe
olives are first dipped in a ferrous gluconate solution to fix the pigment,
then treated with lye, and immediately packed in brine. Because they are
not allowed to ferment for a few weeks, these olives have neither the
pickled flavour nor the resistance to spoilage of theother kinds, and so
must be sterilized in the can. The cooking makes some contribution to
their characteristically mild flavour."