On Food and Cooking

Roman times, the soaking water was often
supplemented with alkaline wood ashes,
which cut the debittering period from weeks
to hours. (The modern industrial treatment is
a 1–3% solution of sodium hydroxide, or lye.)
Alkaline conditions actually break bitter
oleuropein down, and also breach the waxy
outer cuticle and dissolve cell-wall materials.
These effects make the fruit as a whole more
permeable to the salt brine that follows (after
a wash and acid treatment to neutralize the
alkalinity), and help the fermentation proceed
faster. Lactic acid bacteria are the main
fermenters, though some yeasts also grow and
contribute to the aroma. Olives may be
debittered and fermented while still green
(“Spanish” style, the major commercial type)
or once their skin has turned dark with
purplish anthocyanins, when they are less
bitter.
Olives are also fermented without any
preliminary leaching or alkaline treatment,