Olives need to undergo a curing process before they’re ready to eat, due to the bitterness of oleuropein. If you’ve accidentally bitten into a raw olive, you are familiar with the bitterness that follows.
Olive curing is the conversion of the olive’s natural sugars into lactic acid – a process carried out by lactic acid bacteria. Harsh-tasting oleuropein and phenols get extracted from olives in the following ways:
Brine-curing: Fully ripened, dark purple or black olives are gradually fermented in brine (i.e. salt water). This can take up to a year. Brine-cured olives are often sweet and full of depth, since the fermentation process acts to intensify the fruit’s natural flavours.
Water-curing: Soaking, rinsing in plain water, and repeating for about 20 days. This method uses a lot of water and actually washes out the fermentable sugars together with the bitterness. Hereafter the fermentation rarely can take place.
Dry-curing: These olives are packed in salt for a month or longer. The salt pulls the moisture and bitterness from the olives. The salt is then removed, and sometimes the olives get bathed in olive oil to keep them juicy and plump. Dry-cured olives have a deeply concentrated flavour, and a wrinkly, prune-like appearance. Oil-cured olives are dry-cured olives that get macerated, or softened, in oil for several months.
Lye-curing: Large commercial olive producers make use of this time and cost-effective method. Invented in Spain, the process calls for immersing raw olives in vats of alkaline lye solution. This is used for green olives, to hasten the fermentation process – once the fruit is debittered with the lye, they are immediately placed into acidified brine to ferment. The lactic acid produced during the fermentation process neutralizes the excess lye. The important aspect is to check the acidity often.
Sun/air-curing: In some rare cases, olives can be debittered either on the branch or, once picked, by basking in the sunshine.