🧪 Sodium hydroxide (lye)

Sodium hydroxide (lye) is a strong alkali used to remove bitterness from table olives. As a food additive it has the code E524, and the International Olive Council allows it as a processing aid in the amount needed under good manufacturing practice.

For Spanish-style green olives a weak solution is used, from 1.3 to 3.5 per cent according to different reviews. The olives stay in it for several hours, until the lye has soaked through two thirds of the flesh. For Californian black olives, treatment with a 1–2 per cent solution is repeated several times. Exactly how much lye is needed depends on the cultivar, the ripeness of the fruit, the temperature and the water.

Lye makes the skin permeable and breaks down the bitter oleuropein. After that the olives are rinsed with water several times to wash out all the lye, and with it some of the sugars. As a side effect, lye lowers the content of beneficial phenols and destroys some of the chlorophyll.

Lye also has an environmental side. Methods that use it need up to five times more water than the natural method and produce the dirtiest wastewater: for Californian olives, 2–6 litres per kilogram. That is why lye is reused and rinsing is replaced by neutralisation with acids.