🧪 Sodium hydroxide (lye)

Sodium hydroxide (NaOH, food additive E524), also called caustic soda, is a strong alkali used to remove bitterness from table olives. Seville-style green olives are treated with a weak 1.3–3.5% solution, after which the lye is thoroughly rinsed out with water. It is a caustic substance that causes severe skin burns and eye damage, so it is handled with care.

For Seville-style green olives a weak solution is used — from 1.3 to 3.5%. The olives stay in it for several hours, until the lye has penetrated two thirds of the flesh; for Californian-style black olives, treatment with a 1–2% solution is repeated 2–5 times. The lye makes the skin permeable and breaks down bitter oleuropein, after which the olives are rinsed several times with water to wash out all the lye.

Lye has uses beyond table olives too: it neutralises oil during refining, and with olive oil and water it makes soap. Lye methods use up to five times more water than the natural method and produce the dirtiest wastewater — 2–6 litres per kilogram for Californian-style olives. That is why the lye solution is increasingly reused.

Sodium hydroxide is hazardous: under the Globally Harmonized System of classification of chemicals (GHS) it causes severe skin burns and eye damage and is corrosive to metals. For treating olives at home, University of California specialists advise working in safety goggles and rubber gloves, always adding lye to water rather than water to lye, and not using containers made of aluminium, tin or galvanised metal. If lye gets on the skin or in the eyes, rinse them with water for at least 15 minutes; if it is swallowed, do not induce vomiting — seek medical help at once.

Why it matters to you. The lye itself does not stay in the olives: it is washed out, and at home rinsing goes on until the soapy taste is gone; a pinkish brine means more rinsing is needed. For home treatment only pure sodium hydroxide will do, not drain cleaners containing other substances; keep it in a tightly closed container out of children's reach.