E524 - Sodium hydroxide

At a glance

• Strong alkali used as a pH regulator and processing aid
• Also known as lye or caustic soda; labeled as E524 in the EU
• Common in lye-curing olives, lye-peeling of tomatoes and peaches, and pretzel making
• Food-grade use is regulated; concentrations are low and products are washed and neutralized before eating
• Not an allergen; the main risk is from improper handling of the concentrated solution

Why is Sodium hydroxide added to food?

Food makers use sodium hydroxide to control acidity (pH), to aid peeling, and to change surface texture and color. In the United States, it is affirmed as GRAS—Generally Recognized As Safe—when used in line with good manufacturing practice as a pH control agent or processing aid.¹ As an example, FDA rules explicitly allow sodium hydroxide solutions for washing and for assisting in peeling certain fruits and vegetables.²

Because it strongly raises pH, a light dip of dough in dilute lye speeds up browning reactions during baking, giving pretzels their signature color and flavor. In olives, a controlled lye treatment helps remove natural bitterness before the final brining step.

“pH” means “potential of hydrogen.” It is a scale from 0 to 14 that shows how acidic or basic something is. Lower numbers are more acidic, higher numbers are more basic (alkaline).

What foods contain Sodium hydroxide?

You’re most likely to find E524 in:

• Lye-cured table olives (the fruit is later washed and brined)
• Lye-peeled tomatoes and peaches used for canning
• Pretzels and some specialty baked goods with an alkaline dip before baking

In the EU and many other regions, lye-curing is a standard step for certain olive styles.³ In the U.S., the FDA allows sodium hydroxide as a peeling aid for fruits and vegetables, with a required potable-water rinse afterward.²

On labels, it may appear as “sodium hydroxide,” “lye,” or “E524.”

What can replace Sodium hydroxide?

Depending on the job, alternatives include:

• For alkaline doughs and browning: sodium carbonate or sodium bicarbonate can mimic some effects, though they are gentler than lye.
• For alkaline processing or nixtamalization-style treatments: calcium hydroxide (also called pickling lime) is a common substitute in traditional grain processing.
• For pH control toward the acidic side: citric acid, lactic acid, or sodium citrates are frequently used.
• In some formulations, buffered systems such as sodium phosphates can help manage pH and texture.

Swapping in an alternative depends on the target pH, flavor impact, texture, and any regulatory or label needs.

How is Sodium hydroxide made?

Most food-grade sodium hydroxide is produced by the chlor‑alkali process. In this method, an aqueous salt solution (brine) is electrolyzed to yield chlorine gas, hydrogen gas, and sodium hydroxide solution, which is then concentrated and purified.⁴

Is Sodium hydroxide safe to eat?

When used correctly at low levels as a pH control agent or processing aid, sodium hydroxide is recognized as safe in the U.S. under 21 CFR §184.1763.¹ The EU also authorizes E524 and sets identity and purity requirements for the additive.⁵

For lye-peeling fruits and vegetables, FDA regulations require a potable-water rinse to remove peel and any remaining solution before further processing, which keeps residues out of the finished food.² Similar wash steps are used in lye-cured olives before brining.³

Does Sodium hydroxide have any benefits?

• Quality and process efficiency: It makes peeling faster and gentler on the fruit compared to mechanical methods alone, supporting better yields and appearance.²
• Flavor development: In pretzels, a brief alkaline dip speeds browning in the oven, creating a distinct crust and taste.
• Bitterness reduction: In olives, lye helps remove bitter compounds before brining, making the fruit palatable.³

Who should avoid Sodium hydroxide?

• Home cooks using lye should handle it with care. Concentrated solutions are highly corrosive and can cause burns to skin and eyes; proper gloves, eye protection, and ventilation are important.⁶
• People on sodium-restricted diets generally do not need to avoid foods processed with lye, because the solution is washed off or neutralized and contributes little sodium to the finished food. If in doubt, check the nutrition label for total sodium.

Myths & facts

• “It’s an industrial chemical, so it can’t be safe.” Fact: Many food additives are also used industrially. Safety depends on grade, dose, and how it’s used. Food-grade sodium hydroxide is regulated and used at very low levels.
• “Pretzels are full of lye.” Myth: Pretzel dough is only dipped briefly in a dilute alkaline bath, and the surface is then baked; there is no pool of free lye in the finished product.
• “Lye-curing leaves olives caustic.” Myth: Olives are thoroughly washed and then brined after lye treatment, removing the caustic solution and balancing flavor.

Sodium hydroxide in branded foods

You may see E524 on labels of:

• Jarred or canned green olives (certain styles)
• Canned tomatoes or peaches (peeled)
• Packaged pretzels and pretzel buns

How it appears on the label:

• Sodium hydroxide
• Lye
• E524 (in regions using E-numbers)

References

Footnotes

1. Sodium hydroxide (21 CFR §184.1763) — U.S. FDA. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-184/subpart-B/section-184.1763 ↩ ↩²

2. Chemicals used in washing or to assist in the peeling of fruits and vegetables (21 CFR §173.315) — U.S. FDA. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-173/section-173.315 ↩ ↩² ↩³ ↩⁴

3. Standard for Table Olives (CXS 66-1981) — FAO/WHO Codex Alimentarius. https://www.fao.org/fao-who-codexalimentarius/sh-proxy/en/?lnk=1&url=https%253A%252F%252Fworkspace.fao.org%252Fsites%252Fcodex%252FStandards%252FCXS%2B66-1981%252FCXS_066e.pdf ↩ ↩² ↩³

4. Sodium hydroxide — PubChem Compound Summary (NIH). https://pubchem.ncbi.nlm.nih.gov/compound/Sodium-hydroxide ↩

5. Commission Regulation (EU) No 231/2012 — EU specifications for food additives. https://eur-lex.europa.eu/eli/reg/2012/231/oj ↩

6. Sodium hydroxide — NIOSH Pocket Guide to Chemical Hazards (CDC). https://www.cdc.gov/niosh/npg/npgd0566.html ↩