What is it?
Sodium hydroxide, also called lye or caustic soda, is a strong alkali, the opposite of an acid. In food it is used to make things less acidic. It is one of the strongest alkalis allowed in food production.
E-numbers / Other additives / E524
Sodium hydroxide
also: Caustic soda · lye
Caustic soda, used in small amounts to change how acidic food is and to brown pretzels. It reacts away during processing, leaving a sodium salt.
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Sugar, Palm Fat, Water, Invert Sugar Syrup, Emulsifiers (E471, E435, E481), Sea Salt, Preservative (E202), Stabiliser (E450), Flavouring, Acidity Regulators (E524, E330), Colour (E160a)
E524
Sodium hydroxide
Sodium hydroxide, also called lye or caustic soda, is a strong alkali, the opposite of an acid.
VeganVegetarian
235
of the 43,325 UK supermarket foods we checked declare E524 in their ingredients
That is 0.5%, counted by its code or its name.
Only 23% of these labels print “E524”. Most use the name: look for “sodium hydroxide”.
Counted by NutraSafe from UK supermarket ingredient lists, September 2026. How we counted
How common it is in UK food, by aisle
- Bread and bakery65 products1.8% of 3,682
- Cakes, biscuits and desserts63 products1.7% of 3,715
- Chocolate and sweets38 products1.9% of 2,045
- Crisps and savoury snacks32 products0.9% of 3,428
What sodium hydroxide is, what it does and where you will see it
What does it do?
It cancels out acids. As a surface treatment it reacts with the outside of food. That is what gives lye pretzels their dark, shiny crust. In cocoa it takes the edge off bitterness and darkens the colour, and in olive curing it breaks down the bitter compounds. Because it reacts with the food itself, what is left in the finished product is a sodium salt, not the caustic chemical.
Where you will see it
Lye-dipped pretzels and other baked goods, cocoa labelled Dutch-process or alkalised, cured olives, some noodles (it gives them their yellow colour and springy bite), and as an acidity control in soft drinks, jams and processed cheese. Labels call it sodium hydroxide or E524.
What the science says
It is used up in processing, so the caustic chemical does not reach you
Sodium hydroxide reacts with acids and other substances in the food and turns into salts. The free chemical that makes it caustic is not in the finished food. European reviewers, and the food scientists before them, noted that because it is used up in the reaction, people take in almost none of it from properly made food.
Reviewers found that when sodium hydroxide is used to control acidity, people are exposed to almost none of it, because it reacts with the food as it is used.
2013Regulator reviewEFSA ANS Panel, re-evaluation of sodium hydroxide (E524)
It leaves behind a little sodium
Although the chemical itself is used up, it turns into sodium salts, which add a little sodium to the diet. In most foods the amount is small next to other sources such as table salt.
Reviewers noted that sodium from sodium hydroxide used to control acidity adds a negligible amount to total sodium intake.
2013Regulator reviewEFSA ANS Panel, re-evaluation of sodium hydroxide (E524)
Where it stands with the regulators
Approved in the UK and the EU
Daily limit. None set.
- Legal basis
- On the Food Standards Agency's approved list for Great Britain, as an acidity regulator
- Permitted foods
- Baked goods and dough · Cocoa and chocolate products · Processed olives · Soft drinks · Jams · Processed cheese · Many other foods, to control acidity
- Maximum levels
- No set maximum, as long as makers use no more than they need.
- History
- Sodium hydroxide has been allowed in food in the EU and UK for decades. European reviewers looked at it again in 2013 as part of reviewing every approved additive, and found no grounds to restrict it. No bans or end dates have been set.
Who should be careful
No group is advised to avoid it in normally processed food. People cutting down on sodium should know it turns into sodium salts in food, though the amount is small next to the salt used in cooking. Look for sodium hydroxide or E524 if you are counting every sodium source.
The honest read
Sodium hydroxide has a frightening name and burns skin in its pure form, which drives alarm online. In food the picture is simple: it reacts with the food and is used up during processing, so the caustic chemical is gone by the time the product reaches the shop. The 2013 European review found nothing to flag. It has been used in food for centuries, from lye-cured olives around the Mediterranean to German pretzels.
Common questions about sodium hydroxide
Is E524 banned in the UK?
No. Sodium hydroxide is an approved additive in the UK and EU, used to control acidity and in processing. It is on the Food Standards Agency's approved list.
Is it dangerous to eat sodium hydroxide?
Pure sodium hydroxide is a caustic industrial chemical. In food it is used in weak solutions and reacts with the food during processing, so the finished product does not contain it in free form. What you eat is the sodium salt left by that reaction, not the caustic chemical.
What foods contain E524?
Lye-dipped pretzels, Dutch-process cocoa, cured olives, some Asian-style noodles, and a range of processed foods that need their acidity controlled during production.
Is E524 vegan?
Yes. It is a mineral compound with nothing from animals.
Sources
- UK FSA: Approved additives and E numbers
- EFSA ANS Panel: Re-evaluation of sodium hydroxide (E 524) as a food additive
- Assimilated EU Regulation 1333/2008 on food additives (UK retained law)
Last reviewed: 20 June 2026
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