Where to Buy Food-Grade Lime in the UK, and How to Nixtamalise Two Kilos of Corn

We have written about nixtamalisation twice already. Why it matters explains the chemistry and what the process does to maize; making nixtamal at home is the step-by-step. Neither of them solves the problem that actually stops British readers: finding the lime in the first place, and knowing what to do when the batch comes out wrong. That is this article. It is a sourcing and troubleshooting piece, not a tutorial, so read one of the other two first if you have never done it.

What the stuff is called in Britain

The ingredient is calcium hydroxide. In Mexico it is cal, or cal para nixtamal, sold in small bags in any market. In Britain it appears under at least four names, and the labelling is the main source of confusion.

• Calcium hydroxide is the chemical name and the one to search for. If the listing specifies food grade, or carries the additive number E526, it is the right thing.
• Pickling lime is the North American name. Listings aimed at home preservers use it, and it is the same compound.
• Hydrated lime and slaked lime are the industrial names for calcium hydroxide. Sometimes the bag is food grade and sometimes it is not, and the only way to know is the label. A builders' merchant bag that says nothing about food use is not food grade, whatever a forum post tells you.
• Cal or cal mexicana is how the imported bags are labelled in Latin American grocers, often with a brand name and a picture of maize.

Two products share the word lime and are the wrong chemistry entirely. Garden lime is calcium carbonate, which is chalk; it will not raise the pH of your cooking water anywhere near enough and the maize will simply boil. Quicklime or burnt lime is calcium oxide, which reacts violently with water and is not a kitchen ingredient. Bicarbonate of soda is also sometimes suggested as a substitute: it is alkaline, it does soften maize, and it does not produce nixtamal. The calcium is half the point, and bicarbonate has none.

Where to buy it

Latin American grocers. The most reliable route, and the cheapest per use, because the bags are sized for repeated batches rather than for a novelty purchase. London has the densest cluster of these shops, but they exist in most large British cities. Our mapped store directory is the place to start, and it includes the mail-order ones, which matter if your nearest Mexican grocer is two hours away.

Online importers of Mexican groceries. Several ship UK-wide and list cal alongside masa harina, dried chillies and tinned chipotles. Ordering all four together is the sensible move, because the postage on a single small bag of lime is the worst value in this entire article.

Brewing, cheesemaking and water-treatment suppliers. Food-grade calcium hydroxide is used to adjust water chemistry in brewing and as a calcium source in some cheesemaking. The homebrew trade sells it in reasonable quantities, properly labelled, and is often the fastest route if you want it this week. Check that the listing says food grade and not technical grade.

Where not to look. Supermarkets do not stock it. Pharmacies do not stock it in a form you should cook with. Garden centres stock the wrong compound. Builders' merchants stock the right compound at the wrong grade, and the impurities in a construction product are the reason grade exists as a concept.

The maize is the harder half of the problem

Nixtamal needs field maize: a dry, hard, starchy grain, dent or flint, harvested mature and dried. Sweetcorn is a different plant bred for sugar and water, and no amount of lime will turn it into masa. What is available in Britain:

• Imported Mexican maize, including the large white cacahuazintle used for pozole, from the specialist grocers and online importers. This is the right ingredient and the most expensive route per kilo.
• Popping maize from any supermarket. It is flint maize, it is food grade, it is cheap, and it genuinely nixtamalises. The kernels are small, so the yield of masa per kilo is lower and the grind takes longer, but as a way to learn the process without a specialist order it works.
• Whole dried maize from health food shops and Polish or African grocers, sold for porridge or samp. Quality varies a great deal; look for whole, unbroken, uniform kernels.
• Animal feed maize. Avoid. It is not produced to food standards, it is often treated, and the saving is not worth it.

Two kilos, start to finish

The working ratio is roughly one per cent lime by the dry weight of maize, so twenty grams for two kilos, in about five litres of water. Whisk the lime into cold water first so it disperses rather than clumping, then add the maize. Bring it to the boil, hold a gentle simmer, and then turn it off and leave it.

How long you simmer depends on the maize and on what you are making. For masa for tortillas, somewhere between fifteen and forty-five minutes of simmering, then eight to fourteen hours of resting off the heat, is the range most home cooks land in. For pozole, you want the grain to stay whole and bloom later in the broth, so it gets less cooking. Use a stainless steel or enamelled pot, never aluminium, which reacts with the alkaline solution.

The kernel is ready when you can rub the skin off between your fingers and the centre is softened through but not mushy. Then wash: drain, rinse in several changes of cold water, rubbing handfuls between your palms to shed the loosened skins. The water runs cloudy and yellow and then gradually clear. Stop rinsing when the maize smells of maize and not of anything else.

What goes wrong, and what to do about it

It smells soapy, or like a swimming pool

Too much lime, not enough rinsing, or both. Rinse again, through more changes of water, with vigorous rubbing. If the smell persists after four or five changes, the ratio was too high and the batch will carry it into the masa. Next time weigh the lime rather than measuring it by spoon, because bulk density varies enormously between brands.

The kernels turned bright yellow and feel slippery

Same cause: too much lime. A faint yellowing is normal and expected. Slippery, almost soapy kernels are over-limed. Rinsing recovers most batches.

The centres are chalky and the dough is gritty

Undercooked, or rested for too short a time. The soak does as much work as the simmer. Return it to the pot with its liquid, bring it back to a simmer briefly, and rest it longer. Chalky centres are the single most common fault and the easiest to fix.

The maize disintegrated into porridge

Overcooked, usually because the simmer was a rolling boil. The batch is not wasted: it is now the base of an atole or a thick soup. Next time, bring it to the boil and then drop the heat until the surface barely moves.

The skins will not come off

Either undercooked or the maize is old and very dry. Old maize benefits from a longer rest and a little more cooking. If a whole bag behaves this way, the bag is the problem, not your method.

The masa cracks at the edges when pressed

Almost always hydration rather than nixtamal. The dough wants to feel like soft modelling clay, and it keeps absorbing water for several minutes after you think it is done. Let it rest fifteen minutes, then add water a tablespoon at a time. A coarse grind cracks too, which is the usual limit of a domestic blender or food processor.

The tortillas taste faintly bitter

Over-liming again, or scorched grain from a dry pot. Keep the maize covered with liquid throughout.

Handling it safely

Calcium hydroxide is strongly alkaline. It is not dangerous in the way a solvent is, but it is an irritant: it dries and burns skin on prolonged contact and it should never go near your eyes. Work with it dry over a sink, do not breathe the dust, wear gloves if you are mixing a large batch, and keep the bag sealed and labelled away from food in a cupboard where nobody will mistake it for flour. Store it dry; it slowly absorbs carbon dioxide from the air and reverts towards chalk, which is why an old open bag works less well than a fresh one.

Is it worth it, honestly

For tortillas eaten the same day, home nixtamal is a genuine step up over masa harina, and the difference is in aroma more than texture. For anything where the masa is a vehicle rather than the point, such as a thick tamal with a strongly flavoured filling, the gap narrows to the point where good masa harina is the sensible choice on a weeknight. For pozole, nixtamalising your own maize is the only way to get the grain that opens like a flower, and tinned hominy is not close.

Do it once for the knowledge. Then decide, with the pot in front of you, how often you want to spend a day on it. If the answer is rarely, that is not a failure; it is the same calculation every cook in Mexico City makes when they walk to the tortilleria instead.