Nixtamal: The 3,500-Year-Old Chemistry Inside a Tortilla

“Corn treated with lime” is four words on the back of a tortilla package, and a lot of people read them as a warning. They’re the reason a corn tortilla is a food you can live on and plain cornmeal isn’t, and the chemistry behind them is older than the tortilla.

Lime here means slaked lime, calcium hydroxide, the white powder sold as cal in a small bag near the dried chiles. Dried field corn simmers in water with a spoonful of it and then sits overnight. By morning the kernels have swelled, the water has gone the color of weak tea, and the hull slips off between your fingers. What’s left is nixtamal. Grind it wet and you have masa. Press the masa and set it on a hot comal and you have a tortilla. If you try it at home, buy food-grade cal, keep the powder out of your eyes, and rinse the kernels well before you grind.

“The difference was a pot of alkaline water and a night's soak.

What the lime does

Corn has niacin, vitamin B3, but it holds most of it in a locked form, bound up in compounds called niacytin and niacinogen that human digestion can’t open.¹ An alkaline soak breaks that bond and frees the niacin so the body can absorb it.¹² The same soak loosens the hull and leaves the kernel steeped in calcium from the lime.¹ An adult needs around 14 to 16 milligrams of niacin a day, and corn that hasn’t met lime gives up only a fraction of what it holds.¹² On the calcium the numbers aren’t subtle. Whole corn carries about 7 milligrams of calcium per 100 grams and degerminated cornmeal 2 or 3; masa flour made from nixtamal carries about 136, and the difference is the lime, left behind in the corn.³ That’s the dry flour. A finished tortilla is about half water, so a single one carries somewhere around 10 to 20 milligrams.⁷

There’s no sign anyone in Mesoamerica was soaking corn for the B vitamins. They wanted a hull that came off and a dough that held together on the griddle.¹ The vitamin was a side effect of wanting a better tortilla.

How old it is

Older than anyone can prove exactly. Nixtamal doesn’t fossilize, so archaeologists have leaned on a proxy, the flat milling stones used to grind cooked corn, which show up in the record as early as 1500 to 1200 BCE.¹ That’s where the 3,500 years in the title comes from, and it rests on the grinding tools rather than on a dated batch of corn. In 2020 two archaeologists, Emily Johnson and John Marston, published a way to spot nixtamalized corn directly, from a starch byproduct the alkaline cooking leaves behind.¹ Expect the dates to move as the method gets used. Those spherulites, as they’re called, survive in the residue on pots and grinding stones and in the dirt itself, which means older kitchens can now be checked.

The idea traveled, too. In the American Southwest and the Eastern Woodlands, cooks reached for lye leached from wood ash instead of lime, and the result was hominy rather than masa.¹ A different alkali does the same chemistry, and the niacin comes free either way.

What happened without it

Corn crossed the Atlantic after 1492 and became a staple in the wet parts of Spain and northern Portugal, then well beyond.⁴ The lime stayed behind. In Italy the poor came to eat it mainly as polenta.⁴ A Spanish physician named Gaspar Casal described the illness that followed in the eighteenth century, and over the next two hundred years pellagra followed maize from Spain to Italy, France, central Europe, Romania, Turkey, Greece, and parts of southern Russia.⁴ It showed up as a rough, darkening rash where the sun hit the skin, with diarrhea and, in time, confusion.² Those are the three Ds of pellagra, dermatitis, diarrhea, and dementia, and untreated it was fatal.² In the parts of Italy that lived on maize, pellagra stayed endemic for more than two centuries and had reached over 100,000 people by the 1880s.⁴

The same thing happened in the American South in the early 1900s. The diet of poor Southern families had narrowed to meal, meat, and molasses, the three Ms of poverty that Joseph Goldberger tied the disease to, with the meal being plain cornmeal.⁵ Between 1906 and 1940 roughly three million Americans developed pellagra and about 100,000 died of it.⁵ For years the leading theories blamed spoiled corn or an infection. Goldberger, a Public Health Service doctor, spent fifteen years taking those theories apart. He produced pellagra in prison volunteers fed grits, pork fat, and syrup, six of eleven within six months, cleared it in patients with a daily dose of brewer’s yeast, and injected himself with the blood of the sick to show it didn’t spread.⁵ He died in 1929, sure it was a missing vitamin and not yet able to say which one.⁵ Researchers at the University of Wisconsin pinned it to niacin in 1937.⁵

The corn in Italy and Alabama was the same plant as the corn in Oaxaca. The difference was a pot of alkaline water and a night’s soak.

Finding it in a Houston store

Fresh corn tortillas are easy to find in Houston, and nearly every commercial one starts as nixtamal, because that’s what masa is. I still read the back. On the ingredient panel look for “corn treated with lime,” “trace of lime,” or “corn masa flour,” since masa harina is nixtamalized by definition.⁶ On a Spanish-language label the word is cal. If the list says corn flour or cornmeal and lime appears nowhere, it isn’t nixtamal, and unless that corn is labeled whole or enriched, the USDA won’t count it as a grain in school meals either.⁶ A short list is a good sign. Corn, water, and lime is the whole recipe, and a gum or a preservative after that is about shelf life.

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A good tortilla smells like sweet corn and bends without cracking, and a packaged one comes most of the way back if you warm it on a dry skillet until it softens and blisters in a few spots. Keep the stack wrapped in a towel while you finish the rest. Cold, they go stiff, and heat brings them back.

People ask whether a corn tortilla still counts as a whole grain, since the lime bath loosens the hull and some of it comes off in the rinse.¹³ In 2019 the USDA answered it for school and child nutrition programs. Hominy, corn masa, and masa harina all count as a whole grain there; the memo notes that the soak raises the availability of nutrients the kernel already had.⁶ Don’t expect the package to say so. Most don’t. The lime on the label is the tell.