It’s a good question, because on an intuitive level one would think that if a corn tortilla is a simple pancake made of ground corn, there really shouldn’t be that much difference between finely ground corn meal and masa harina (literally “dough flour”). In reality there’s a huge difference, which can be summed up in a single word: nixtamalization. Yeah, that’s a mouthful. It comes from the Aztec word “nixtamal” which, loosely translated, means “ash dough.”
Not very appetizing by the sound of it. However what ancient Mesoamericans discovered 3,500 years ago is that when you boil and then soak raw corn kernels together with ashes, some very interesting things happen. Notably, the outer hulls (pericarps) of the kernels loosen to the point that they can be slipped off. Further, what’s left — the endosperm and germ of the kernels — becomes quite soft and pliable, easily ground into a dough that’s perfect for making tortillas.
So nixtamalization is all about heat and water then? Er, not quite. The key component is really the ash, which, long-time readers of joepastry.com may remember, has a high pH. Add a handful to a pot of water and you get an alkaline solution that goes to work dissolving the glues that hold the walls of the pericarp cells together (the hemicellulose and pectins). The pericarp loosens and falls off, and the endosperm swells as it takes in more and more of the chemical-laden water. As it does, the tight bundles of starch molecules in the endosperm start to come apart. What you get, once the corn is finally drained and washed, is a processed corn product that’s easily ground into a dough that’s just smooth enough and just elastic enough to be formed into a thin cake like a tortilla.
Neat. But it raises the question: how did the ancient Mesoamericans hit on this process? Unknown. It may have started when someone mixed a few wood ashes in with some corn gruel (a common technique that indigenous American peoples used to lighten — really leaven — cakes made from grains). But who knows? Wood ashes (which contain potassium carbonate) weren’t the only alkaline compounds available to the peoples of that time. Naturally occurring lime (calcium carbonate-rich limestone and/or chalk deposits) were also used, as were the ashes of burned mussel shells.
These days, most people who want to make their own corn masa use a commercially-made product that simply goes by the name of “cal” in Mexico. Calcium hydroxide is what it is, commonly known as “slaked” (dissolvable) lime in America. It can be found in most Mexican markets in the spice section.
Buying the cal and boiling and steeping the corn is the easy part, however. The real challenge when it comes to making one’s own corn masa is the grinding. Getting it fine enough to make tortillas is no easy task. Food processors aren’t good enough. What’s needed is either a plate grinder or a good, old-fashioned metate. I’ve never used one, but I can imagine they call for more elbow grease than I’d be willing to put in for home-made tortillas. But those of you who might be interested, knock yourselves out! You can get them in Mexican markets as well. They cost less than you’d think.