In Spanish, masa harina literally means “dough flour,” but many people use it specifically to refer to a type of coarse meal made from corn which has been subjected to the nixtamalization process. This specially treated corn is used to make tortillas, fillings for tamales, and a variety of other traditional Mexican foods. If you live in a region with a Mexican grocery, you can probably find masa harina there, and some larger markets also carry this food product.
The process of making masa harina begins with cooking whole grains of corn in an alkali solution which loosens their outer casings. The corn is soaked after cooking and then rinsed repeatedly to remove the caustic alkali. After this process is complete, the corn is ground to create masa, a coarse dough which can be used fresh or dried and sold as masa harina.
Some cooks feel that fresh masa is superior to masa harina. If you happen to live near a tortilla factory, you may be able to obtain some fresh masa to test the difference for yourself. Other people feel that masa harina is perfectly adequate for making tortillas and other Mexican dishes. It can also be used somewhat like cornmeal, although it has a much finer texture than cornmeal.
Because masa harina is made from corn, it is typically gluten free, and some gluten intolerant cooks and bakers like to experiment with it as a result. It definitely has a corny flavor, which can cause it to clash with some ingredients, but it can be used in things like cookies and pies as well as in more conventional tortillas.
It is important to avoid confusing masa harina with cornmeal or corn flour. Cornmeal is made by grinding corn, either dry or wet, but the corn is not treated with lime first. As a result, cornmeals and flours behave different than masa harina in baking and cooking. They are also not as nutritionally valuable as nixtamalized masa.
The nixtamalization process was developed in Mesoamerica, and it is thousands of years old. In addition to softening the corn for grinding into masa, it also changes the structure of the corn, freeing the nutritionally valuable niacin and adding calcium from the lime used as an alkali. This process made corn an excellent nutritional addition to the diet of early Americans, and it explains why corn became such an important crop across the Americas.