Field corn grain is dried and then treated by soaking and cooking the mature, hard grain in a diluted solution of slaked lime (calcium hydroxide) or wood ash. This process is nixtamalization, and it produces hominy, which is ground into flour to create masa harina.
Lime and ash are highly alkaline: the alkalinity helps the dissolution of hemicellulose, the major glue-like component of the maize cell walls, and loosens the hulls from the kernels and softens the corn. Some of the corn oil is broken down into emulsifying agents (monoglycerides and diglycerides), while bonding of the corn proteins to each other is also facilitated. The divalent calcium in lime acts as a cross-linking agent for protein and polysaccharide acidic side chains. The soaked maize is washed, and then ground into masa. When fresh masa is dried and powdered, it becomes masa seca or masa harina.
The chemical changes in masa allow dough formation, and also allows the nutrient niacin to be absorbed by the digestive tract. By contrast, untreated cornmeal is unable to form a dough on the addition of water, and a diet heavily reliant on its consumption is a risk factor for pellagra.