A tortilla is made by curing maize in limewater in the nixtamalization process, which causes the skin of the corn kernels to peel off (the waste material is typically fed to poultry), then grinding and cooking it, kneading it into a dough called "masa nixtamalera", pressing it flat into thin patties using a rolling pin or a tortilla press, and cooking it on a very hot "comal" (originally a flat terra cotta griddle, now usually made of light sheet-metal instead). The process, called nixtamalization, was developed indigenously by pre-Columbian cultures and predates European contact by many centuries, if not millennia. Soaking the maize in limewater is important because it makes available the B vitamin niacin and the amino acid tryptophan. When maize was brought to Europe, Africa and Asia from the New World, this crucial step was often omitted. Those whose diet consisted mostly of corn meal often became sick — because of the lack of niacin and tryptophan — with the deficiency disease pellagra, which was common in Spain, Northern Italy, and the southern United States.