Corn is unusual among the grains for its large size, and for the thickness and toughness of its outer pericarp, or hull. Early corn eaters developed a special pretreatment to ease the removal of the hull, which is called nixtamalization (from an Aztec word): they cooked the kernels in water made alkaline with a variety of substances. The Mayas and Aztecs used ashes or lime; North American tribes, ashes and naturally occurring sodium carbonate deposits; and a contemporary Mayan group burns mussel shells for the same purpose. One of the major glue-like components of plant cells walls, hemicellulose, is especially soluble in alkaline conditions. Nixtamalization softens the hull and partly detaches it from the rest of the kernel so that it can be rubbed off and washed away. It also helps transform the kernels into a cohesive dough for making tortillas and other preparations (see below), and it releases much of their bound niacin so that we can absorb and benefit from it.