Corn tortillas: Physicochemical, structural and functional changes

Maize (Zea mays L) is the third most important food crop in the word and a major source of energy, protein and other nutrients for both human and livestock. Starch and non-starch polysaccharides (dietary fiber) are the major carbohydrate constituents. Starch is a polymeric mixture of essentially linear (amylose) and branched (amylopectin) a-glucans. Starch owes much of its functionality and physical organization into a granular structure to these macromolecules. In Mexico, and gradually in other countries like the United States, consumption of maize tortillas becomes common. The process to convert maize into tortillas, called nixtamalization, starts when the grain is cooked in lime [Ca(OH)2] to produce nixtamal. The moist nixtamal is stone-ground into a dough (masa), that is further shaped into disks and baked on a hot griddle or gas-fired oven to produce tortillas. Tortillas can be produce from fresh masa using traditional nixtamal, or instant corn flour, which is dehydrated masa. During maize tortillas production, many changes occur, especially in starch, and the changes that this polymer suffers during processing are responsible for the textural and sensorial properties of masa and tortillas. The masa obtained from the nixtamalization process is a network of disperse and soluble starch polymers; starch granules, partially gelatinized in a continuous water phase, supported by free starch granules, other pieces of endosperm and lipids. The reassociation of the starch granules can also significantly affect the rheological properties of products made from masa. On the other hand, reassociation of the starch molecules may occur on cooling. Retrogradation, the ability of starch chains to form ordered structures in pastes, gels, and baked foods during storage, greatly influences the texture and shelf life of these products. In starch-rich products, amylose retrogradation is a rapid process taking only a few hours. Cereal-based foods such a tortillas can contain appreciable amounts of resistant starch (RS) which survives prolonged incubation with amylolitic enzymes. The occurrence of RS has important implications for human bowel physiology. Diets rich in RS produce a larger faecal mass with lower pH and increase daily butyrate production. The presence of fermentable substrates helped in preventing inflammatory intestinal diseases. The study of the reactions leading to the staling of stored tortillas has considerable technological, commercial and health implications, because different physicochemical and structural changes in RS formed during nixtamalization, tortillas elaboration and storage happen.