Effect of storage time on in vitro digestibility and resistant starch content of nixtamal, masa, and tortilla

Nixtamal, masa, and tortilla samples were stored for 24-96 hr and their chemical composition, retrogradation, and in vitro starch digestibility features were evaluated. Ash and fat contents in the three products were smaller than in the original corn sample, but protein levels were higher, all in accordance with previous studies. In general, a minor decrease in available starch (AS) content was observed with storage time. Masa showed the greatest AS values, followed by tortilla and nixtamal. Tortilla presented slightly higher retrograded resistant starch (RS3) values (1.1-1.8%, dmb) than masa (0.7-0.9%) and nixtamal (0.7-0.8%) and only minor increases were observed after 24 hr of storage, suggesting that retrogradation phenomenon in these samples takes place very rapidly and is more pronounced in the final product (tortilla). The development of RS3 explains the observed decrease in AS. Higher total resistant starch values were found in all samples at a range of 2.1-2.6% for nixtamal and masa, and a range of 3.1-3.9% in tortilla. This indicates that, apart from retrograded resistant starch, some ungelatinized fractions appear to contibute to the indigestible content of these products. The α-amylolysis rate of the three materials decreased with storage. Tortilla showed the greatest hydrolysis indices. Differential scanning calorimetry (DSC) analysis showed that the nixtamal, masa, and tortilla did not show differences in amylopectin crystal melting temperature with storage time, but tortilla exhibited higher enthalpy values after 72 hr of storage, in accordance with the greater total RS contents recorded after prolonged storage.