This work presents the study of the structural changes of the endosperm of Quality Protein Maize (QPM H‐368C), modified by alkaline cooking at two different temperatures (72 and 92°C) and steeping time of 0–7 hr. Structural changes in the outermost 10% layers, the subsequent 10%, and the remaining 80% of the endosperm as a function of the steeping time were studied using scanning electron microscopy (SEM), X‐ray diffraction, and differential scanning calorimetry (DSC) techniques. SEM images revealed that soft and hard endosperm have different shapes and packing factors. The X‐ray diffraction patterns of the hard and soft endosperm from raw corn suggest that the hard endosperm consists mainly of amylopectin and has a bigger relative crystallinity quality than the soft endosperm. Samples cooked at 72 and 92°C with and without the (Ca(OH)2 and steeped for 0, 3, and 7 hr, showed structural changes, X‐ray diffraction patterns from the outermost 10% layers and subsequent 10% of the endosperm were completely amorphous. This fact is related to the total or partial gelatinization of the starch. The crystallinity in the internal layers of endosperm (remaining 80%) did not have significant changes after the treatments and exhibited the characteristic patterns of crystalline amylose and amylopectin. DSC measurements in the outermost layers of the endosperm did not exhibit the characteristic endothermic peak of starch (from 64 to 81°C) compared with the raw sample, while the endotherm peak for 80% of the endosperm internal layers appears in all cases (72 and 92°C). According to these results, a new definition of the nixtamalization process can be developed as follows. During the nixtamalization process there is a total gelatinization of the starch granules from the most external layers, and a partial gelatinization of the innermost internal layers of the endosperm.

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In this study, scanning electronic microscopy, X‐ray diffraction, and thermogravimetric analysis were used in order to study the main characteristics of starch from green‐banana, corn, potato, and cassava, and to analyze the process of thermal degradation of these starches. The Flynn–Wall–Ozawa and modified Coats–Redfern methods were used to determine the activation energy (E) of the degradation process, by using thermogravimetric data. These studies have shown that potato starch exhibited the highest activation energy values during the entire thermal degradation process while corn starch exhibited the lower values. Banana, corn, and potato starches showed that its activation energy was not dependent on the conversion extension, and therefore degradation corresponds to a simple process. In contrast, cassava starch showed that E was dependent of the conversion level, indicating that this degradation was more complex which involved more than one reaction. Potato and banana starches had higher values of the activation energy in its thermal degradation, which could be attributed to greater heterogeneity in granule sizes. Corn starch, which has small and homogeneous granules and major contents of the crystalline fractions, presented a lower activation energy in thermal degradation process.

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