To obtain the nixtamalized masa, the maize grains are subjected to three types of processes: cooking (thermo-alkaline), steeping time (ionic interchange) and milling (thermo-mechanical) which give rise to modifications in the starch (Villada et al., 2017). Starch, the main component of maize, is a complex micro or sub-micro particle composed of amylose, amylopectin, fat, protein, and bound and free water, but mainly of nanocrystals with orthorhombic and/or hexagonal crystal structure and other smaller components such as water, proteins, lipids, and minerals (Esquivel-Fajardo et al., 2022; Rodriguez-Garcia et al., 2020). The nixtamalization process leads to partial gelatinization of the starch, a complex endothermic process in which some components of the starch granules (nanocrystals) change from an ordered to a disordered state. This is because the starch granules swell in the presence of water, and through heating. When they are swollen to their maximum capacity, the granules are dissolved (Ratnayake & Jackson, 2008; Wang & Copeland, 2013). There are several methods to evaluate the gelatinization process, such as viscosity profile, differential scanning calorimetry (DSC), and X-ray diffraction (Villada et al., 2017). Nixtamalized masa is a network of solubilized starch polymers with gelatinized, uncooked and swollen starch granules, cell fragments, proteins, and lipids (Quintanar-Guzmán et al., 2011).

Diver's foods that have as base nixtamalized masa as tortillas, tamales, and snacks. Additionally, nixtamalized masa can be subjected to the fermentation process to make other foods such as beverages (Fernández-Suárez et al., 2013). In this case, the solid-state fermentation (SSF) is implemented to improve nutritional characteristics. SSF is a fermentation involving solids in the absence (almost absence) of free water. Though, the substrate must have sufficient moisture to support the growth and metabolism of microorganisms. SSF stimulates the growth of microorganisms in nature on moist solids, this fermentation is considered to be responsible for the beginning of fermentation in ancient times (Pandey, 2003). In this sense, around the world there are different traditional beverages, for example pozol and atole agrio in Mexico.

The nixtamalized masa is the basis for the preparation of various food products or is used as a substrate for the production of fermented products (Fernández-Suárez et al., 2013), starch being the main carbohydrate (Robledo-Márquez et al., 2021) that is hydrolyzed by enzymes such as α-amylase, β-amylase, or pullulanase, these enzymes can arrive to the substrate from starch or microorganisms to process molecules such as maltose, dextrins, and glucose for the fermentation of the nixtamalized masa (Díaz-Ruiz et al., 2003). Sefa-Dedeh et al. (2003) studied the SSF of mixtures with steeped and nixtamalized maize (0:100, 25:75, 50:50, 75:25, 100:0, steeped: nixtamalized maize) for different time (0, 24, 48 h). The sample derived from 100 % nixtamalized masa showed a decrease in viscosity and increased titratable acidity. However, the changes that occur in the starch of nixtamalized masa during SSF in terms of morphology, structure, and certain physicochemical properties remain unclear. Afoakwa and Aidoo (2006) evaluated blends compositions with steeped y nixtamalized maize (0:100, 50:50, 100:0) during 48 h used water and coco water. The results indicated a decreasing in the pH, colour, and texture, and increasing in titratable acidity, water absorption capacity but without analyzing the structural properties of nanocrystals with orthorhombic crystal structure, which play an important role in fermentation.

On the other hand, Phiri et al. (2019) subjected maize to a cooking process to obtain gelatinized starch, which was hydrolyzed by the amylolytic enzymes and converted into fermentable sugars. The fermentable sugars were used in spontaneous fermentation to produce beverages. Recently, Espinosa-Ramírez et al. (2021) determined different degrees of gelatinization to compare the quality of fresh nixtamalized masa for tortilla production and studied the thermal properties, crystallinity, and RVA parameters. Nevertheless, the effect of the degree of gelatinization on nixtamalized masa as a substrate in fermentation has not yet been clarified. Therefore, the present study aims to evaluate the effect of two cooking times of nixtamalization (30 and 90 min) and solid-state fermentation (0 to 240 h) on the morphology, structure, and physicochemical properties of starch in masa.