Effect of moisture content and temperature, on the rheological, microstructural and thermal properties of MASA (dough) from a hybrid corn (Zea Mays sp.) variety

The effect of the moisture content (55, 60 and 65% (w/w)) and temperature (30, 40 and 50 °C) on the X-ray diffraction (XDR), microstructural, pasting, thermal and rheological properties of nixtamalized dough (masa) from a corn hybrid variety (Zea mays sp.) is reported here. A complete set of rheological tests including temperature and frequency sweeps, steady shear and transient shear flow was performed in order to get detailed information on the food processing issues. The nixtamalization process affected significantly (P < 0.05) the microstructural and thermal properties of masa. Polarized Light microscopy and X-ray diffraction (XDR) showed that crystallinity in starch granules decreased for the masa; whereas, Scanning Electron Microscopy (SEM), showed swollen granules dispersed into a plasticized surface. Moreover, Differential Scanning Calorimetry (DSC) showed significant (P < 0.05) differences on the gelatinization enthalpy (ΔH_gel) of masa, where it increased with increasing the moisture content from 6.7 ± 0.84 J/g for 55% (w/w) to 10.2 ± 0.4 J/g for 60% (w/w). Frequency sweeps showed a predominant elastic behavior where the storage modulus (G′) was higher than the loss modulus (G″) and they were significantly (P < 0.05) affected by the moisture content and temperature into a small range. The steady shear data exhibited a shear-thinning flow behavior and followed a power law equation, where the power law index (n) decreased when the temperature and the moisture content increased. The effect of temperature on the shear-viscosity (η) was well described by the Arrhenius equation, exhibiting energy activation energy (E_a) values in the range from 25.52 to 59.35 KJ/mol. For transient shear test, all masas presented a stress overshoot at high shear rate before reaching a steady state. It was found that the amplitude of this overshoot depends on the shear rate. On the other hand, the stress relaxation test, which gives the main relaxation time (τ), showed fast relaxation decay at short times, whereas at long times, a slow relaxation was observed. The τ values ranged from 10.46 to 0.43 s, which decreased with increasing shear rate. The rheological behavior of masa was related to a weak gel-like structure formation, composed by dispersed swollen starch granules into a cross-linked starch network, similar to a biocomposite material.