**2.4. Corn masa viscoelasticity**

Corn masa was prepared from each ENCF using 100 g sample and adding distilled water. The quantity of water utilized corresponded to the WAC. Once prepared masas, they were allowed to stand for 30 min in a plastic bag at room temperature (25 °C). Samples of 2 g of masa each were weighed out to be utilized in the rheometer.

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 241

Response surface methodology (RSM) was used, and the process variables were: temperature of the fourth zone of the extruder (T, 110-130 °C), moisture content of conditioned ground corn (MC, 25-35 % of the corn weight), and concentration of xanthan gum (XG, 0.3-0.7 % of corn weigth). A central composite rotatable design was utilized (Table 1), with three factors and 5 levels (Montgomery 2001; Myers 1971). In order to observe the difference between specific treatments, Tukey's test was used at a level of significance of p= 0.05 . To find the best variables combination of the process to obtain the extruded corn flour, the conventional graphical method was used and maximizing WAI, Tan δ; minimizing Gʹ, Gʹʹ and MF. To obtain contour plots for the visualization and selection of the best combination of T, MC and XG to prepare the extruded nixtamalized corn flour, the contour plots of each of the response variables were utilized, by means of the method of superimposition of surfaces. Data analysis and the elaboration of surface response and contour plots were performed using Design Expert version 6.0.7 software (Design Expert,

WAI is a parameter that gives an idea of the absorption of water of corn flour, and is an indicator of yield of fresh masa (Molina et al. 1977). The highest value of WAI in ENCFs was of 3.6 g of gel/g of dry matter, and was obtained at high concentration (0.84%) of xanthan gum (treatment 14, Table 1), which would indicate the capacity of the gum to form gels. This could be due to the high affinity of hydrocolloids for water, because of its branched structure. During hydration, water molecules hydrogen bond with hydroxyl (or carboxyl) groups found in the unit components (sugars) of hydrocolloid molecules, inducing this association with increased capacity for water retention (Dickinson 2003). Aguirre-Crus et al. (2005) observed in their research an increase in the capacity for water retention in suspensions of masa of corn dehydrated with hydrocolloids at different

WAC is the quantity of water that is absorbed by the flour to obtain a masa of appropriate consistency for the preparation of tortillas and is a subjective test. WAC was affected very significantly (p < 0.01) By the treatment. The WAC range in the ENCFs was between 74.8 - 89 mL water/100 g flour (Table 1). Arámbula et al. (1999) reported in extruded flours with xanthan gum a high WAC value of 88.5 mL/100g. However, the value for extruded flour reported by González (2006) was 72 mL water/100 g flour, which was low; probably due to that none type of gum was added. Arámbula et al. (2002) found in extruded flour a WAC of 70 mL/100g with 0%, and 80 mL/100g with 3% of addition of corn pericarp,

**2.7. Experimental design and statistical analysis** 

2002).

temperatures.

respectively.

**3. Results and discussion** 

**3.1. Corn flours evaluation** 

The oscillatory dynamic scanning test was performed utilizing a dynamic mechanical spectrometer (Rheometrics Scientific, model RSF III. Piscataway, NJ, USA) equipped with parallel plates of 25 mm diameter and a chamber for temperature control (peltier). A sample was placed between the plates separated by a gap of 2.5 mm. The excess of masa was cut off using a plastic instrument. Next, petroleum jelly was applied where the sample was air exposed to prevent loss of moisture. The frequency sweep test was carried out using a software (RSI Orchestrator, Rheometrics Scientific). Each test was run to a deformation of 0.04% and at 25°C, which gave a minimum of structure disorder and with sufficient assurance of the level of torsion (Broulliet-Fourmann et al. 2003). The deformation used was previously determined to work in the viscoelastic linear region in a frequency range from 0.1 to 100 rad/s. The viscoelastic parameters obtained in the frequency range used were the storage modulus (Gʹ) and loss modulus (Gʹʹ) in kPa, and the tangent of the phase angle (Tan δ).
