**2. Experimental procedure**

#### **2.1. Raw material**

238 Viscoelasticity – From Theory to Biological Applications

industry (Aguirre-Cruz et al. 2005).

(Broulliet-Fourmann et al. 2003).

Hoseney 1990).

(Gʹʹ /G<sup>ʹ</sup>

gum, alginates, carrageenans and xanthan gum (Gurkin 2002).

some additives such as hydrocolloids which are water-soluble, high-molecular weight heteropolysaccharides. These compounds vary in shape and function and add flexibility to the tortillas produced, acting as a fat replacement, water binder, texturizer and adhesive. Among the main hydrocolloids utilized in tortillas is carboxymethylcelullose (CMC), guar

Roman-Brito et al (2007) studied the effect of xanthan gum on nixtamalized corn flour with 0.25%(w/w), 0.50%(w/w), and 0.75%(w/w) of xanthan gum to make tortillas. These authors observed a decrease in hardness and increase in flexibility in the tortillas during their storage at 4°C. Yau et al. (1994) also extended the stability of tortillas made from corn flour during their storage at 25°C with xanthan gum at 1% (w/w), along with other additives. Arämbula et al. (1999), prepared tortillas with extruded corn flour with the addition of hydrocolloids: CMC, arabic gum, guar gum and xanthan gum at 0.5% (w/w) with different concentrations of lime. Tortillas were obtained with good results regarding to their textural characteristics (rollability, extensibility and shear force) with masa containing 0.2 %( w/w) of lime and xanthan gum added before extrusion. The unique structure and properties of xanthan gum account for its potential in versatile applications in the food industry (Hanna et al. 1997). The effect of hydrocolloids has not been studied at any concentration or mixture of them in the production of tortillas from extruded corn flour. It could help to keep a soft texture of tortillas during storage. The addition of hydrocolloids can modify the rheological (viscoelasticity) properties of corn masa, which are important in the tortilla making process. The understanding of these parameters is crucial for the design of equipment and definition of operation parameters in the tortilla

On the other hand, one of the methods most frequently utilized for the study of viscoelastic properties of masa is the dynamic method. In this test, results obtained refer to parameters that help to characterize appropriately the materials whose rheological behavior is complex, such as dynamic moduli of storage and loss, respectively the energy stored elastically and that dissipated as heat during a cycle of deformation (Faubion and

The storage modulus G<sup>ʹ</sup> is an indicator of the elastic component of the material, and the loss modulus G**ʹʹ** is an indicator of the viscous component. The storage and loss moduli are usually reported as a function of frequency. The phase angle represents a simple mean of elastic and viscous natures of the material. On some occasions, this property is expressed as the tangent of the phase angle (Tan δ), that is, the ratio between the loss and storage moduli

). Moisture content is an important element in the determination of viscoelastic properties of cereals, which are reduced proportionally with moisture content (Masi et al. Measurement of viscoelastic characteristics have been utilized in dispersions in masas of nixtamalized corn dehydrated with hydrocolloids (Aguirre-Cruz et al. 2005), and of commercial corn flour at different moisture content: 35%, 40% and 50%, respectively White corn (Dekalb variety) from Sinaloa de Leyva, Sinaloa, México was used. Corn was cleaned using a vibrating cleaner (Blount, model M2BC. Bluffon, IN, USA), and stored at 5 °C until used.

## **2.2. Extrusion process**

Samples of corn (2 kg) were ground in a mill (Pulvex, model 200, Mexico, D.F.), with a 0.8 mm sieve. The ground corn was mixed with commercial grade lime (Ca (OH2)) at 0.3 % (w/w) and xanthan gum (Spectrum Chemical, Gardena, CA, USA) at different concentrations (0.3-0.7 % of corn weigth) and using an industrial mixer (Hobart, model AS200T. Troy, Ohio, USA). Next, distilled water was added to this blend up to reach the moisture content appropriated (range 25-35 % of the corn weight). To obain a complete hydration of the ground corn particles, samples were packed in a polyethylene bag and stored for 12 h at 5 °C. Before extrusion, samples were tempered at 25°C during 4 h.

The extrusion process was carried out in a single-screw laboratory extruder (Brabender, model 837416. Duisburg, Germany) with a 19 mm screw-diameter, length-to-diameter 25:1, nominal compression ratio 1:1, and a die opening of 3 mm. The first three zones of the extruder were maintained at 60 ºC, and the fourth zone was varied (110-130 °C), with a screw speed of 112 rpm. A screw-operated feed hopper fed the extruders at 45 rpm. Extrudates were dried at 60 °C for 1 h in a tunnel type dryer (no brand), and cooled at room temperature (25°C). To obtain the extruded nixtamalized corn flour (ENCF), the extrudates were ground in a mill (Pulvex, model 200, Mexico, D.F.) with a sieve 0.8-mm diameter**,** and packed in plastic bag at 5°C.

#### **2.3. Corn flours evaluation**

The ENCFs were analyzed for water absorption index (WAI) and subjective water absorption capacity (WAC). These response variables are the most critical for making tortillas at commercial level. WAI was measured using the method of Anderson et al. (1969) with a modification: The distilled water temperature was 25 °C instead of 30 °C, and theWAI was expressed as g of gel / g of dry matter. WAC was determined using the method described by Flores-Farías et al. (2002). The quantity of water added was recorded as the capacity for water absortion of the flour in mL of water / 100 g of flour.
