**3. Results and discussion**

240 Viscoelasticity – From Theory to Biological Applications

the tangent of the phase angle (Tan δ).

**2.6. Corn tortilla textural evaluation** 

treatment in each test.

**2.5. Corn tortilla preparation** 

masa each were weighed out to be utilized in the rheometer.

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

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

Two kg of masa from each ENCF were mixed with distilled water to obtain masa. The amount of water was based on the WAC of each extruded treatment. Masas from ENCF were transported to a commercial factory. A roller machine (Rodotec, model RT-100, Guadalupe, N.L., México) was used with a mold of 14 cm diameter, and was adjusted to a masa weight of 25 g. Tortillas were baked in an oven (integrated to the roller machine) with 3 temperature zones: zone 1, 270 ± 10 °C; zone 2, 320 ± 30 °C; and zone 3, 300 ± 25°C, with a residence time of 60 s. The baked tortillas were cooled at room temperature (25 °C).

To determine firmmess and rollability, tortillas packaged in plastic bags were placed at room temperature (25°C). Firmness and rollability were measured at 2 h, 24 h and 48 h after baking. The firmness test was carried out according to the procedure reported by Ramírez-Wong et al. (2007), modifying the cross head speed of the texturometer (Instron, model 4465. Canton, MA, USA) to 50 cm/min. Firmness was expressed as maximal force (MF) in kPa. Regarding tortilla rollability, three strips 2 cm wide were cut from each tortilla and individually tested (Waniska 1976). Each strip of tortilla was rolled up in a wooden cylinder 2 cm in diameter, and was examined for degree of rupture, which was established on a scale of 1 to 5, where 5 indicated no tear of the tortilla (maximum rollability), 3 partial tear, and 1 complete tearing. Five tortillas were used for each

**2.4. Corn masa viscoelasticity** 

## **3.1. Corn flours evaluation**

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 temperatures.

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, respectively.


Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 243

small gradual changes occurred at high frequencies. Hence, the values of the viscoelastic parameters of 10 rad/s were selected for the optimization of the variables of the process for making tortillas from ENCFs. Values of Gʹ were higher than those of Gʹʹ (Table 1), indicating that the elastic behavior predominated over the viscous behavior of masas. Similar trend was found by Aguirre-Cruz et al. (2005) in samples of suspension of corn masa at 10% (w/w) of solids in their heating/cooling kinetics, and by Broulliet-Fourmann et al. (2003) in corn

**Figure 1.** Storage modulus (Gʹ) vs frequency for extruded nixtamalized corn flours in treatments:

(a) 1-10 and (b) 11-20. Error bars indicate standard error of means.

flours at different moisture contents (35%, 40% and 50%).

242 Viscoelasticity – From Theory to Biological Applications

aNumbers do not correspond to the order of processing. bT, extrusion temperature (°C); MC, moisture content (%, w/w); XG, xanthan gum (%, w/w); values in parentheses are the coded levels. c WAC, subjetive water absorption capacity (mL water/100/g flour; WAI, water absorption index (g gel/g dry matter); Gʹ, storage modulus (kPa), Gʹʹ, loss modulus (Kpa); Tan δ, tangent of the phase angle; MF, maximal force (kPa). dMean of three replicates.

**Table 1.** Experimental designa used to obtein different combinations of extrusion temperature/moisture content/xanthan gum for production of extrusion-nixtamalized corn flour.

#### **3.2. Masa viscoelasticity evaluation**

In Figures 1(a, b), 2(a, b) and 3(a, b) are presented the storage modulus (Gʹ), loss modulus (Gʹʹ) and tangent of the phase angle (Tan δ), respectively. It is observed that all of these viscoelastic parameters increased with frequency. The range 0.1-10 rad/s was the most susceptible to structural changes (Broulliet-Fourmann et al. 2003). In this frequency range, there was a considerable increase in the viscoelastic parameters Gʹ, Gʹʹ and Tan δ, whereas small gradual changes occurred at high frequencies. Hence, the values of the viscoelastic parameters of 10 rad/s were selected for the optimization of the variables of the process for making tortillas from ENCFs. Values of Gʹ were higher than those of Gʹʹ (Table 1), indicating that the elastic behavior predominated over the viscous behavior of masas. Similar trend was found by Aguirre-Cruz et al. (2005) in samples of suspension of corn masa at 10% (w/w) of solids in their heating/cooling kinetics, and by Broulliet-Fourmann et al. (2003) in corn flours at different moisture contents (35%, 40% and 50%).

242 Viscoelasticity – From Theory to Biological Applications

<sup>9</sup>103.18 (-

<sup>10</sup>136.82

11 120 (0) 21.59 (-

12 120 (0) 38.41

**3.2. Masa viscoelasticity evaluation** 

13 120 (0) 30 (0) 0.16 (-

14 120 (0) 30 (0) 0.84

**Treatmenta Process Factorsb Response Variablesc,d**

**T(X 1) MC (X2) XG(X3 ) WAC WAI Gʹ Gʹʹ Tan δ MF** 

1.681) 30 (0) 0.50 (0) 82.8 3.5 231.0 50.7 0.22 44.9

(+1.681) 30 (0) 0.50 (0) 76.6 3.4 219.3 52.7 0.24 51.0

15 120 (0) 30 (0) 0.50 (0) 87.3 3.4 218.7 54.7 0.25 46.0 16 120 (0) 30 (0) 0.50 (0) 87.0 3.2 190.3 45.7 0.24 43.6 17 120 (0) 30 (0) 0.50 (0) 87.3 3.5 225.3 55.8 0.25 46.5 18 120 (0) 30 (0) 0.50 (0) 87.0 3.5 218.7 49.3 0.23 49.1 19 120 (0) 30 (0) 0.50 (0) 87.3 3.2 176.6 46.5 0.26 41.2 20 120 (0) 30 (0) 0.50 (0) 87.0 3.1 210.3 53.5 0.25 41.2

aNumbers do not correspond to the order of processing. bT, extrusion temperature (°C); MC, moisture content

modulus (Kpa); Tan δ, tangent of the phase angle; MF, maximal force (kPa). dMean of three replicates.

capacity (mL water/100/g flour; WAI, water absorption index (g gel/g dry matter); Gʹ, storage modulus (kPa), Gʹʹ, loss

**Table 1.** Experimental designa used to obtein different combinations of extrusion temperature/moisture

In Figures 1(a, b), 2(a, b) and 3(a, b) are presented the storage modulus (Gʹ), loss modulus (Gʹʹ) and tangent of the phase angle (Tan δ), respectively. It is observed that all of these viscoelastic parameters increased with frequency. The range 0.1-10 rad/s was the most susceptible to structural changes (Broulliet-Fourmann et al. 2003). In this frequency range, there was a considerable increase in the viscoelastic parameters Gʹ, Gʹʹ and Tan δ, whereas

(%, w/w); XG, xanthan gum (%, w/w); values in parentheses are the coded levels. c

content/xanthan gum for production of extrusion-nixtamalized corn flour.

1.681) 0.50 (0) 80.1 2.5 257.0 61.6 0.24 61.3

(+1.681) 0.50 (0) 79.0 2.9 294.7 72.1 0.24 55.9

1.681) 83.1 3.0 243.7 50.7 0.21 49.7

(+1.681) 89.0 3.6 211.0 47.8 0.23 43.7

WAC, subjetive water absorption

1 110 (-1) 25 (-1) 0.30 (-1) 76.9 3.0 274.3 58.3 0.21 53.3 2 130 (+1) 25 (-1) 0.30 (-1) 77.0 2.8 269.3 59.8 0.22 59.0 3 110 (-1) 35 (+1) 0.30 (-1) 76.3 3.2 223.0 54.8 0.25 47.0 4 130 (+1) 35 (+1) 0.30 (-1) 78.6 3.3 302.0 60.0 0.20 48.5 5 110 (+1) 25 (-1) 0.70 (+1) 74.8 2.6 231.0 51.6 0.22 58.5 6 130 (-1) 25 (-1) 0.70 (+1) 79.6 2.2 270.3 63.3 0.23 58.6 7 110 (-1) 35 (+1) 0.70 (+1) 79.0 3.4 213.3 48.2 0.23 54.3 8 130 (+1) 35 (+1) 0.70 (+1) 79.6 3.2 234.0 51.2 0.22 41.8

**Figure 1.** Storage modulus (Gʹ) vs frequency for extruded nixtamalized corn flours in treatments: (a) 1-10 and (b) 11-20. Error bars indicate standard error of means.

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 245

**Figure 3.** Tangent of the phase angle (Tan δ) vs frequency for extruded nixtamalized corn flours in

treatments: (a) 1-10 and (b) 11-20. Error bars indicate standard error of means.

**Figure 2.** Loss modulus (Gʹʹ) vs frequency for extruded nixtamalized corn flours in treatments: (a) 1-10 and (b) 11-20. Error bars indicate standard error of means.

**Figure 2.** Loss modulus (Gʹʹ) vs frequency for extruded nixtamalized corn flours in treatments: (a) 1-10

and (b) 11-20. Error bars indicate standard error of means.

**Figure 3.** Tangent of the phase angle (Tan δ) vs frequency for extruded nixtamalized corn flours in treatments: (a) 1-10 and (b) 11-20. Error bars indicate standard error of means.

In the storage modulus (Gʹ), the lowest value (176.6 kPa) was with treatment 19 (Table 1) with no significant difference (p < 0.05) with treatment 16, whose conditions were the same. When the concentration of xanthan gum increased, at the same conditions of temperature and moisture content, Gʹ decreased. It probably was due to that the water molecules and bound components (amylose-hydrocolloid) of masas with hydrocolloids formed a gel with a softer structure, which affected Gʹ values (Aguirre-Cruz et al. 2005). Regarding to the loss modulus (Gʹʹ), the lowest value (45.7 kPa) was obtained for treatment 16 (Table 1), however, there were not significant diferences (p < 0.05) with treatments 7, 14 and 19. The highest value of Tan δ was observed in treatment 19 with 0.26 (Table 1), which differed significantly (p < 0.05) to the rest of the treatments. Since Tan δ values for all masas were in the range of 0.2-0.3, it is corresponding to that of an amorphous polymer (Ferry 1980). Similar values were obtained by Aguirre-Cruz et al. (2005) during cooling of diluted corn masa.

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 247

stored for 3 days at 4°C; however, the increase was less due to the lower storage temperature. Results obtained for tortillas made from ENCFs are similar to those reported by Galicia (2005) and Gonzalez (2006) after 2 h of storage. Neverthaless, at 24 h and 48 h of storage the MF values obtained in the present study were lower than those reported for

these authors because they did not use gum in masas preparation.

In each storage time, MF values with the same letter are not statistically significant (p > 0.05).

nixtamalized corn flours.

used the same concentration and the same gum.

**Figure 4.** Effect of storage time on the maximum force (MF) to rupture for tortillas made from extruded

Figure 5 presents tortilla rollability for all ENCF treatments after 2 h, 24 h and 48h of storage at room temperature (25°C). In general, for all the treatments (Table 1), as time progressed tortilla rollability decreased. For all ENCF treatments, the best tortilla rollability was obtained at the 2 h of storage. The most rapid loss of tortilla rollability occureed within 24 h of storage time, such as in the study of tortillas made from corn flour with xanthan gum added and stored at 4°C by Roman-Brito et al. (2007). It could be due to the very rapid association (retrogradation) of amylose and of amylose and amylopectin in starch (Fernández et al. 1999). At 2 h of storage time, the highest rollability (a value of 5) was obtained for all treatments, except for 11 and 12 (Table 1). At 24 h of storage, treatments 15, 19 and 20 showed the highest rollability values (3.6- 3.8). For 48 h of storage, treatment 15 showed the highest value (3.6). The highest values for rollability during storage corresponded to the xanthan gum concentration of 0.5%, which offers greater flexibility to the tortillas as in the study made by Arámbula et al. (1999), who

#### **3.3. Corn tortilla evaluations**

Values for the physical characteristics of tortillas from ENCFs after two hours they were made, time at which they are eaten, were similar to those of commercial tortillas. The physical characteristics evaluated were: weight, (range 21.3-25.2 g); diameter (range 12.2-12- 7 cm); and thickness (range of 1.4-1.9 cm). Similar results were reported by Ramirez-Wong (1989) who evaluated corn tortillas obtained by the traditional process of nixtamalization.

Figure 4 presents the tortilla firmness as maximum force (MF) to rupture the tortillas made from the different ENCFs after 2 h, 24 h and 48 h of storage at room temperature (25°C). In general, for all the treatments (Table 1), as time progressed tortilla firmness increased. At 2 h of storage time, the lowest MF (41.2 kPa), which is the best value for tortilla firmness (softer) were observed in treatments 8, 9, 14, 15, 16, 17, 19 and 20 (Table 1), and they were not significantly different (p > 0.05). At 24 h of storage time the lowest MF (62.8 kPa) was observed for treatment 20. At 48 h of the storage time the lowest value (62.3 kPa) was similar to that of 24 h, corresponding to the same treatment (20), where there was a significant difference (p<.05) compared to all other treatments. The lowest values of firmness in tortillas during storage corresponded to high concentrations of xanthan gum (0.5% to 0.84%). Treatments with high xanthan gum concentrations retained the tortilla moisture and improving its textural characteristics. This finding is similar to that reported by Arámbula et al. (1999), who obtained better results when utilizing xanthan gum compared to other addtitives such as guar gum, CMC and Arabic gum.

Tortillas made with extruded nixtamalized corn flours showed an increase in their MF during their storage at 25°C. Regarding to texture, the most important changes during storage occurred the first 24 h (Fernández et al. 1999; Ramírez-Wong 1989). Since some starch crystals are retained after baking of the tortilla, they serve as nuclei which facilitate the rapid association of starch molecules, and structural changes occur during the initial 24 h following baking, which in turn leads to rapid retrogradation or increasing the texture of this product (Fernández et al. 1999). A similar tendency for MF was obtained by Roman-Brito et al. (2007) for tortillas made with nixtamalized flour containing xanthan gum and stored for 3 days at 4°C; however, the increase was less due to the lower storage temperature. Results obtained for tortillas made from ENCFs are similar to those reported by Galicia (2005) and Gonzalez (2006) after 2 h of storage. Neverthaless, at 24 h and 48 h of storage the MF values obtained in the present study were lower than those reported for these authors because they did not use gum in masas preparation.

246 Viscoelasticity – From Theory to Biological Applications

**3.3. Corn tortilla evaluations** 

addtitives such as guar gum, CMC and Arabic gum.

In the storage modulus (Gʹ), the lowest value (176.6 kPa) was with treatment 19 (Table 1) with no significant difference (p < 0.05) with treatment 16, whose conditions were the same. When the concentration of xanthan gum increased, at the same conditions of temperature and moisture content, Gʹ decreased. It probably was due to that the water molecules and bound components (amylose-hydrocolloid) of masas with hydrocolloids formed a gel with a softer structure, which affected Gʹ values (Aguirre-Cruz et al. 2005). Regarding to the loss modulus (Gʹʹ), the lowest value (45.7 kPa) was obtained for treatment 16 (Table 1), however, there were not significant diferences (p < 0.05) with treatments 7, 14 and 19. The highest value of Tan δ was observed in treatment 19 with 0.26 (Table 1), which differed significantly (p < 0.05) to the rest of the treatments. Since Tan δ values for all masas were in the range of 0.2-0.3, it is corresponding to that of an amorphous polymer (Ferry 1980). Similar values

were obtained by Aguirre-Cruz et al. (2005) during cooling of diluted corn masa.

Values for the physical characteristics of tortillas from ENCFs after two hours they were made, time at which they are eaten, were similar to those of commercial tortillas. The physical characteristics evaluated were: weight, (range 21.3-25.2 g); diameter (range 12.2-12- 7 cm); and thickness (range of 1.4-1.9 cm). Similar results were reported by Ramirez-Wong (1989) who evaluated corn tortillas obtained by the traditional process of nixtamalization.

Figure 4 presents the tortilla firmness as maximum force (MF) to rupture the tortillas made from the different ENCFs after 2 h, 24 h and 48 h of storage at room temperature (25°C). In general, for all the treatments (Table 1), as time progressed tortilla firmness increased. At 2 h of storage time, the lowest MF (41.2 kPa), which is the best value for tortilla firmness (softer) were observed in treatments 8, 9, 14, 15, 16, 17, 19 and 20 (Table 1), and they were not significantly different (p > 0.05). At 24 h of storage time the lowest MF (62.8 kPa) was observed for treatment 20. At 48 h of the storage time the lowest value (62.3 kPa) was similar to that of 24 h, corresponding to the same treatment (20), where there was a significant difference (p<.05) compared to all other treatments. The lowest values of firmness in tortillas during storage corresponded to high concentrations of xanthan gum (0.5% to 0.84%). Treatments with high xanthan gum concentrations retained the tortilla moisture and improving its textural characteristics. This finding is similar to that reported by Arámbula et al. (1999), who obtained better results when utilizing xanthan gum compared to other

Tortillas made with extruded nixtamalized corn flours showed an increase in their MF during their storage at 25°C. Regarding to texture, the most important changes during storage occurred the first 24 h (Fernández et al. 1999; Ramírez-Wong 1989). Since some starch crystals are retained after baking of the tortilla, they serve as nuclei which facilitate the rapid association of starch molecules, and structural changes occur during the initial 24 h following baking, which in turn leads to rapid retrogradation or increasing the texture of this product (Fernández et al. 1999). A similar tendency for MF was obtained by Roman-Brito et al. (2007) for tortillas made with nixtamalized flour containing xanthan gum and

In each storage time, MF values with the same letter are not statistically significant (p > 0.05).

**Figure 4.** Effect of storage time on the maximum force (MF) to rupture for tortillas made from extruded nixtamalized corn flours.

Figure 5 presents tortilla rollability for all ENCF treatments after 2 h, 24 h and 48h of storage at room temperature (25°C). In general, for all the treatments (Table 1), as time progressed tortilla rollability decreased. For all ENCF treatments, the best tortilla rollability was obtained at the 2 h of storage. The most rapid loss of tortilla rollability occureed within 24 h of storage time, such as in the study of tortillas made from corn flour with xanthan gum added and stored at 4°C by Roman-Brito et al. (2007). It could be due to the very rapid association (retrogradation) of amylose and of amylose and amylopectin in starch (Fernández et al. 1999). At 2 h of storage time, the highest rollability (a value of 5) was obtained for all treatments, except for 11 and 12 (Table 1). At 24 h of storage, treatments 15, 19 and 20 showed the highest rollability values (3.6- 3.8). For 48 h of storage, treatment 15 showed the highest value (3.6). The highest values for rollability during storage corresponded to the xanthan gum concentration of 0.5%, which offers greater flexibility to the tortillas as in the study made by Arámbula et al. (1999), who used the same concentration and the same gum.

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 249

0 0 <sup>2</sup> Y 1 35 – 64XG .97 MC *<sup>G</sup>* (2)

<sup>0</sup> <sup>2</sup> Y 235 .22 MC *<sup>G</sup>* (3)

The highest value of WAI (3.34 g gel/g dry matter) was observed at T=110- 120.94°C/MC=29.68-35% (w/w)/XG=0.48-0.70% (w/w) (Figs. 6 (a,b,c)). Similar values of WAI

Storage modulus. The linear term of xanthan gum (XG, p<0.01) and quadratic term of moisture content [(MC)2, p < 0.01) affected significantly G' in masa of ENCFs. The prediction

The regression model explained 70% of the total variation (p < 0.05) in G' of extruded nixtamalized corn flours. Figs. 7 (a,b,c) show the effect of G' in masa of ENCFs as a function of T, MC and XG, observing that with any value of moisture content from approximately the central point (30%), G' increaces, due to its quadratic effect, and that increasing the concentration of XG decreases Gʹ, because of its linear effect. The lowest value (214.58 kPa) was observed at T=110-130°C/MC=28-32.1% (w/w)/GX=0.57-0.70%

Loss modulus. The viscous modulus Gʹʹ in masa of ENCFs depended on the quadratic term of moisture content [(MC)2, p < 0.01)]**.** The prediction model in terms of original variables for

The regression model explained 72% of the total variation (p < 0.01) in G'' of masa from extruded nixtamalized corn flours. Figs. 8 (a,b,c) show the effect of G'' in masa from ENCFs as a function ot T, MC and XG, noting that for any value of moisture content starting at the central point (30%), Gʹʹ increased due to the quadratic effect. The lowest value (49.35 kPa) was observed at T=110-128.32°C/MC=27.4-32.72% (w/w)/XG=0.52-0.70% (w/w)/ (Figs. 8

Tangent of the phase angle. Analysis of variance showed that Tan δ of masa from ENCFs depended on the quadratic terms of temperature (T2, p < 0.05) and xanthan gum [(XG)2, p < 0.01], and on the interaction temperature-moisture content [(T)(MC), p < 0.05)]. The

The regression model explained 71% of the total variation (p < 0.05) in Tan δ of masa from extruded nixtamalized corn flours. Figs. 9 (a,b,c) show the effect of Tan δ of masa from ENCFs as a function of T, MC and XG, observing that at any concentration of xanthan gum from approximately the central point (0.5%), Tan δ decreases due to its quadratic effect. The highest value (0.239) was shown at T=116.61-124.55°C/MC=0.25-0.35% (w/w)/XG=0.47-0.59%

0 000 0 0000 0 2 2 Y 1.59 . 2 T MC . 7 T .31 XG Tan*<sup>δ</sup>* (4)

prediction model in terms of orginal variables for Tan δ was:

for extruded flours were obtained by Reyes-Moreno et al. (2003)

model in terms of original variables for G' was:

(w/w) (Figs. 7 (a,b,c).

G'' was:

(a,b,c)).

(w/w) (Figs. 9(a,b,c)).

In each storage time, rollability values with the same letter are not statistically significant (p > 0.05).

**Figure 5.** Effect of storage time on the rollability of tortillas made from extruded nixtamalized corn flours.

#### **3.4. Extrusion process optimization**

To find the best variables combination of the process (T, MC and XG) to obtain the extruded nixtamalized corn flour, response surface methodology (RSM) was used. The evaluations to optimize the extrusion process were: WAI, G', G'', Tan δ and MF.

Water absorption index. Analysis of variance showed that WAI of extruded flours depended on the linear term of the conditioning moisture content (MC, p < 0.01), quadratic term of MC [(MC)2, p < 0.01] and combined term of moisture content with xanthan gum [(MC)(XG), p < 0.10]. The prediction model in terms of original variables for WAI was:

$$\text{Y}\_{\text{WAI}} = -4.37 + 0.583 \text{MC} + 0.137 \left( \text{MC} \right) \left( \text{GX} \right) - 0.01 \left( \text{MC} \right)^2 \tag{1}$$

The regression model explained 75% of the total variation (p<0.01) in WAI of extruded nixtamalized corn flours. Figs. 6 (a,b,c) show the effect of WAI extruded flours as a function of T, MC and XG, noting that an increase conditioning moisture content of flour increase the water absorption index, as it does the increase in xanthan gum concentration at high levels of moisture content. Vargas-López (1987) mentioned that extruded grits of corn-sorghum and corn starch exhibit a maximum WAI at high temperatures and high moisture content. The highest value of WAI (3.34 g gel/g dry matter) was observed at T=110- 120.94°C/MC=29.68-35% (w/w)/XG=0.48-0.70% (w/w) (Figs. 6 (a,b,c)). Similar values of WAI for extruded flours were obtained by Reyes-Moreno et al. (2003)

248 Viscoelasticity – From Theory to Biological Applications

**3.4. Extrusion process optimization** 

flours.

In each storage time, rollability values with the same letter are not statistically significant (p > 0.05).

optimize the extrusion process were: WAI, G', G'', Tan δ and MF.

p < 0.10]. The prediction model in terms of original variables for WAI was:

**Figure 5.** Effect of storage time on the rollability of tortillas made from extruded nixtamalized corn

To find the best variables combination of the process (T, MC and XG) to obtain the extruded nixtamalized corn flour, response surface methodology (RSM) was used. The evaluations to

Water absorption index. Analysis of variance showed that WAI of extruded flours depended on the linear term of the conditioning moisture content (MC, p < 0.01), quadratic term of MC [(MC)2, p < 0.01] and combined term of moisture content with xanthan gum [(MC)(XG),

The regression model explained 75% of the total variation (p<0.01) in WAI of extruded nixtamalized corn flours. Figs. 6 (a,b,c) show the effect of WAI extruded flours as a function of T, MC and XG, noting that an increase conditioning moisture content of flour increase the water absorption index, as it does the increase in xanthan gum concentration at high levels of moisture content. Vargas-López (1987) mentioned that extruded grits of corn-sorghum and corn starch exhibit a maximum WAI at high temperatures and high moisture content.

0 0 0 0 <sup>2</sup> Y 4.37 .583MC .137 MC GX . 1 MC WAI (1)

Storage modulus. The linear term of xanthan gum (XG, p<0.01) and quadratic term of moisture content [(MC)2, p < 0.01) affected significantly G' in masa of ENCFs. The prediction model in terms of original variables for G' was:

$$\text{Y}\_{\text{G}'} = \text{1035}-64\text{XG} + 0.97 \text{(MC} \text{)}^2 \tag{2}$$

The regression model explained 70% of the total variation (p < 0.05) in G' of extruded nixtamalized corn flours. Figs. 7 (a,b,c) show the effect of G' in masa of ENCFs as a function of T, MC and XG, observing that with any value of moisture content from approximately the central point (30%), G' increaces, due to its quadratic effect, and that increasing the concentration of XG decreases Gʹ, because of its linear effect. The lowest value (214.58 kPa) was observed at T=110-130°C/MC=28-32.1% (w/w)/GX=0.57-0.70% (w/w) (Figs. 7 (a,b,c).

Loss modulus. The viscous modulus Gʹʹ in masa of ENCFs depended on the quadratic term of moisture content [(MC)2, p < 0.01)]**.** The prediction model in terms of original variables for G'' was:

$$\text{Y}\_{\text{G}} = \text{235} + \text{0.22(MC)}^2 \tag{3}$$

The regression model explained 72% of the total variation (p < 0.01) in G'' of masa from extruded nixtamalized corn flours. Figs. 8 (a,b,c) show the effect of G'' in masa from ENCFs as a function ot T, MC and XG, noting that for any value of moisture content starting at the central point (30%), Gʹʹ increased due to the quadratic effect. The lowest value (49.35 kPa) was observed at T=110-128.32°C/MC=27.4-32.72% (w/w)/XG=0.52-0.70% (w/w)/ (Figs. 8 (a,b,c)).

Tangent of the phase angle. Analysis of variance showed that Tan δ of masa from ENCFs depended on the quadratic terms of temperature (T2, p < 0.05) and xanthan gum [(XG)2, p < 0.01], and on the interaction temperature-moisture content [(T)(MC), p < 0.05)]. The prediction model in terms of orginal variables for Tan δ was:

$$\text{Y}\_{\text{Tanb}} = \text{ } - \text{ } 1.59 \text{ } - \text{ } 0.0002 \text{ (T)} \text{(MC) } - \text{ } 0.00007 \text{(T)}^2 \text{ } - \text{ } 0.31 \text{(\chi G)}^2 \text{ } \tag{4}$$

The regression model explained 71% of the total variation (p < 0.05) in Tan δ of masa from extruded nixtamalized corn flours. Figs. 9 (a,b,c) show the effect of Tan δ of masa from ENCFs as a function of T, MC and XG, observing that at any concentration of xanthan gum from approximately the central point (0.5%), Tan δ decreases due to its quadratic effect. The highest value (0.239) was shown at T=116.61-124.55°C/MC=0.25-0.35% (w/w)/XG=0.47-0.59% (w/w) (Figs. 9(a,b,c)).

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 251

**Figure 7.** Response surface and contour plots for the effect of extrusion process factors on storage modulus (Gʹ) of masa from extruded nixtamalized corn flours. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on Gʹ; (b) Effect of xanthan gum (XG, %) and T on Gʹ; (c) Effect of XG

and MC on Gʹ.

**Figure 6.** Response surface and contour plots for the effect of extrusion process factors on water absortion index (WAI) from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, % ) and extrusion temperature (T, °C ) on WAI; (b) Effect of xanthan gum (XG, % ) and T on WAI; (c) Effect of XG and MC on WAI.

**Figure 6.** Response surface and contour plots for the effect of extrusion process factors on water absortion index (WAI) from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, % ) and extrusion temperature (T, °C ) on WAI; (b) Effect of xanthan gum (XG, % ) and T on WAI; (c) Effect

of XG and MC on WAI.

**Figure 7.** Response surface and contour plots for the effect of extrusion process factors on storage modulus (Gʹ) of masa from extruded nixtamalized corn flours. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on Gʹ; (b) Effect of xanthan gum (XG, %) and T on Gʹ; (c) Effect of XG and MC on Gʹ.

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 253

**Figure 9.** Response surface and contour plots for the effect of extrusion process factors on tangent of the phase angle (Tan δ) of masa from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on Tan δ; (b) Effect of xanthan gum (XG, %) and T on Tan δ;

(c) Effect of XG and MC on Tan δ.

**Figure 8.** Response surface and contour plots for the effect of extrusion process factors on loss modulus (Gʹʹ) of masa from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on Gʹʹ; (b) Effect of xanthan gum (XG, %) and T on Gʹʹ; (c) Effect of XG and MC on Gʹʹ.

**Figure 8.** Response surface and contour plots for the effect of extrusion process factors on loss modulus (Gʹʹ) of masa from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on Gʹʹ; (b) Effect of xanthan gum (XG, %) and T on Gʹʹ; (c) Effect of XG and MC on Gʹʹ.

**Figure 9.** Response surface and contour plots for the effect of extrusion process factors on tangent of the phase angle (Tan δ) of masa from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on Tan δ; (b) Effect of xanthan gum (XG, %) and T on Tan δ; (c) Effect of XG and MC on Tan δ.

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 255

**Figure 11.** Regions of best combinations of process factors for producing optimized extruded nixtamalized corn flour, using a single-screw extruder. (a) Effect of moisture content (MC, % ) and extrusion temperature (T, °C ); (b) Effect of xanthan gum (XG, % ) and T; (c) Effect of XG and MC.

**Figure 10.** Response surface and contour plots for the effect of extrusion process factors on maximum force (MF) for tortilla made from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on MF; (b) Effect of xanthan gum (XG, %) and T on MF; (c) Effect of XG and MC on MF.

**Figure 10.** Response surface and contour plots for the effect of extrusion process factors on maximum force (MF) for tortilla made from extruded nixtamalized corn flour. (a) Effect of moisture content (MC, %) and extrusion temperature (T, °C) on MF; (b) Effect of xanthan gum (XG, %) and T on MF; (c) Effect of

XG and MC on MF.

**Figure 11.** Regions of best combinations of process factors for producing optimized extruded nixtamalized corn flour, using a single-screw extruder. (a) Effect of moisture content (MC, % ) and extrusion temperature (T, °C ); (b) Effect of xanthan gum (XG, % ) and T; (c) Effect of XG and MC.

Maximum force**.** Changes in MF of tortilla made from ENCFs were affected by the linear term of moisture content (MC, p < 0.01), quadratic of moisture content (MC)2, p < 0.01) and the combinations of temperature-moisture content [(T)(MC), p < 0.10] and temperature-gum [(T)(XG), p < 0.01]. The prediction model in terms of original variables for MF was:

$$\text{Y}\_{\text{MF}} = 14.75 \text{ -- } 7.36 \text{MC -- } 0.04 \text{(T)} \text{(MC) -- } 1.17 \text{(T)} \text{(XG) } + 0.2 \text{(MC)}^2 \tag{5}$$

Viscoelastic and Textural Characteristics of Masa and Tortilla from Extruded Corn Flours with Xanthan Gum 257

The Authors want to thank to SEP-PROMEP for the financial support of the Project "Aplicación de Métodos Físicos, Reológicos y Biológicos en el Procesamiento de Maíz",

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**Acknowledgement** 

**5. References** 

The regression model explained 82% of the total variation (p < 0.01) of MF of tortilla made from ENCFs. Figs. 10(a,b,c) show the effect of MF in tortilla made from ENCFs as a function of T, MC and XG, observing that at any value of moisture content for conditioning starting at approximately the midpoint of the matrix, the maximum force is increased due to the quadratic effect. The lowest value (46.16 kPa) was shown at T=115 - 130°C/MC=30.69-34.87% (w/w)/GX=0.42-0.7% (w/w) (Figs. 10 (a,b,c).

The superimposition of contour plots of the effect of variables of the extrusion process (T, MC and XG) on WAI of flour, Gʹ, Gʹʹ and Tan δ of masa, and MF of tortilla made from ENCFs, was used to obtain Figs. 11 (a,b,c), which in turn was utilized to determine the best combinations of the extrusion process variables. The central points of the regions of optimization in Figs. 11 (a,b,c) correspond to the values of the process variables of T=111.77°C/MC=28.84%, XG=0.66%/T=122.17°C and XG=0.65%/MC=29.42%, respectively. The optimal combination for the operation conditions of the single-screw extruder derived from the averages of those values were: T=116.67°C/MC= 29.13%/XG = 0.65%. The optimal conditions were validated using experiments, which were similar to those values predicted by RSM. These values can be used to obtain ENCF with the highest WAI and WAC, and tortillas with less firmness (softer) and more flexibility (more rollable).
