**2. Experimental procedure**

## **2.1. Raw material**

260 Viscoelasticity – From Theory to Biological Applications

lipids of the flour than for its gluten.

stress-relaxation test (Mohsenin, 1978).

less for strong dough than that moderately strong.

Another factor affecting the rheological properties of dough is starch. It has been seen that damage to starch and the presence of other minor components affect rheological properties (Dexter et al., 1994; Lynn & Stark, 1995). Campos et al. (1997) and Chiotelli et al. (2004) suggested that the rheological changes observed in dough subjected to low heating tests may be due to starch molecules present. Zeng et al. (1997) found that 80% of the variation in the viscosity of starch paste of 13 cultivars of wheat was due to the concentration of amylose and amylopectin, and Morris et al. (1997) observed that this variation is due to the presence of isoenzymes synthesized in relation to the starch granule. Georgopulus (2006) observed that the rheological properties of dough are affected more by the removal of the native

Rheological properties of wheat flour are determinants of the desired characteristics in the final product, as well as for the design of equipment and processes. Therefore, it is necessary to find reliable rheological tests that are a useful tool for characterizing dough of wheat flours (Safari-Ardi & Phan-Thien, 1998). Rheology of wheat dough is broad, and the tests utilized are diverse. Initially, empirical tests were more used than the fundamental tests, but disadvantages were seen such as: dependence on the instrument, the form and quantity of the sample utilized and lack of theoretical basis (Faubion & Hoseney, 1990), which led to the development of fundamental tests. The most important fundamental viscoelastic tests utilized for wheat dough are the force-deformation ratio, creep test, the dynamic test and

Recently, the dynamic test and the stress relaxation test have been used to characterize the dough viscoelasticity. Safari-Ardi & Phan-Thien (1998), utilizing the dynamic test observed that there were no differences among dough elaborated with different flours, probably due to the small deformation used (<1%). Therefore, it was decided to evaluate viscoelastic properties of dough applying a stress-relaxation test. In this technique, the dough is rapidly deformed at a predetermined level and the stress is measured over time, where

Several researches have utilized and recommended the stress-relaxation test (Lee and Mulvaney, 2003; Rao et al., 2000; Uthayakumuran et al., 2002; Wikström & Eliasson,, 1998; Yadav et al., 2005). Safari-Ardi et al. (1997) found when using the stress-relaxation test, it was possible to differentiate dough from distinct wheat flours, and demonstrated that besides the method being consistent, the data of the stress-relaxation test obtained at high amplitudes (1-15% deformation in 3x103 s) were very precise (Safari-Ardi & Phan-Thien, 1998). Utilizing the stress-relaxation test, it has been found that the distribution of the quantity of protein (high molecular weight glutenin) and its molecular weights are related to the relaxation time (Rao et al., 2000; Uthayakumaran et al., 2002). In addition, it has been shown that moisture content and strain affect the relaxation characteristics of dough by doing them no linear (Yadav et al., 2005). Similarly, Smith et al. (1970) observed that the relaxation time increases with mixing, and Rao et al., (2000) concluded that this parameter is

deformations are >1% (Faubion & Hoseney, 1990; Mohsenin, 1978; Rao et al., 2000).

Samples of soft wheat from four cultivars were used: Barcenas, Cortazar, Salamanca and Saturno. Samples were obtained in the Central part of Mexico (El Bajío Zone), and they were sent to the Departamento de Investigación y Posgrado en Alimentos from the University of Sonora at Hermosillo Sonora, México, where was carried out this study.

## **2.2. Flours elaboration**

Wheat samples of the four cultivars were cleaned (Blount/Ferrell-Ross, model M2BC), and placed in plastic bags, which were stored at refrigeration temperature (4°C) until use. Wheat samples were conditioned based on the international approved method number 26-95 (American Asociation of Cereal Chemists, [A.A.C.C], 2000), at a moisture content of 16%, utilizing a conditioner (Chopin Instruments, Villeneuve-La-Garenne, France). Samples were allowed to stand for a period of 24 h before preparation of flours. The conditioned samples were milled using the approved method number 26-10 (A.A.C.C., 2000), and an experimental mill (Brabender, model Quadrumat Senior, South Hackensack, NJ). For maturation, flours were allowed to stand for a period of 15 days.

## **2.3. Flours quality analysis**

For proximate chemical analysis of flours, the official methods of the A.A.C.C. International (2000) were used, and the following determinations were made: protein content (approved method number 46-13) using a nitrogen analyzer (LECO, model FP-528, MI, USA), and the protein factor was N x 5.7; ash content (approved method number 08-03); and moisture content (approved method number 44-40).

Wet gluten content was determined utilizing the approved method number 38-11 (A.A.C.C., 2000) and the apparatus Glutomatic (Falling Number, model 2100, Huddinge, Sweden). Sedimentation volume was determined using the approved method number 56-61A (A.A.C.C., 2000). The falling number was measured utilizing approved method number 56- 81b (A.A.C.C., 2000) and the apparatus Falling Number (model 1400, Huddinge, Sweden).
