Knowledge of dough rheology will give information about the final product quality. The instruments to investigate dough behavior are the farinograph, mixograph, extensograph, and alveograph. These instruments measure power input during dough development caused by either mixing action or extensional deformation.
Both the farinograph and the mixograph are torque measuring devices that provide empirical information about mixing properties of flour by recording resistance of dough to mixing. These instruments differ in their mixing action. In the farinograph there is a kneading type of mixing. There are two Z-shaped blades that rotate at different speeds at different directions. The mixograph involves a planetary rotation of vertical pins (lowered into the dough) attached to the mixing bowl.
The farinograph gives information about mixing properties of flour by recording the resistance of the dough to mixing blades during prolonged mixing. The output of this instrument is known as a farinogram, which is a consistency versus time curve. Consistency is expressed as Brabender units (BU). The shape of the farinogram is interpreted in terms of dough development time, tolerance to overmixing, stability, and optimum water absorption (Fig. 2.31).
The time required for dough to reach the maximum consistency is called peak time. It is also called dough development time. Tolerance of the dough to overmixing can be expressed as stability, mixing tolerance index (MTI) and departure time. Stability is the duration at which the dough consistency is >500 BU. It is an indication of flour strength. Higher values indicate stronger dough. Mixing tolerance index is the change in dough consistency 5 min after it reaches its maximum value. The time for the consistency to decrease below 500 BU is known as departure time. Water absorption (%) is obtained by measuring the amount of water required to produce dough having a consistency of 500 BU.
Departure lime Peak lime ■
Departure lime Peak lime ■
Figure 2.31 A general farinogram.
Table 2.1 Farinograph Analysis of Dough Containing Hydrocolloidsa
Water Absorption (%) DDT (min) Stability (min) MTI (BU)
Control 54.9 2.8 10.5 40
Alginate 59.5 9.0 15.5 30
K-carragenan 56.5 2.5 5.8 60
Xanthan 56.7 11.5 20.0 20
aFrom Rosell, C.M., Rojas, J.A, & de Barber, C.B. Influence of hydrocolloids on dough rheology and bread quality. Food Hydrocolloid, 15 (1), 75-81. Copyright © (2001), with permission from Elsevier.
The shape of the farinogram curve depends on the variety of wheat, environmental conditions, and type of the flour produced during milling. Hard wheat or bread type flours have longer stability values than flours from soft wheat or cake type flour.
Hydrocolloids are usually added to bread formulations to improve dough handling properties, to increase quality of fresh bread, and to extend shelf-life of bread. Table 2.1 shows the effect of hydro-colloid addition on farinograph measurements (Rosell, Rojas, & de Barber, 2001). Water absorption increased when hydrocolloids was added. The extent of increase was dependent on the structure of hydrocolloid added. Dough development time (DDT) was affected differently by each hydrocolloid. The strongest dough was obtained by addition of alginate or xanthan which, was reflected in high stability and low MTI values.
In the mixograph, torque is recorded while a fixed amount of flour and water is mixed. Output of this instrument is known as mixogram. The mixograph is more complicated than the farinograph since there is no predetermined optimum consistency that will be taken as a reference. Therefore, other methods should be used to determine the amount of water required to produce dough of optimum absorption. For a mixograph, the tolerance of flour can be seen in weakening angle (W), the area under the curve, the height of the curve at a specified time after peak, and the angle between the ascending and descending portions of the curve which is known as tolerance angle (T) (Fig. 2.32).
Figure 2.32 A general mixogram.
Tolerance angle (T) is obtained by drawing a line from center of the curve at its peak down the center of the curve in both directions. A large tolerance angle indicates more tolerant flour. Weakening angle (W) is formed by drawing a line for center of the curve at its peak down the descending portion of the curve and a line horizontal to the baseline through the center of the curve at its maximum height. The size of the angle (W) is inversely related to mixing tolerance. The development angle (D) is formed by a line drawn horizontal to the baseline through the center of the curve at its maximum height and a line drawn through the center of the ascending part of the curve.
Measurement of the rheological properties of the dough after mixing is made using an extensograph and an alveograph, which measure stress-strain relationships, thus providing information about elasticity.
An extensograph gives information about dough resistance to stretching and extensibility (Fig. 2.33). It measures the force to pull a hook through a rod-shaped piece of dough. The extensograph gives resistance to constant deformation after 50 mm of stretching (R50) and extensibility. The resistance to stretching (B) is related to elastic properties and the extensibility (C) is related to the viscous component. The ratio of resistance to stretching to extensibility (B/C) is a good indicator of the balance between elastic and viscous components of the dough. The area under the extensogram (A) indicates energy and is related to the absolute levels of elastic and viscous components of the dough. A combination of good resistance and good extensibility results in desirable dough properties. It is also possible to determine stretching and extensibility values with a texture analyzer if Kieffer dough and a gluten extensibility rig is used.
An alveograph, which is also called the Chopin Entensograph, provides similar information by measuring the pressure required to blow a bubble in a sheeted piece of dough. An alveogram is shown in Fig. 2.34. The advantage of an alveograph over an extensograph is the mode of expansion. There is a constant rate of extension in only one direction with an extensograph but an alveograph expands the
Figure 2.33 Example of an extensogram.
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