Mixing is arguably the most critical process in bread production. As a result, much research has been conducted to investigate the parameters that lead to an optimally developed dough that will result in a high quality bread product that satisfies not only processing requirements but satisfies customer expectations. The study of dough development and formation includes investigation of a dough’s rheological properties. Rheology is the study of how materials deform, flow, or fail when force is applied (Hoseney, 1994). While this definition applies to many materials subjected to mechanical processing it is particularly applicable dough mixing. In the US the most common instrument used to measurement the rheological properties of doughs is the farinograph.



Dough mixing involves the combining and blending of ingredients with the application of sufficient physical energy (work) that will transform the mixture into a cohesive mass with the required viscoelastic properties (Pyler, 1986). Specifically for bread dough formation, mixing is the process of converting flour and water into a dough by both blending and distributing the dough ingredients and developing the gluten protein into a continuous phase possessing viscoelastic properties (Hoseney, 1974). What makes bread dough a viscoeleastic material is the derivation of it physical properties primarily from two states of mater: liquid and solid. A dough exhibits plasticity that combines the attributes of both fluids and solids, elasticity which is a property generally exhibited by solids, and viscosity, a characteristic of liquids (Pyler, 1986).



The two principle components of bread dough are flour and water. As water is added to flour the particles are wetted and slowly hydrated. With mixing comes the application of physical energy and several physical and chemical changes occur.
The flour-water mass gradually becomes a coherent mixture, loses its wet, sticky appearance and becomes a smooth and homogeneous dough. This transformation of a mixture from a wet sticky mass
which exhibits a high degree of extensibility to a dry and increasingly elastic material involves the presence of free water in the dough (Hoseney, 1974). The presence of free water allows for the lubrication of the constituent particles of a mixture or dough and allows for some degree of flow. Upon mixing the protein and other constituents of the mixture become hydrated, decreasing the level of free water present and leading to the development of a drier feeling dough. As a result resistance to extension increases and dough mobility or extensibility decreases.



Obviously this transformation in the mixture is not only the result of the decreasing presence of free water. Other physical and chemical changes occur as a result of mixing. Upon hydration of the flour particles the protein structure becomes altered. This protein structure begins (just after the start of mixing and the hydration of the flour) as a tangled mass. As mixing proceeds a gradual orientation of the long linear protein molecules occurs with a simultaneous decrease in chain entanglements (Pyler, 1986). If mixing is halted before the development of this ordered protein structure then the dough will be less able to retain the gases of fermentation and result in reduced loaf volume (Hoseney, 1974).



As mixing continues beyond the optimum development point of the protein network dough breakdown will occur. During breakdown the dough begins to lose the elastic component of its viscoelastic characteristics and become more extensible, and progressively softer (Pyler, 1986). Experimental evidence suggests that with continued mixing beyond optimum development the protein network becomes even less entangled allowing for increased laminar flow and thus the increase in extensibility. Hoseney argues that this orientation of protein molecules would greatly increase the probability of protein-protein interactions that could result in the release of bound water. This increase in free water would result in a decrease in elasticity (decreased viscosity) and account for the wet and sticky appearance of the dough (Hoseney, 1987).

It is the complex nature of the dough development process and the needs of the baking industry for a reliable set of tests that are able to determining the mixing and baking qualities of flour that the mixograph farinograph was developed.



The farinograph as developed by Hankoczy, a Hungarian in 1928 (MacRitchie, 2000a) and later refined by Brabender has gained wide acceptance and is used more often in cereal analysis than any other piece of experimental dough testing instrument (Pyler, 1986). The farinograph was initially designed to test the rheological properties of the lower protein European wheats but today it has been modified and found to be an accurate instrument for assessing the qualities of North American wheats as well.



The farinograph is used to assess the rheological properties of wheat flours as they are mixed and developed into a dough. As we have seen above this is a complex process which can be broken down into the following three processes: absorption of water, dough development, and dough break down (Preston, 1984). The farinograph measures the energy required to mix a dough as it progresses through these three stages of development. From this information the dough’s relative quality characteristics can be assessed.

The Brabender Farinograph consists of a high speed mixer with two ‘z’ shaped paddles which rotate in opposite directions at a differential of 1.5:1 (MacRitchie, 2000a). Flour and water are mixed together at a constant temperature to form a dough. This mixing apparatus is designed in a manner such that the resistance of the dough against the constant mechanical shear is accurately transferred to a dynamometer and recorded by a computer which translates this information into a trace producing a visual chart of the quality characteristics of the flour. It is this very setup, however, that makes the farinograph a poor instrument for studying the “basic” properties of dough rheology. The ‘z’ shaped paddles contribute a complicated mixing action that does not impart homogeneous stress and strain forces over time making mathematical analysis impossible. The farinograph trace, as a result, can not be broken down into its fundamental rheological components; rather, the trace is a composite of these forces (Preston, 1984).



That said the farinograph is useful in the study of a dough’s rheological properties. Typically, the most commonly gathered information from the farinograph trace consists of flour absorption, and the mixing characteristics of the dough (Tanaka, 1969). As the flour and water are mixed into a dough, development progresses through the three stages mentioned above. Obviously in such a complex system as dough development it is not a case of seamlessly moving from one stage to the next. Rather, each stage goes through a period of dominance as mixing progresses.



The farinograph is used extensively for determination of a flour’s water absorption characteristics. Several studies have indicated that flour absorption is a function of protein content, starch (damaged starch in particular), pentosans, and gluten strength (Preston, 1984). Flour absorption, as determined by the farinograph is the amount of water required to develop a dough that centers the trace on the 500 BU line. Use of the farinograph as a method for determining flour absorption is not accepted by all cereal scientists. Hoseney indicates that there is little evidence that the farinograph absorption value is related to that as determined during baking. He also notes that in choosing an arbitrary dough consistency of 500 BU and adjusting the water to meet that consistency assumes that water is the only factor affecting optimum dough consistency, and he feels that that is an incorrect assumption (Hosney, 1974). Even with such criticism the absorption values determined by the farinograph are widely used and are useful in determining relative water absorption characteristics between flour samples.



In addition to identifying a flour’s absorption for baking purposes it is also necessary to have a standardized absorption for the evaluation of a flour’s mixing qualities at a particular consistency so that results can be compared across a range of flours. As we will see relative quality characteristics of a wide range of doughs require that the consistency be such that the “peak” of the farinograph trace is centered on the 500 BU line. From this peak consistency several quality characteristics are determined. Over absorption of the flour will cause the trace not to reach the 500 BU line (too high a level of extensibility or too low of a resistance to mechanical sheer) and under absorption will cause the trace to be centered above the 500 BU line (too high a level of elasticity or too great of a resistance of mechanical sheer) (Pyler, 1986).



The other area of use for the farinograph is in relative flour quality characteristics. An example of a farinograph trace is shown here.

The mixing action of the farinograph develops the dough through the stages outlined above. The resistance of the dough to mechanical development is recorded by the dynamometer and recorded on a chart measured in arbitrary “Brabender Units” (BU). From the farinograph trace a number of flour quality characteristics have been derived. The most common use of the trace is in the characterization of the inherent mixing properties of the flour. These parameters are outlined as follows:

• Arrival time - the time required for the top of the curve to reach the 500 BU line. This is a generalized measurement of the rate of water take up by the flour. It has been shown that as protein content increases arrival time increases as well.

• Dough Development Time - This is the time between the first addition of water and the dough’s development to minimum extensibility.

• Stability - This is the difference in time between the point at which the peak first crosses the 500 BU line and the point at which the top of the curve leaves the 500 BU line. This is taken to be an indication of the flour’s mixing tolerance level.

• Departure time - The time from the first addition of water until the top of the curve leaves the 500 BU line. As this values increases it indicates increasing dough strength.

• Twenty minute drop - Difference between the height of the curve at peak development and the height of the center of the curve after twenty minutes. This is an indication of the rate of breakdown and the strength of the flour.

• Mixing tolerance index - This is the difference in BU’s between the height of the curve at the peak and the top of the curve measured five minutes after the peak. This is a composite value which indicates mixing tolerance and flour strength (Shuey, 1984)

These values can be used as a relative composite description of a flour’s overall quality characteristics. Flours can be generally placed into one of the following descriptive categories as outlined by Preston:

• Weak - Flours with short ( less than 2.5 min) development times, high MTI values ( > 100) and low water absorption ( less than 55%).

• Medium - Flours having a dough development time ranging from 2.5 to 4.0 min, MTI values in the range of 60 - 100, and absorptions of 54% - 60%.

• Strong - Flours having long development times (4.0 - 8.0 min), low MTI values (15 - 50) and absorptions generally above 58%

• Very strong - Flours having very long development times ( > 10 min) and very low MTI scores ( less than 10 min )(Preston, 1984)