Rheology has been used for a long time to study the behavior of wheat flour dough and the breadmaking process. The need to predict wheat performance and formulation parameters first led to the development of empirical methods, de-
Figure 20 Basic mechanism for expansion and shrinkage of bubbles in extruded starchy products. (Adapted from Refs. 54, 121, and 123.)
scribed in Sec. II.A. Then the spreading of objective rheological methods favored the use of dynamic oscillatory shear measurements at low strains (less than 1%) to better evaluate the dough structure (11). By application of polymer science concepts, these studies have allowed us to underline the role of protein composi- tion and molecular features in the gluten network (133, 134) and the influence of other components, like water (135). In the last ten years, there has been grow- ing attention to performing objective rheological measurements in conditions of strain and strain rate more similar to those encountered in breadmaking. This is also due to a better understanding of the various phenomena that occur at the different stages of breadmaking: mixing, fermentation, and baking (13). If simple shear deformation may dominate during the first stage, elongational strain be- comes more significant during the two others, mainly because of gas-cell creation and growth, the dynamics of which will be partly controlled by the elongational properties of the dough matrix.
1. Elongational Tests and Results
a. Uniaxial Deformations. Various systems were employed to measure uniaxial elongational properties. De Bruijne et al. (136) used a mercury bath extensometer in which the sample was floating, to discard gravity effects. This sample had a dumbbell shape to minimize end effects on strain. Constant strain rates (ε˙ ⫽10⫺4–10⫺2 s⫺1) were obtained by modifying exponentially the speed of the Instron machine head. Although the test allowed them to discriminate doughs with various breadmaking performances, the final bread volume could not be predicted, which was attributed to the fact that the deformation was uniaxial.
Morgenstern et al. (137) measured average elongation and elongation rates in the case of a dough sheet deformed by means of the crosshead of a universal testing machine. In these types of experiments, correction has to be made on the sample section area to compute the stressσfrom the measured forceF:
σ⫽F
A (29)
where sectionA:
A(t)⋅h(t)⫽A0h0 (30)
where A0 andh0are, respectively, the initial cross-sectional area of the sample and its initial length andhis the sample length at timet.
Stressσ, deformationε, and deformation rateε˙ are all functions of time, defined by:
σ(t)⫽ F(t) A0
expε(t) (31)
ε(t)⫽lnh(t) h0
(32) ε˙ (t)⫽ 1
h(t) dh(t)
dt (33)
Depending on the machine used, one can impose either a constant force [F(t)⫽ F0] or a constant speed [(dh(t)/dt⫽V0] or a constant strain rate [ε˙ (t)⫽ε˙0]. A transient elongational viscosity ηE(t) can be always defined by the ratio σ(t)/
ε˙ (t). De Bruijne et al. (136) and Morgenstern et al. (137) found that, for flours at 45% moisture content but with differing protein content, this viscosity followed a power law (ηE(t)⫽Kε˙n⫺1) with a power law indexnof 0.24. Strain hardening was observed, which means that the stress increased with strain more rapidly than would be predicted by the theory of linear viscoelasticity.
Schweizer and Conde´-Petit (57) used a Meissner caterpillar-type elonga- tional rheometer, which allows one to operate at constant strain rate, controlled by a video system. The measurement of the traction force directly provides the elongational viscosity as a function of time (or deformation). They observed strain-hardening behavior above a critical strain, equal to 0.3 and independent of the elongation rate. The strain-hardening index (d lnσ/dlnε) varied from 3 to 7, according to the elongation rate (0.03–0.3 s⫺1).
b. Biaxial Deformations. The most widely employed method for mea- suring the biaxial properties of a dough is lubricated squeezing flow, presented in Sec. II.B.1.c. In this case, for a constant compression speedV0, the strain and strain rate are linked by the following relationship:
ε˙B⫽ V0
2h0
exp 2εB (34)
where the biaxial deformation is defined as:
εB⫽ ⫺1 2lnh(t)
h0
(35) Janssen et al. (24) and Kokelaar et al. (29) have applied this technique to various flours and gluten doughs, with different compression speeds in order to separate the effects of strain and strain rate. Both types of materials presented the same characteristics, also similar to the ones described before:
For a constant total strain, elongational viscosity decreased with strain rate, according to a power law (n⫽ 0.11–0.30)(Fig. 21a).
For a constant strain rate, they all exhibited strain hardening, the index of which (dlnσ/dεB) varied in the range 1.3–3 (Fig. 21b).
The same kind of observations were done by Launay and Bure´ (26), as well as by Dobraszczyk and Roberts (12), using the Alveograph test (see Sec.
II.A). However, in this last work, the strain-hardening index was derived directly from the stress–strain experimental curve, i.e., not for constant strain rates, which makes difficult the comparison of the values obtained with those from other works.
2. Application to the Breadmaking Process
Kokelaar et al. (29) found that gluten doughs behaved more differently than wheat flour doughs, according to wheat origin. At first glance, this result challenges the possible use of extensional tests for predicting flour breadmaking performance.
However, this discrepancy may be due to the way the samples were prepared, since various amounts of water had to be added to wheat flour to reach a 500- BU consistency in the Farinograph test. For flours, stress was found to increase when the temperature increased from 20 to 55°C, which may be due to the begin- ning of starch granule swelling, since the opposite trend was noticed for gluten dough. Arabinoxylan solubilization and the occurrence of enzyme activities should not be neglected either. A significant increase in the strain-hardening index was also found when ascorbic acid was added (57).
Gas retention is important for the stability of the foam created by dough fermentation. Indeed, cell-wall rupture should not occur too early before solidifi- cation. Van Vliet et al. (138) proposed using extensibility and the strain-hardening characteristics of the dough in biaxial extension as a criterion for gas retention, expressed by:
冤ddlnεBσ冥ε˙B⫽cst
⫹冤ddlnlnε˙σB冥εB⫽cst冤ddlnεBε˙B冥⬎2 (36)
The first term of the left-hand member is the strain-hardening index (m), whereas the first part of the second term is the elongational flow index (n). Van Vliet et al. (138) suggested that the last term (d ln ε˙B/dεB) could equal ⫺3 during fermentation and that this value would be closer to 2 during baking. A summary of the results obtained by this group is presented inFigure 22.Regions below the value of 2 indicate poor gas retention properties of the dough, either during fermentation (form⫺3n) or during baking (form⫹2n). Except for some values
Figure 21 Examples of experimental results of wheat flour doughs in biaxial extension:
(a) Viscosity as a function of elongation rate, for different values of total strain; (b) stress as a function of strain, for different values of elongation rate. (Reprinted from Ref. 29, with permission.)
Figure 22 Summary of the gas retention criterion results (Eq. 36). Open symbols corre- spond to good baking quality, closed symbols to poor baking quality.䊉,䊊: flour dough (Ref. 29);■,䊐: gluten dough (Ref. 29);䉱,䉭: flour dough (Ref. 24). (Data from Refs.
24 and 29.)
obtained at 50°C, the criterion seems able to discriminate varieties of gluten and wheat flour doughs.
Other information could be derived from results gained in extensional mea- surements. Recently, Launay and Bartolucci (139) showed that the analysis of stress relaxation curves obtained after a squeezing-flow or alveograph test could help in predicting some dimensional characteristics of biscuits, for which oven rise is not as important as for bread. If gas retention is linked to cell-wall rupture, then the strain at break should also play an important role. Opposite variations of strain at break and strain rate have been found, according to the flour quality (136). The relevance of such measurement is enhanced by the fact that, near the rupture, dough cannot be considered a continuous homogeneous medium, but rather a composite material (protein network–starch granules–water), the me- chanical behavior at rupture of which is influenced by inhomogeneities.
Several techniques are available to measure the elongational properties of dough. However, their extensive use as tools for improving the knowledge and control of the breadmaking process require progress in two directions. First, com- parison studies should be made between these different tests to solve some techni- cal points, such as the decoupling between the effects of strain and strain rates.
Sample preparation is the first difficulty to overcome in this way. In some cases (biscuits, for instance), these rheological data could be directly used for modeling the dough-shaping process. But generally, and this is the second direction, the conditions of use, especially temperature, should be closer to those of the baking process, in order to provide results in actual relation to the performance of the
material. This would provide useful information to improve the global modeling of heat and mass transfers during baking, which are presently limited to a global scale (140), by taking into account local phenomena, such as cell-wall rupture.
IV. CONCLUSIONS
Rheology is undoubtedly a major issue in cereal processing. The knowledge of rheological properties in connection with structural features at different scales (macromolecular structure, network and complexes, macroscopic properties) is absolutely necessary to understand the elementary mechanisms involved in the processes and thus to optimize processing conditions, equipment size, and final end-use properties. In the last 20 years, a progressive shift has been observed from empirical techniques (which are still widely used and offer interesting possi- bilities) to more objective measurements, partially through input from synthetic polymer science. This shift is all the more difficult because cereal products are very complex materials for which classical approaches are sometimes unsuitable and consequently that often need the development of new methodologies.
Another important issue is the rise of numerical modeling and the use of continuum mechanics to compute the flow parameters in processing operations.
Extrusion cooking has already been approached by such methods, but other pro- cesses may also be characterized: bread baking (140), pasta extrusion (141), mix- ing (142), dough sheeting (143), and dough extrusion (144). For these applica- tions, it is obvious that the rheological properties of the products one wants to study have to be perfectly known.
Although less investigated than biopolymer solutions, the viscous shear behavior of molten cereals and doughs is now beginning to be well understood.
Viscoelastic properties are sometimes more difficult to measure, mainly at high temperatures and for large deformations. The characterization of elongational properties, which play a major role in some cereal-forming processes, remains a real challenge for the future.
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