On the Use of Conventional Dough Extension Tests in Characterising Flours for Dough Sheetability. II. Simulations

Alveograph test

The alveograph test joins the extensigraph test as a predominant method of flour quality testing for end-use applications. The implications of gravity-induced slump and more generally dough elasticity for measuring dough strength were investigated numerically for this test. Sources of shape differences between samples include the following:

1. The test samples being left suspended under the influence of gravity during preparation. As seen for the real extensigraph samples [2], slump might require only a matter of seconds to become significant.

2. The initial sample thickness being uncontrolled and not compensated for during the analysis of results.

The above effects are demonstrated via FE simulation and the ABBM model; see Figure 8. The method is described elsewhere [8], although the dough model used in the previous work was replaced with the ABBM model for a “weak” dough (rested BR).

Figure 8:

Predicted effect of (a) initial sample thickness and (b) gravity-induced slump on bubble inflation pressures in the alveograph (rested BR dough). The initial sample shapes are in inset. (c) Distribution of principal extensional strain for the two bubble shapes. The dashed white lines indicate the vertical symmetry axis of each sample.

As Figure 4 demonstrates for the extensigraph test, apparent dough strength is predicted to increase with initial sample thickness for the alveograph test. The effect is predicted to be quasi-linear, i. e. a 5–10 % increase in thickness leads to a similar increase in apparent dough strength (bubble pressure). However, the effect of initial thickness is dwarfed by the effect of slump. Using a similar method to the extensigraph simulations, an initially flat sample (disc) was drawn within the FE package and subjected to gravity for 45 min to obtain a slumped sample shape; see Figure 8(b). Slump was predicted to reduce the apparent dough strength by roughly one-third in the alveograph simulation, which is roughly similar to the effect on hook force predicted by the extensigraph simulations for the rested BR dough.

Kieffer test

The Kieffer micro-extensigraph test is a popular flour quality test for dough strength. A virtual sample with the rested BR rheological properties was drawn using the dimensions of the sample forming mould used in a previous study [10], i. e. a straight cuboidal bar 4 mm wide, 4 mm tall and 18 mm long. The sample dimensions in reality change relative to the mould dimensions due to sample preparation, and do so differently between doughs. However, this effect was beyond the scope of this brief numerical study.

The hook speed used was as for the extensigraph simulations but in the opposite direction (14.5 mm/s vertically upwards). The interaction between the virtual sample and the sample holder was modelled using a simple “hinge” rather than by drawing the sample holder explicitly. Essentially, the virtual sample was defined as being fixed in place at each end at a pin joint. The sample ends were then allowed to rotate freely around the pins during hook stretching. This setup is presented in Figure 9. Note that this approach greatly simplified the simulations, and as a result these could be conducted in 2D (plane stress) rather in full 3D as was done for the extensigraph simulations.

Figure 9:

Predicted effect of gravity-induced slump on hook force in the Kieffer dough extension test (rested BR dough). The initial and final sample shapes are shown in the inset.

A slumped sample shape was generated by simulating the effect of gravity on a straight sample of rested BR dough for 45 min of physical time. This process generated an initial sample shape similar to the photographs of sagged samples presented in the previous study [10].

Figure 9 presents the hook force for the rested BR dough for the straight and slumped sample shapes. The predicted hook forces (~25 mN) are at the lower limit of those found experimentally [10] (~20–500 mN). However, given the approximations in the simulation method and the obvious differences in flours, dough formulations and mixing protocols, this level of agreement is considered close enough for the current purpose of determining if slump has a noticeable effect for this test.

Slump is predicted to cause a significant drop in hook force. While further analysis is limited without real test data, these results agree with those presented for the extensigraph and alveograph tests in that a method of testing immune to slump and other variations in sample dimensions is needed.