1New Zealand Institute for Crop & Food Research Limited, Private Bag 4704, Christchurch, 8020, New Zealand.
2Plant Industry, CSIRO, Canberra, ACT, Australia.
Elongational flow plays an important role in the production of bread, pastry and pasta. For example, the moulding and sheeting operations impose elongational flow on to the wheat flour doughs. Likewise, the expansion of dough gas bubbles during fermentation and baking imposes biaxial elongation on the dough surrounding these bubbles. An understanding of the elongational properties of wheat flour doughs is important if baking processes and the functionality of flours during processing are to be understood. As a result, various dough elongational instruments are used by the cereal industries and researchers. These instruments impose different elongational flows, for example the Brabender extensograph imposes uniaxial flow whilst the Chopin Alveograph imposes biaxial flow. Differences in these elongation flows may also be reflected in the measured rheological responses.
The purpose of this study was twofold: firstly, to use fundamental stress-strain calculations in elongational measurements of wheat flour doughs under two elongational flow regimes; and, secondly, to compare the behaviour of these flours under these different elongational flows.
Methods and materials
Australian wheat varieties Chara and Janz (Prime Hard) and Kukri (strong) were used to prepare wheat flour doughs. A null line set of wheats derived from a cross between the cultivars Gabo and Olympic in which only the genes on chromosome 1 controlling expression of high molecular weight glutenins were modified were also studied. These null line flours are indicated by plus and minus signs to symbolise which of the respective Glu-1A, Glu-1B and Glu-1D genes are active. For example, the null line described by (-, +, -) has an active Glu-1B gene whilst the Glu-1A and Glu-1D genes are inactive.
All doughs were prepared on a 10 g MDD Mitchell type mixer (Mitchell, 1989) and mixed at optimal water addition and work input levels.
All doughs were tested at constant speed and constant strain rate on the Stable Microsystems Kieffer microextensograph rig using a 6 mm diameter hook (Mann et al, 2004) and on the sheet deformer system (Morgenstern et al, 1996). The microextensograph subjects the dough to uniaxial elongation whilst the sheet deformer technique imposes planar elongational flow. Both systems were mounted on to an Instron Model 4444 (Instron, High Wycombe, UK) Universal Testing Machine controlled by a PC running in-house software routines written in LabVIEW (National Instruments, Austin, TX, USA). The force-distance data were converted into strain-stress units for both elongational geometries using the equations devised by Morgenstern et al (1996) for the sheet deformer, and Dunnewind et al (2004) microextensograph equations modified to take account of the larger hook diameter. Dough samples were sheeted, prepared and rested for 45 min before elongation.
Results and discussion
Uniaxial elongational flow of the microextensograph resulted in higher rupture strains and four times higher rupture stresses and rupture strains then the planar elongation imposed by the sheet deformer (Figure 1). Such differences between these elongational flows is not surprising given that the viscosities of rheologically 'simple' Newtonian fluids depend on the particular elongational flow regime to which they are subjected. This concept is demonstrated by comparing the Trouton ratio (ratio of elongational viscosity to zero shear viscosity) of Newtonian liquids under different elongational flows. Newtonian liquids deformed under uniaxial flow have a Trouton ratio of 3, whereas under biaxial and planar flows the ratio is, respectively, 4 and 6. Whilst wheat flour doughs are much more complex materials, the dependence of dough elongational behaviour on the flow geometry is therefore not surprising, nor is the fact that the exact relationship between the geometries differs from that of simpler Newtonian materials.
Figure 1. Comparison between typical stress-strain curves obtained for cv. Chara when elongated at a constant strain rate of 0.1 s-1 using the microextensograph (uniaxial flow) and the sheet deformer (planar flow).
Although the rheological behaviour of the flours were quantitatively different for the uniaxial and planar flows, the behaviour was, however, qualitatively similar. The classic J-shape of the stress-strain curves, typical of strain-hardening materials were observed under both flow regimes (Figure 1) and the relative stresses at which the flours underwent rupture were also similar (Figure 2). The rupture stresses increased when more of the HMW-GS genes were expressing in the null lines while the commercial cultivars had the highest rupture stresses under both uniaxial and planar flows (Figure 2).
Figure 2. Rupture stress verses the optimal 125 g MDD work input for flours deformed under (a) planar elongation flow, and (b) uniaxial elongation both at a constant strain rate of 0.1 s-1.
The differing abilities of the flours to undergo strain hardening, as indicated by the J-shape stress-strain curves, are clearly demonstrated in Figure 3. The commercial cultivars and the null lines with more HMW-GS-expressing genes strain harden to a large extent which allows these doughs to elongate under greater strains and stresses. The weaker null lines, however, did not display any strain-hardening behaviour and had substantially inferior elongational properties.
Figure 3. Stress-strain curves of flours deformed under planar elongational flow at a constant strain rate of 0.1 s-1.
The Considère criterion relates the point at which an elongating dough is about to rupture to the force acting on the dough. This provides a means of quantifying the strain-hardening behaviour. When the stress-strain curves are modelled with a power law ( ) the Considère criterion dictates that the strain at which dough ruptures is equal to the power law exponent, , which is referred to as the strain-hardening index (Dobraszczky and Roberts, 1994). Fitting the power law to the stress-strain curves reveals a clear linear relationship between the dough rupture strain and the strain-hardening index (Figure 4a), with the commercial cultivars and stronger null lines strain-hardening to a greater degree and rupturing at higher strains. However, the relationship is not as clear under uniaxial flow, largely due to the poorer differentiation in the rupture strains (Figure 4b). The cause of this behaviour is not known. These differences may relate to the flow geometries or to the greater degree of sheeting applied to the samples measured with the planar flow sheet deformer.
Figure 4. Rupture strain verses the strain-hardening index obtained from the power law fits for flours deformed under (a) planar elongation, and (b) uniaxial elongation at a constant strain rate of 0.1 s-1.
This study clearly reveals that the different elongational flows imposed by the microextensograph (uniaxial flow) and sheet deformer (planar flow) result in quantitatively dissimilar elongational characterisations. However, at the qualitative level both techniques yielded similar information in terms of the relative elongational properties of the flours. Those doughs that ruptured at higher stress showed greater propensity to strain harden than those that ruptured at low stress. The rupture stresses measured by both uniaxial and planar flows were capable of differentiating between the flours. While planar flow rupture strains were also able to differentiate between the flours, uniaxial flow rupture strains were poorly differentiated. These findings of differences between elongational flows highlights the need to compare various elongational flows both to better understand the imposed flows and to gain a greater understanding of the rheological properties of wheat flour doughs.
The authors thank Marcela Ross and Lidia Motoi for their assistance in preparing the samples.
Dobraszczyk, B., & Roberts, C.A. (1994) J. Cer. Sci. 20: 265-274.
Dunnewind, B., Sliwinski, E.L., Grolle, K., & Van Vliet, T. (2003) J. Text. Stud. 34: 537-560.
Mann, G., Békés, F., & Morell, M.K. (2004) in 'The Gluten Proteins”, Lafiandra, D., Masei, S., & D'Ovidio, R.D. (eds.), Cambridge, UK: Royal Society of Chemistry. 215-218.
Mitchell, T.A. (1989) in 'Modern Methods of Plant Analysis', Linskens,H.F., & Jackson, J.F. (eds.), Heidelberg, Germany: Springer-Verlag: 313-331.
Morgenstern, M.P., Newberry, M.P., & Holst, S.E. (1996) Cer. Chem. 73: 478-482.