1Food Science Australia, Werribee, Vic., 3030
2Food Science Australia, North Ryde, NSW, 1670
3Value-Added Wheat CRC, North Ryde, NSW, 1670
The rheological properties of the gluten component of dough are critical to baking quality. When the protein content in flour is low, vital wheat gluten (VWG) may be added to increase flour “strength”, to meet dough-quality specifications for baking. For this purpose, gluten producers and the bakers must be able to determine the rheological properties of commercial vital wheat gluten and its suitability for baking applications. Traditionally, an Extensograph, which measures extensibility and maximum resistance of flour dough by mixing an appropriate amount of gluten with wheat starch, is used to access the rheological quality of gluten. This conventional method is well established and widely accepted by the industry for determining the performance of gluten. However, this method requires relatively large qualities of dry gluten. For laboratory-based studies, this approach presents difficulties, as the production of gluten is laborious and time consuming, particularly when modification of gluten is involved.
Several techniques such as dynamic oscillatory measurements, stress-relaxation and creep-recovery tests have been used for fundamental rheology studies of wheat flour dough and isolated gluten (Amemiya and Menjivar, 1992; Campos et al, 1997; Khatkar and Schofield, 2002; Tronsmo et al, 2003). These methods which measure small deformational rheological changes in polymer materials at low strains, not only require small amounts of material, but also provide information about the fundamental structure behaviour of the material. Although correlations have been poor between dough rheological properties measured by these small deformation measurements and breadmaking quality, the application of these techniques to gluten has showed agreement with the traditional large deformation rheological measurements (Khatkar et al, 1995; Edwards et al, 1999). There have been indications that the creep strain of isolated gluten could be used to discriminate the extensibility and baking potential of wheat flour (Janssen et al, 1996).
The aims of this work were to develop a small-scale creep-recovery test that could be used routinely in laboratory studies to examine the fundamental rheological properties of gluten and to determine its potential as an alternative to the use of the Extensograph for the specification of gluten quality.
A set of commercial vital wheat glutens was obtained from Manildra Group (produced in years 2002 and 2003). Their protein contents, determined according to the AACC method 46-30, were within the range of 70-79% (w/w). No protein correction was made for the Extensograph test, nor for the creep-recovery test. Two industrial flour samples were also obtained from Manildra Group (years 2002 and 2004). Hand-washed glutens were prepared with and without modification.
Analysis of extensibility and strength
The extensibility and strength of gluten were measured using the Extensograph, according to AACC method 54-10 with slight modification. Flour (300 g) was made by mixing gluten powder (40 g) with wheat starch (260 g) (Manildra Group, Nowra, NSW). The viscosity of the dough was adjusted to be 400 (±20) Brabender Units (BU) at the end of mixing.
Creep and recovery test
Dry gluten (0.6 g) was mixed with water (0.9 mL) using a pestle and mortar. The gluten dough was then wrapped up in Cling wrap and allowed to rest for one hour at room temperature. The creep test was carried out using a dynamic stress rheometer (Paar Physica MCR 300, PHYSICA Meßtechnik GmbH, Stuttgart, Germany). The rheometer was equipped with 10-mm diameter serrated upper and lower parallel plates that were maintained at 25°C. The gluten dough was placed between the plates, and the upper plate being lowered to a fixed gap of 2.0 mm. A home-made ring surrounded the measuring plates using water-saturated ring-shaped filter paper to minimise drying of the gluten sample during measurements. The creep and recovery tests were carried out by applying a constant shear stress of 200 Pa for 200 sec. The shear stress was then taken off to allow the sample to recover, also for 200 sec.
Curves from the two steps of the creep-recovery test can be seen in the profiles (Figure 1), namely, a constant shear stress during an initial creep time, followed by no stress to allow recovery. Two parameters are recorded: 1) the maximum creep strain, i.e. the height at the end of creep time, and 2) the relative recovery, expressed as the percentage of strain at the end of recovery time, divided by the maximum creep strain. Higher values for the creep strain indicate greater softness and better flow, while higher values for the recovery relate to greater elasticity. The stress of 200 Pa, a relatively high stress for small deformation measurement, was used for the initial creep test. The stress is likely to exceed the region of linear viscoelasticity for wheat flour dough (Edwards et al, 1999). However gluten lends itself more readily to fundamental rheological characterisation, therefore the linear domain of its viscoelastic behaviour is much larger than that of dough (Lefebvre et al, 2003). It has been reported that a much wider range of stress can be applied to gluten, compared to dough, with viscoelastic behaviour being still maintained (Khatkar and Schofield, 2002; Lefebvre et al, 2003).
For the range of commercial gluten samples tested, there was a good linear correlation between extensibility and the maximum creep strain (Figure 2). A correlation co-efficient R2 of 0.9 was achieved. The results show that the gluten sample with good extensibility would have soft and good flow behaviour, therefore resulting in a higher maximum creep strain. Maximum creep strain (or compliance) is related to the baking quality of wheat flour according to some reports (Khatkar and Schofield, 2002; Schober et al, 2002 and Edwards et al, 1999). It has been reported that gluten isolated from a ‘strong’ flour had a higher resistance to deformation, therefore a lower maximum creep strain than the gluten from a ‘weak’ flour (Janssen et al, 1996). A similar trend has been observed for dough made from moderate strength and relative extensible cultivars which had higher creep strain than the doughs from stronger, but less extensible wheat (Campos et al, 1997; Edwards et al, 1999).
Gluten and modified gluten preparations were produced from two commercial flours by hand-washing in the laboratory. The gluten controls from the poor- and good-quality flours had contrasting extensibilities of 4.5 and 12.5 cm (Figure 2, S008 GC and S031 GC, respectively). The modified glutens produced from both flours (S008 S1G and S031 S1G) had improved extensibilities. The creep-recovery results for these gluten samples follow the trend established for the commercial samples, but the correlation line was higher.
Figure 1. Creep-recovery curves of rehydrated vital wheat gluten (60% moisture) with different extensibilities. A constant shear stress of 200 Pa was applied for 200 seconds, followed by the release of stress to allow the sample to recover for another 200 seconds.
The upward-shift of the apparent correlation curves between the commercial and laboratory-prepared gluten samples may be due to the differences caused by the drying methods. In commercial practice, a ring dryer is used with drying at temperatures of 55-60°C; while the laboratory glutens were freeze dried. The drying method, particularly differences in temperatures, is known to cause changes in protein structure. Heat-treatment of hydrated gluten can cause an increase in the number of rheologically effective cross-links (Apichartsrangkoon et al, 1999). Irreversible alterations in the structural properties of gluten can occur at high temperature. In this case, the rheological properties of gluten at large deformation (as measured by their extensibility) are not clearly affected by the drying methods, but differences were observed at small deformation. The maximum creep strains for the commercial vital glutens were lower than for the laboratory freeze dried glutens. The creep-recovery tests were sensitive to the molecular bonds within the protein structure influenced by water; as a result, they were more sensitive to heat damage. The main contribution to gluten rheological properties at large deformation would be inter-molecular links, such as disulphide bonds between the glutenin subunits, that are mainly affected through energy input by mechanical mixing. Therefore, changes in protein structure due to the heat damage seem much less important.
Another important parameter to assess the suitability of flour or gluten for particular end-uses is gluten ‘strength’ measured as the Rmax of the Extensogram. However, a relationship between the strength of the gluten samples and either creep or recovery in the small-scale test was not found.
Figure 2. Correlation of the extensibility (cm) measured by the Extensograph method (13.3% gluten in gluten/starch dough) and the maximum creep strain reached after 200 sec under the constant stress by the creep test (0.6g 60% rehydrated gluten) for the commercial vital gluten (diamonds) and the laboratory prepared gluten (triangles).
A small-scale deformation test (“creep and recovery”) was developed to assess the rheological properties of isolated gluten, using a set of commercial dry-gluten samples. The method requires less than one gram of dry gluten, and it provides information relating to dough extensibility obtained by the traditional empirical methods. The rheological properties of gluten, measured by the creep test, can be used to predict their extensibility performance in the traditional large-scale test. A distinct relationship was obtained for freeze-dried glutens. The test offers a relatively simple method for research on gluten rheological behaviour and for studying the structural basis of gluten properties.
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