744 CEREAL CHEMISTRY Basic Rheology of Bread Dough with Modified Protein Content and Glutenin-to-Gliadin Ratios S. Uthayakumaran, 1–3 M. Newberry, 1,3 M. Keentok, 1,3 F. L. Stoddard, 1,4 and F. Bekes 1,5 ABSTRACT Cereal Chem. 77(6):744–749 The uniaxial elongational and shear rheology of doughs varying in either the protein content or glutenin-to-gliadin ratio were investigated. Increasing the protein content at constant glutenin-to-gliadin ratio increased the strain- hardening properties of the dough, as shown by increasing elongational rupture viscosity and rupture stress. Glutenin and gliadin had a more com- plex effect on the elongational properties of the dough. Increased levels of glutenin increased the rupture viscosity but lowered the rupture strain, while elevated gliadin levels lowered the rupture viscosity but increased the rupture strain. These observations provide rheological support for the widely inferred role of gliadin and glutenin in shaping bread dough rheology, namely that gliadin contributes the flow properties, and glutenin contri- butes the elastic or strength properties. The shear and elongational prop- erties of the doughs were quite different, reflecting the dissimilar natures of these two types of flow. Increasing protein content lowered the maxi- mum shear viscosity, while increasing the glutenin-to-gliadin ratio increased maximum shear viscosity. Strong correlations between the results of basic and empirical rheology were found. These basic, or fundamental, rheological measurements confirmed prior empirical studies and supported baking industry experience, highlighting the potential of basic rheology for bread and wheat research. Dough measurements, as determined on equipment such as the mixograph, farinograph, extensigraph and alveograph, all utilize arbi- trary units that cannot readily be converted to more useful scientific dimensions (Levine 1987). Basic rheological instruments are cap- able of providing the essential, or fundamental, details of the material’s rheological properties, unlike empirical tests. Rheological properties can be considered as a continuum between two ideal states, those of pure elasticity and of pure viscosity. A purely elastic rubber band does not flow but snaps back to its starting position, while purely viscous water flows without recovery. Most materials, including bread dough, demonstrate behavior that is a combination of both states and basic rheometry can elucidate and quantify these properties. Wheat dough is a unique material formed when wheat flour is mixed with water creating a viscoelastic dough that retains gas (Walker and Hazelton 1996). The rheological characterization of wheat flour dough is essential to produce information concerning the quality of the raw material and the textural characteristics of the finished product. Dough studies in which the basic principles of physics have been applied have involved basic rheological measurements of shear stress in steady shear, creep, stress relaxation and extension (Bloksma and Bushuk 1988, Janssen et al 1996a, Safari-Ardi and Phan-Thien 1998). These studies have had the aim of completely characterizing dough and of finding reliable rheological tests that can differentiate dough types. Strain sweep experiments and stress-relaxation tests on Australian strong flour indicated that tests at higher shear strains can differentiate flour types (Phan-Thien and Safari-Ardi 1998, Safari-Ardi et al 1998). A study on the effects of starch-protein interaction on rheological properties has also been performed on two South Australian wheat cultivars where a synthetic aqueous mixture of dough was used (Chiruta et al 1997). Khatkar et al (1995) and Janssen et al (1996a) studied the effect of varying the glutenin-to-gliadin ratio on rheo- logical property of gluten dough and showed that glutenin contri- buted to elastic and gliadin to the viscous property of hydrated gluten. Though it has been shown that basic rheological tests can differentiate flour samples, very little work of this nature has been reported so far on the relationship between different protein quan- tities, different glutenin-to-gliadin ratios, and their functional properties. The objective of this study was to use basic rheological tests (elongational and shear viscometry) to separate the effects of protein quantity and composition on dough properties. MATERIALS AND METHODS Samples Wheat flours Banks, Hartog, Rosella, and Sunbri were obtained from BRI Australia Ltd., North Ryde, NSW, for the study. The high molecular weight glutenin subunit (HMW-GS) composition of the flours is given in Table I. The flour components (starch, gluten, glutenin-rich and gliadin- rich fractions) for enrichment studies were prepared as described by MacRitchie (1987) and Uthayakumaran et al (1999). The nitrogen content of the components was determined by the Dumas total combustion method using an elemental analyzer (CHN-1000, Leco Inc., St. Joseph, MI). Protein (%) was estimated as N × 5.7. Altering Protein Content of Flour at Constant Glutenin-to-Gliadin Ratio Blends (10.0 g) of each of the base flours, using gluten and starch isolated from that flour, were prepared as previously des- cribed (Uthayakumaran et al 1999). Based on the protein content of each flour, gluten and starch blends of flour and gluten isolated from it were prepared to have 110, 120, and 130% of the protein content in the base flour (increasing protein). Formulations contain- ing 80 and 90% of the protein of the parent flour were prepared by blending the flour with starch isolated from that flour (to dilute the protein). Altering Glutenin-to-Gliadin Ratio at Constant Protein Content Gluten, glutenin, or gliadin prepared from the parent flour were added to the flour to vary the glutenin-to-gliadin ratio while keeping the protein content constant at 120% of the protein content of the parent flour. Measurement of Functional Properties The amounts of water to be added were calculated from the protein content and the moisture of the blend using the standard method (AACC 2000). Blend, water, and salt solution (6.67% 1 Quality Wheat Cooperative Research Centre Ltd., Locked Bag 1345, North Ryde, NSW 1670, Australia. 2 Corresponding author. Phone: 61 2 9351-7141. Fax: 61 2 93517060. E-mail: suthay@mech.eng.usyd.edu.au 3 Department of Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia. 4 Plant Breeding Institute, The University of Sydney, NSW 2006, Australia. 5 CSIRO Plant Industry, Grain Quality Research Laboratory, P.O. Box 7, North Ryde, NSW, 1670, Australia. Publication no. C-2000-1011-03R. © 2000 American Association of Cereal Chemists, Inc.