H. Wieser, Deutsche Forschungsanstalt für Lebensmittelchemie, Garching, Germany; W. Bushuk, Food Science, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada; F. MacRitchie, Kansas State University, Manhattan, Kansas, USA
Gliadin and Glutenin: The Unique Balance of Wheat Quality
Pages 213-240
DOI: https://doi.org/10.1094/9781891127519.010
ISBN: 978-1-891127-51-9
Abstract
Traditionally, gluten proteins have been divided into roughly equal fractions according to their solubility in alcohol-water solutions: the soluble gliadins and the insoluble glutenins. The major portion (~80%) of the gliadin fraction consists of monomeric proteins with molecular weights (MW) in a range from around 30,000 to 55,000 (MW based on complete amino-acid sequences and mass spectrometry). Disulfide bonds are either absent (ω-gliadins) or present as intrachain crosslinks (α/β-, γ-gliadins). The smaller portion (~20%) contains oligomeric proteins (MW ~100,000–500,000) linked by interchain disulfide bonds. This fraction has been called high-molecular-weight (HMW) gliadin, aggregated gliadin or ethanol-soluble glutenin (Shewry et al 1983; Huebner and Bietz 1993) and was shown to consist of α/β-and γ-gliadins (probably with an odd number of cysteines) and low-molecular-weight glutenin subunits (LMW-GS). The glutenin fraction contains alcohol-insoluble polymeric proteins of varying size with MWs ranging from about 500,000 to more than 10 million. They are mainly formed by LMW-and HMW-GS linked by interchain disulfide bonds.
Both gliadin and glutenin fractions are important contributors to the rheological properties of dough, but their functions are divergent. Purified hydrated gliadins have little elasticity and are less cohesive than glutenins; they contribute mainly to the viscosity and extensibility of the dough system. In contrast, hydrated glutenins are both cohesive and elastic, and are responsible for dough strength and elasticity. To simplify matters, gluten is a “two-component glue”, in which gliadins can be understood as a “plasticizer” or “solvent” for glutenins. A proper mixture of the two is essential to impart the viscoelastic properties of dough and the quality of the end product. The uniqueness of dough properties and of the breadmaking quality of wheat have mainly been associated with glutenin's polymeric structure. Even though closely related to wheat, rye storage proteins are not able to generate similar amounts of large molecular-sized polymers (Gellrich et al 2003). Native glutenins are amongst the most complex protein networks in nature due to numerous different subunits and variable disulfide bonds participating in polymerization, and due to variability caused by genotype, growing conditions and technological processes. The structure of glutenins is not in a stable state, but undergoes a continuous change from the maturing grain to the end-product (e.g., bread). In this chapter, we try to summarize knowledge about structural features and quantitative aspects of glutenin polymers in relation to physical dough properties.
