New, Uncommon, and Specialised Words in the World of Wheat
Gluten protein represents ∼85% of the wheat proteins and is made up of a complex mixture of proteins, containing many, probably several hundred, polypeptides of different molecular weights (Fig. 10.4).
About half of the proteins are monomeric gliadins, with the remainder being disulfide cross-linked polypeptides that form the polymeric glutenin fraction, whose size can range up to tens of millions of Daltons (Shewry et al., 2002; Wrigley, 1996). Together, they provide gluten with its unique physical properties – gliadin for viscosity and extensibility and glutenin for elasticity. The appropriate balance of the two is important for baking.
The gliadin fraction contains mainly single polypeptide chains of molecular weight (Mr) in the range of 30,000–75,000 Da. The gliadins associate with each other and with glutenin proteins through non-covalent hydrogen bonds and hydrophobic interactions. The ω-gliadin contains a large proportion of the amino acids glutamine, proline and phenylalanine; together they account for about 80% of the total residues. They constitute the so-called S-poor prolamins fraction because they have few or no residues of sulphur-containing amino acids cysteine and methionine (Shewry, et al., 1986).
In contrast, α-, β- and γ-gliadins have less proline, glutamine and phenylalanine, but 2–3 mol.% cysteine plus methionine. The cysteine residues are located towards C-terminal ends and are involved in intramolecular disulphide bonds (Shewry and Tatham, 1997). The α/β- and γ-gliadins are structurally related with their Mr ranging from 30,000 to 45,000 Da.
The polymeric glutenin proteins are divided into high molecular weight (HMW) and low molecular weight (LMW) subunits. The HMW subunits account for about ∼12% of the total gluten protein. Their size (Mr > 100,000 Da) and their ability to form an intermolecular network give the gluten framework its structure. Thus, it is the HMW subunits that are largely responsible for providing gluten viscoelastic properties (Shewry, et al., 2002). The remaining LMW glutenins are polymeric proteins that have amino acid compositions and structure similar to the α/β- and γ-gliadins, with a slightly higher Mr of ∼45,000 Da. But their ability to form intermo- lecular disulphide bonds with each other and/or with HMW glutenins, is important for the formation of the glutenin macropolymer.
Gluten - Gluten may be defined as the ‘cohesive, visco-elastic proteinaceous material prepared as a by-product obtained by isolation of starch from wheat flour. A biological definition might include the origins of the gluten–protein complex as being derived from the ‘storage proteins of the wheat grain'. The minimum gluten content of wheat flour should be about 24 % (wet) and 8 % (dry).
Gluten Index - The gluten index (GI) is a measurement of wheat protein that provides a simultaneous determination of gluten quality and quantity (AACC, 2000). The GI value expresses the weight percentage of the wet gluten remaining on a sieve after automatic washing with salt solution and centrifugation.
Dry Gluten - It gives the dry matter of gluten protein present in flour and water absorption capacity of gluten present in flour.
Wet Gluten - Wet gluten content is determined by washing the dough obtained from flour or wheat meal, with water or other solution (e.g. NaCl solution), in certain conditions, to remove the starch and other soluble compounds of the sample. The rubbery mass that remains after washing is the wet gluten.
Glutenin - Glutenin (a type of glutelin) is a major protein within wheat flour, making up 47% of the total protein content. The glutenins are protein aggregates of high-molecular-mass (HMW) and low-molecular-mass (LMW) subunits. Glutenin is responsible for the strength and elasticity of dough. Breadmaking qualities are largely dependent on the number and composition of HMW glutenin subunits. It has been demonstrated that alleles Glu-A1b (Ax2∗) and Glu-D1d (Dx5 + Dy10) are normally associated with superior end-use quality, especially dough strength.
Gliadin - A type of prolamin, which are a component of gluten, are essential for giving bread the ability to rise properly during baking. Gliadins are soluble in 70% aqueous ethanol. The gliadins are intrinsically disordered proteins meaning that they have continuously altering shapes making it difficult to study them. The performed image analysis and computer simulations of the proteins show that the average shape of the gliadins follows an elliptical shape. The gliadins are unable to form polymers in the cell since its cysteines form intra-chain disulphide bonds at synthesis due to hydrophobic interactions.
α-/β-gliadins – soluble in low-percentage alcohols.
γ-gliadins – ancestral form of cysteine-rich gliadin with only intrachain disulfide bridges.
ω-gliadins – soluble in higher percentages, 30–50% acidic acetonitrile
Sodium Dodecyl Sulfate (SDS Solution) Sedimentation Value - The sedimentation value indicating correlation with gluten content, gluten quality and loaf volume depends on the protein composition and is mostly correlated to the protein content. Therefore, it is useful to measure sedimentation value.
Gluten Intolerance - A gluten intolerance is the body’s inability to digest or break down the gluten protein found in wheat and certain other grains. Also known as a gluten sensitivity, gluten intolerance can range from a mild sensitivity to gluten to full-blown celiac disease.
Celiac Disease - Celiac disease is an autoimmune disorder in which your immune system responds abnormally to gluten. Gluten is present in wheat, barley, and rye.