Chemistry Reference
In-Depth Information
6.1.4 Gluten
Wheat protein is seen as the most important factor governing bread-making quality.
3-5
A
high protein content is related to good bread-making quality. There are also a number of other
factors, together described as protein 'quality' that determine the bread-making potential of
a wheat flour.
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In wheat kernels, 80% of the protein is found in the endosperm. Wheat flour proteins
have been classified into four types based on their solubility
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: albumins, soluble in water;
globulins, soluble in salt solutions; gliadins, soluble in 70% ethanol; and glutenins (partly)
soluble in dilute acid or alkali. The bulk of the protein is formed by the latter two: gliadins and
glutenins. All these fractionations lead to broad overlapping classes of proteins, indicating
the complexity and variability of the various classes. There is also only a limited relationship
between the amounts of each of the Osborne fractions and the final bread-making quality.
8
When flour is mixed with water, a viscoelastic mass is formed. From this mass, starch
can be washed out and then gluten remains. On a dry basis, gluten contains around 70-85%
protein, 5-15% carbohydrates (starch and non-starch polysaccharides (NSP)), 3-10% lipids
and 1-2% ash.
5, 9
Gluten proteins contain relatively high amounts of glutamic acid, which exists as glutamine
in the wheat kernel, proline, hydrophobic amino acids and the sulphur-containing cysteine.
Glutamine and also the hydrophobic amino acids ensure sufficient hydrogen bonding during
mixing. This helps in film formation. Proline, because of its ring structure, disrupts the
-
helix formation that normally occurs in protein polymers. The result is a higher percentage
of
α
-sheet structure, which helps in giving gluten the necessary elasticity. The sulphur-
containing amino acids ensure the formation of disulphide bridges between protein chains
and also within protein chains, giving the protein network sufficient strength.
The effect of all these amino acids together is a rapid continuous film formation which
gives wheat dough its unique viscoelastic properties and gas holding capacity.
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The viscoelastic behaviour of wheat gluten sets it apart from other grains or other vegetable
protein sources.
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The formation of gluten is the result of interaction between the two major
classes of wheat proteins, the gliadins and glutenins. When flour is mixed with water, a
viscoelastic mass is formed, to a large extent due to this interaction.
The sulphur-rich glutenins are able to form polymeric networks. The sulphur-poor gliadins
are present mainly as monomers. Glutenins consist of a high molecular weight (HMW)
group and a low molecular weight (LMW) group of proteins. Both groups are able to
form disulphide polymeric networks. Gliadins behave as a viscous liquid, whereas glutenins
behave as a cohesive elastic solid.
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In the gluten network, these properties are combined
into a protein network with viscoelastic properties, which enables gas cells to be retained in
a dough during the bread-making process. This highly specific property allows wheat flour
to be used for production of a variety of yeast leavened or chemically leavened products.
The composition and strength (quality) of the gluten is partly determined by the presence
and abundance of individual glutenin subunits. The relative amounts of each subunit are
determined by genetic factors, growing conditions and fertilization. Glutenin polymers are
very heterogeneous in composition and size. Certain fractions of the HMW glutenins are
highly correlated with loaf volume, whereas others are not.
13, 14
A glutenin fraction that is
insoluble in sodium dodecyl sulphate (SDS) solution, has been called glutenin macropolymer
(GMP).
15
This fraction has been studied in great detail and evidence has been presented
showing that this GMP can be seen as a wheat quality parameter.
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