Biomedical Engineering Reference
In-Depth Information
fermentation and this results in a leavened loaf of bread after baking. It is commonly
accepted that gluten proteins (gliadins and glutenins) decisively account for the physi-
cal properties of wheat dough. Both protein fractions are important contributors to
these properties, but their functions are divergent. Hydrated monomeric and oligo-
meric proteins of the gliadin fraction have little elasticity and are less cohesive than
glutenins; they contribute mainly to the viscosity and extensibility of dough. In con-
trast, hydrated polymeric glutenins are both cohesive and elastic, and are responsible
for dough strength and elasticity. Thus, gluten is a “two-component glue,” in which
gliadins can be understood as a “plasticizer” or “solvent” for glutenins [
65
] . A proper
mixture (~2:1) of the two is essential to give desirable dough and bread properties.
Native gluten proteins are amongst the most complex protein networks in nature
due to the presence of several hundred different protein components. Even small
differences in the qualitative and quantitative protein composition decide on the
end-use quality of wheat varieties. Numerous studies demonstrated that the total
amounts of gluten proteins (highly correlated with the protein content of flour), the
ratio of gliadins to glutenins, the ratio of HMW-GS to LMW-GS, the amount of
GMP, and the presence of specific HMG-GS determine dough and bread quality.
Amongst chemical bonds disulfide linkages (Fig.
2.1
) play a key role in deter-
mining the structure and properties of gluten proteins. Intrachain bonds stabilize the
steric structure of both monomeric and aggregative proteins; interchain bonds pro-
voke the formation of large glutenin polymers. The disulfide structure is not in a
stable state, but undergoes a continuous change from the maturing grain to the end
product (e.g., bread), and is chiefly influenced by redox reactions. These include (1)
the oxidation of free SH groups to S-S linkages, which supports the formation of
large aggregates, (2) the presence of chain terminators (e.g., glutathione and glia-
dins with an odd number of cysteine), which stop polymerization, and (3) SH-SS
interchange reactions, which affect the degree of polymerization of glutenins.
Consequently, oxygen is known to be essential for optimal dough development and
oxidizing agents, for example potassium bromide, azodicarbonimide, and dehy-
droascorbic acid (the oxidation product of ascorbic acid) have been found to be
useful as bread improvers [
88
] .
Conversely, reducing agents such as cysteine and sodium metabisulfite are used
to soften strong doughs, accompanied by decreased dough development and resis-
tance and increased extensibility. They are specifically in use as dough softeners for
biscuits. The overall effect is to reduce the average MW of glutenin aggregates by
SH/SS interchange.
Beside disulfide bonds, dityrosine and isopeptide bonds have been described as
further covalent cross-links between gluten proteins. Compared with the concentra-
tion of disulfide bonds (~10 mmol per g flour) tyrosine-tyrosine cross-links
(~0.7 nmol per g flour) appear to be only of marginal importance [
89
] . Interchain
cross-links between lysine and glutamine residues (isopeptide bonds) are catalyzed
by the enzyme transglutaminase (TG). Addition of TG to flour results in a decrease
in the quantity of extractable gliadins and an increase of the glutenin fraction and
the nonextractable fraction [
90
]. Thereby, dough properties and bread-making qual-
ity can be positively influenced, similar to the actions of chemical oxidants.
