Health-related issues regarding high salt intake including high blood pressure and
associated cardiovascular diseases have been the reason for the growing research on salt
reduction in different food products. However, in dough-based products, reducing or
eliminating salt from the formulation is highly challenging. This is because salt (in the
form of NaCl) is used in the processing of wheat-based foods, not only for enhancing
sensory taste, but also for its technological functions. Previous studies have been
conducted to examine the effect of NaCl on wheat flour dough properties and the
properties of gluten protein fractions and the amount of solubilised or aggregated gluten
proteins as affected by salts. However, it is not clear whether the rheological properties
and structural changes of the gluten in the presence of NaCl are due to the contribution
of either sodium or chloride ions. Therefore, a basic understanding of how salt
influences the formation of gluten network and the rheological behaviour of the dough
is needed. This can lead to developing ways to maximise the technological functionality
of these salts in replacing NaCl.
Accordingly, this thesis is based on the need to establish a knowledge and
understanding of the function of salts on gluten structural network during hydration at
the molecular level. Rheological, microstructural, and chemical properties of the gluten
as a function of salt have been investigated. In addition, the effects of salts belonging to
the Hofmeister series on gluten and wheat flour dough have been evaluated to enhance
the understanding the functionality of gluten as it is controlled by salts.
The effect of salt particularly NaCl on the structure and rheological properties of gluten
were investigated by obtaining gluten samples from two wheat flours, with different
levels of total protein, in the presence or absence of sodium chloride (2% flour base).
The dynamic oscillation rheology, large extensional deformation, confocal laser
scanning microscopy, transmission electron microscopy and chemical analysis of
disulfide bond linkages and the ratio of polymeric glutenins and monomeric gliadins
were used to investigate the effect of salt on the structure and rheological properties of
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gluten. The effect of NaCl on the gluten structure and rheology has shown that NaCl
increased the non-covalent interactions of the gluten and the β-sheet structure which
results in different molecular conformation, fibrous network structure, hence differences
in rheological properties. On the basis of these results, it is proposed that NaCl causes
conformational changes as water molecules are drawn away from the gluten to interact
with sodium and chloride ions. The hydrogen bonding and hydrophobic interactions
could be the reason for the increase in the β-sheet structure within gluten. This proposed
mechanism results in the formation of the gluten with a typically more closely aligned
structure.
The rheological properties of the gluten with and without NaCl during heating have also
been investigated. Both the rehydrated and fresh gluten samples were prepared in the
presence and absence of NaCl during mixing and washing. In addition, the starch was
added back to the gluten samples with and without NaCl to determine the effect of
residual starch in the gluten network as influenced by NaCl. Gluten network formed in
the presence of NaCl determine its rheological properties during heating. Changes in the
gradual decrease in G’ and G’’ values up to certain temperature and the onset of the
sharp increase in those values during heating are the results of the extent of hydrogen
bond formation as a function of NaCl. The delay of the sharp increase in the G’ to
higher temperature during heating is the result of the formation of the gluten network in
the presence of NaCl, rather thanvii of the presence of the residual starch in the gluten
network.
Different cation salts were used to investigate the effects of the Hofmesiter salt series on
gluten network formation. The work was carried out by comparing the effects of cation
salts on both wheat flour dough mixing properties, as well as the rheological, and the
chemical properties of the gluten extracted from the dough with the respective salts. The
effects of different cations on dough and gluten different flours generally followed the
Hofmeister series. Despite the differences observed in mixing properties and
microstructure of dough with different cation salts, the impacts of cations on gluten
structure and dough rheology at large deformation at the levels tested were relatively
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small. Among the different cations, the K+ gave a similar microstructural resemblance
and effect on large deformation rheological properties of both dough and gluten samples
including the extensibility, resistance to extension, and the strain hardening behaviour to
that of NaCl.
Different ani