CHAPTER 8

GENERAL DISCUSSION AND CONCLUSIONS

General Discussion
Conclusions
Future work


General Discussion

A model structure has been proposed for gliadin and deamidated gliadin based on the results from chapters 3 and 4. Molecules of gliadin which are not spherical in shape aggregated through hydrogen bonding to form a unit with a diameter of 32nm. Formation of the unit caused exposure of hydrophobic groups making it insoluble in water. This unit then aggregated with other units through hydrophobic interactions to form large aggregates It is being proposed that this might be a mechanism that nature uses to store proteins so that they do not dissolve and diffuse away from their storage site. Deamidation of the gliadin molecule changed the glutamine residues to glutamic acid which caused electrostatic repulsion between the molecules. The hydrogen bonding holding the molecules together in gliadin were replaced by electrostatic repulsion which caused deamidated gliadin to aggregate through hydrophobic interaction exposing carboxyl groups making the deamidated gliadins soluble in water.

The addition of beta-mercaptoethanol to deamidated gliadin caused a reduction of the disulphide bonds with the exposure of cys-299 and cys-307 which subsequently formed disulphide bonds with other deamidated gliadin molecules. In the presence of both beta-mercaptoethanol and urea, the whole molecule unfolded and fibrils of 343nm thickness were formed.

Below 16% (w/v) concentration the viscosity of SWP decreased with an increase in temperature (Figure 3.25). In addition, exposed hydrophobicity showed greater values than surface hydrophobicity (Figure 3.27). The large aggregates of SWP formed on hydration at room temperature (Plates 3.22 and 3.24) were attributed to polymerisation and interactions between the glutenin fraction (Figure 2.9). The reason for the reduction in viscosity on heating was caused by the breakdown of aggregates, which was observed by microscopy (Plates 3.23 and 3.25). Further evidence by differential scanning calorimetry (DSC) indicated that there was an absence of conformational transition during the heating of SWP proteins (Figure 3.28). This stability can be attributed to intra-disulphide bonds in deamidated gliadins and both intra- and inter-disulphide bonds in deamidated glutenins. A reduction of the disulphide bonds by beta-mercaptoethanol promoted the gelation of SWP at room temperature (Figure 3.35) which otherwise would not gel (Figure 3.34).

The addition of 2% NaCl to SWP at 20°C caused a decrease in viscosity (Figure 6.14). This decrease was proposed to be due to the formation of a diffuse double layer and also to an increase in the dielectric constant of the medium.

Small quantities of SWP (1%) caused a major increase in the storage modulus of BSA (Figure 5.17). It is proposed that SWP electrostatically interacted with domain III of BSA which can be saturated. Above the critical value at which saturation of domain III occurs, the elastic modulus decreased because of repulsion between SWP and negatively charged domains I and II.

In the absence of salt, 50% SWP/WPI mixtures were found to have greater apparent viscosities than the individual proteins (Figures 6.16, 6.17 and 6.19). It was proposed that since SWP is a sodium salt of a protein, WPI scavenged part of the sodium ions to induce its own aggregation and the shielding effect of sodium ions also helped to reduce the repulsion between WPI and SWP leading to aggregation. The gels formed in the SWP/WPI system at 95°C are most likely to be phase separated gels made up of viscous SWP sol trapped in WPI gel network.

The addition of small amounts of basic proteins like clupeine or lysozyme (0.1%) led to the gelation of 19% (w/v) SWP aqueous dispersion at 80°C. SWP on its own, normally gels above 105°C. The lysozyme-induced gelation was proposed to be electrostatic in nature involving proteins which did not unfold prior to gelation (Figure 8.3). SWP was found to protect lysozyme from heat induced denaturation and, therefore, could be useful in the preservation of products to which lysozyme has been added as an anti-bacterial agent.

SWP was found to bind iron, copper, zinc and cadmium. The highest affinity was found to be for cadmium.

Conclusions

In any broad, applied area like Food Science and Technology, an integrated approach comprising several techniques is required to investigate complex systems. With this in mind, the problem of solving the mechanisms of interaction of the selected systems was attacked from different angles using different techniques. The findings using this approach are listed as follows:

Future work Use of neutron scattering to study the interaction of SWP and lysozyme under varying temperatures and the gelation of SWP above 100°C.