Cereal Proteins. Interfacial and Physicochemical Properties

Abstract: The main theme of this thesis is the interfacial properties of cereal proteins from wheat and rye. Some aspects of dough bulk properties are also included. The proteins studied are of interest for the stabilisation of gas cells and the mechanism of gas retention, and the rheological properties of a bread dough. The interfacial properties were investigated by using methods such as film balance, ellipsometry and surface force apparatus. Comparison of protein fractions of wheat (gliadin) and rye (secalin) at the air/water interface showed a higher surface activity for the secalins, probably due to their lower molecular weight. When secalin or gliadin was spread at the liquid/air interface with pH 3.7 or 4.5, a lower surface pressure compared to water was observed. This means that interfacial properties of the secalin will be affected in a sour dough process, and the composition of the interfacial film will probably be related to the pH. Investigation of the interaction between gliadin and L-a-phosphatidyl-choline-dimyrostoyl (DMPC) at the air/water interface indicated that no molecular interaction occurred in the mixed film. Gliadin was spread from both solid and solution, as well as from a mixture with DMPC. Sequential adsorption of a water soluble fraction (WSF) and a gliadin fraction onto methylated silica was studied by ellipsometry, showing that these fractions adsorbed readily and in high amounts to hydrophobic surfaces. Pre-adsorbed WSF seems to prevent further adsorption of gliadins. Pre-adsorbed gliadins, on the other hand, can under certain conditions adsorb WSF. Increased pH increases the adsorbed amount, especially for the addition of WSF to pre-adsorbed gliadin, which indicates that adsorption of wheat proteins will differ in doughs with different pH. Interaction between both Langmuir-Blodgett deposited and adsorbed layers of a-gliadin was investigated with the surface force technique. The deposited layer (measured in 1.0 M NaCl) gave rise to a long range electrostatic repulsion followed by steric forces at short range and the adsorbed layer (1.0 M NaCl) resulted in a much more undefined long range repulsive force, probably due to steric forces. The measurements indicated a layer thickness of 65 Å for the deposited layer. The bulk properties were studied by using a fundamental rheological method (viscosimetry) and differential scanning calorimetry (DSC). The flow properties of alkali-acid treated gluten dispersions, studied by viscosimetry, showed at low shear rates decreased viscosity with increasing concentration of NaOH and HCl. An increase in viscosity was also observed with increasing protein concentration of the dispersions. The flow properties of the gluten dispersions did almost not change upon addition of surfactants. The thermal behaviour of rye flour-milling streams was studied with DSC, showing that the gelatinisation temperature of rye flour-water mixtures was strongly related to the protein content of the flour. This means that a higher protein content could gain the loaf volume, by delaying the moment of gelatinisation of starch.