Large Deformation Tests Research Articles

Gelation properties of deamidated soluble wheat proteins

Compared with native wheat gluten proteins, chemically modified deamidated gluten proteins are more readily dispersed in aqueous solution. In this paper the gelation behaviour of commercial deamidated gluten, commonly referred to as soluble wheat protein (SWP) on heating as well as its interaction with calcium and basic proteins was investigated. Soluble wheat protein decreased in viscosity on heating at 60°C due to the breakdown of protein aggregates which was observed by phase contrast microscopy. Heating to 60°C was accompanied by an increase in hydrophobicity without conformational transitions as observed by differential scanning calorimetry. In aqueous solutions at concentrations greater than 17% wlw and after autoclaving at 120°C, SWP produced firm gels which were examined by large deformation testing. In the presence of calcium strong but coarse gels were formed which exhibited syneresis. The addition of 0.1–0.5% wlw of basic lysozyme or clupeine to the highly negatively charged SWP induced gelation at the lower temperature of 80°C which was detected by small deformation rheological testing and microscopy using basic and acidic stains. The structure and mechanism of lysozyme-induced gelation of SWP is different from the high thermal (>100°C) gelation of SWP alone. In the lysozyme-induced gelation non-covalent mainly electrostatic interactions occur between the positively charged lysozyme and the negatively charged soluble wheat proteins with the absence of conformational changes in the SWP. In contrast, in the heat gelation of SWP proteins at 120°C it is proposed that the intra-disulphide bonds are broken leading to conformational changes and subsequent association and gelation. This mechanism is supported by the fact that gelation occurred spontaneously in the presence of urea and β-mercaptoethanol indicating that in SWP the disulphide bonds normally prevent gelation unless exposed to very high heat (120°C) or disulphide reducing reagents.

EFFECTS OF pH ON WHEY PROTEIN CONCENTRATE GEL PROPERTIES: COMPARISONS BETWEEN SMALL DEFORMATION (DYNAMIC) AND LARGE DEFORMATION (FAILURE) TESTING

ABSTRACTThe effects of pH on the properties of gels prepared from commercially available whey protein concentrate were investigated by nondestructive dynamic shear testing with a Bohlin Rheometer and by large deformation compression, tension and penetration tests with an Instron Universal Testing Machine. Dynamic storage modulus, G′, and the slopes of force‐deformation curves from all three types of large deformation test exhibited maxima at or near both pH 4 and pH 7. Electrostatic interactions were the dominant factor in explaining this behaviour. Failure displacements in the large deformation tests generally increased with increase in pH. It is proposed that the formation of disulphide bonds at pH values above 6.5 is the dominant factor in explaining this behaviour. Failure forces in the large deformation tests remained low up to pH 6.5, increased rapidly to a maximum at pH 7–7.5 and then decreased again. The variation in both electrostatic interactions and disulphide bonds with pH is necessary to explain this behaviour. The combination of penetration failure force, penetration failure displacement and penetration rigidity (slope of force‐displacement curve) provided as much qualitative understanding of the variation of gel properties with pH as similar information from any other single test.

The kinematics of double slip with application to cubic crystals in the compression test

A generalization and extension of the kinematic analysis of double slip in fcc and bcc crystals by A.H. S halaby and K.S. H avner (1978) is presented. Particular emphasis is given to equations for areal changes, which are applied to the calculation of loading axis rotations in the compression test. No assumptions are made as to symmetry of slip and/or axis position. Therefore, purely kinematical solutions are obtained which are not limited to the Taylor hardening model and can be used in the interpretation of experimental data from large deformation tests of anisotropic (prestrained) crystals, as well as in theoretical studies.