Large deformation properties of β-lactoglobulin gel structures

Abstract

Different gel structures formed by β-lactoglobulin dissolved in distilled water (12% w/w) at pH 3.0–7.5 have been investigated using tensile measurements at large deformations. Gels formed at pH 4–6 were opaque, whereas at pH values below or above this range they were transparent. The fracture properties showed large variations over the pH range studied. Gels formed at low pH were brittle with low strain and stress at fracture, as opposed to those formed at high pH, which were rubber-like with high strain and stress at fracture. Gels formed at intermediate pH (pH 4–6) had an intermediate, near-constant, strain at fracture. The fracture stress was, however, higher at pH 5.5–6.0 than at pH 4.0–5.2. The specific fracture energy resembled the stress at fracture, with a maximum of 6 J/m 2 at pH 6.0. Gels formed at pH 4.5, 5.5, 6.5 and 7.5 were all notch-sensitive. The opaque gels were defined as aggregate gels and the transparent gels were defined as fine-stranded gels. The fracture properties clearly showed there are differences between the fine-stranded gels formed at high pH and those formed at low pH. The fracture stress demonstrated that there are structural differences within the pH range in which the aggregate gels are formed. The non-linearity of the stress-strain curve was the same for all fine-stranded gels, which had r-shaped stress-strain curves. The stress-strain curves of the aggregate gels were almost linear. The non-linearity did not influence the fracture properties. Young's modulus showed two peaks, at pH 3.5 and 6.0, coinciding with the range where the structure changes between aggregated and fine-stranded. The stress at fracture also has a maximum at pH 6.0. The high elasticity and fracture stress may depend on strong, elastic areas in the network structure. Apart from the two peaks, Young's modulus shows the same behaviour as the storage modulus, G′, measured at small deformations, but Young's modulus is slightly larger than 3 G′.