Inhomogeneous fine-stranded β-lactoglobulin gels

Abstract

Inhomogeneities occur in the network of fine-stranded β-lactoglobulin gels. They have been characterized by electron microscopy and their influence on the rheological properties at small deformations measured. The inhomogeneities occurred as dense and loose regions in the network and their formation depended on the heating rate during gel formation (0.017–12°C/min) at pH 7.5 or on pH if close to where the network structure shifts from fine-stranded to particulate (pH 5.8–6.5). An inhomogeneous microstructure was formed at pH 7.5 with a slow heating rate, whereas the network was homogeneous after fast heating. These homogeneous gels had higher G′ than the inhomogeneous gels. In addition the character, as expressed by the frequency dependence, changed from strong homogeneous gels to weaker, more frequency-dependent inhomogeneous gels. The formation of the inhomogeneities was monitored at the slowest heating rate by measuring G′ during gelation. G′ showed a maximum around 70°C, indicating a separation into polymer-rich and polymer-poor regions in the network. Electron microscopy showed a transient, homogeneous network before the separation. Both types of inhomogeneous gels had broken frequency curves, i.e. G′ f had two slopes, which were caused by different relaxation times in the dense and loose regions. Relaxation measurements of the inhomogeneous gels showed relaxation time spectra with two peaks. A model was used to confirm that a broken frequency curve may be caused by two peaks in the relaxation time spectrum.