Quantitative analysis of confocal laser scanning microscopy images of heat-set globular protein gels

Gels of the globular protein β-lactoglobulin were made by heating solutions at pH 7 and different NaCl concentrations (Cs). The influence of the ionic strength on the gel structure was studied by confocal laser scanning microscopy (CLSM). For Cs < 0.2 M the images were homogeneous, but at higher NaCl concentrations micro-phase separation was observed. The protein concentration in the two phases was determined from the images. It is shown how CLSM images can be quantitatively analysed in terms of the pair correlation function of the protein concentration fluctuations, yielding results that can be directly compared to those obtained from light scattering. The transition between so-called finely stranded and particulate gels is explained by a switch from net repulsive to net attractive interaction between growing protein aggregates.