Confocal microscopy of heat-induced aggregation and gelation of β-lactoglobulin in presence of non-ionic surfactant

Using confocal laser scanning microscopy, we have followed in time the effect of a non-ionic surfactant Tween 20 on the evolution of the microstructure during the thermal gelation of β-lactoglobulin (β-lg). At pH 6.8, the final gel is fine-stranded, and addition of Tween 20 had no discernible effect on the final microscopic gel structure. However, for surfactant/protein molar ratios between R=0.5 and 2, small aggregates were observed to appear in the protein solution, and they disappeared again upon further heating towards the gel point. This aggregation process was found to be completely reversible if the gel point was not reached. The size and concentration of the aggregates depended on R and on the heating rate. At pH 5.5 with 0.05 M NaCl present, a preliminary study indicated the formation of a coarse β-lg gel on heating, and the degree of inhomogeneity was observed to be greater in the presence of surfactant. These findings demonstrate the potential of confocal microscopy for studying the effect of protein–surfactant interactions on the evolving structure of aggregates during the heat-induced gelation of globular food proteins.