Aggregation, viscosity measurements and direct observation of protein coated latex particles under shear

Abstract

The aggregation under shear, of latex particles coated with whey protein isolate was monitored, in a continuous phase with a complex behaviour in relation to temperature dependence and shear thinning. The monitoring was done with viscosity measurements and microscopy. An aggregating dispersion of whey coated polystyrene latex particles, salt, sucrose and gelatine was sheared in a rheometer at shear rates between 0.05 and 5 s −1. The viscosity was monitored as a function of time during a temperature increase from 30 to 60°C. The viscosity curves were interpreted with the aid of additional information from light microscopy micrographs. The aggregation was clearly visible as an increase in viscosity. Aggregation was observed to initiate at a temperature between 40 and 50°C. Unbound protein, i.e. protein not a part of particle coating, was found to be essential for the aggregation of latex particles. After aggregation, a shear thinning behaviour was detected. This was due to two phenomena: structural changes of the aggregates and shear thinning behaviour of the dispersion. The build-up of the aggregates was followed by direct observation in a confocal laser scanning microscope. A sequence of micrographs was taken, in an unstopped 3-D flow field generated in a four-roll mill, which showed the evolution of the size of the aggregates. The micrographs were in good agreement with the viscosity measurements. This showed that the four-roll mill and a confocal laser scanning microscope is a useful tool for studying aggregation in an undisturbed 3-D flow.