The aim was to investigate the effect of dynamic thermal treatment in a heat exchanger on the denaturation and aggregation of whey proteins. A 2% w/v WPI solution (pH 7.0), with or without NaCl addition (100 mM), was submitted to heat treatment at 100 degrees C. At low ionic strength, a nearly complete denaturation was observed, while the heat-treated samples at 100 mM ionic strength registered only 10% denaturation. It was shown that not only denaturation was reduced by the presence of NaCl, but also that the size of the aggregates formed during heat treatment was far smaller and remained essentially below 1 mu m. Surface tension measurements showed a faster adsorption and lower sigma values for the non-polymerized proteins, whereas insoluble aggregates played only a poor role on the sigma curves. Finally, experimental results demonstrated the strong interaction between ionic strength and dynamic thermal treatment on protein aggregation and properties.