The transformation of liquid food product under heat treatment is often represented by considering average temperature evolution along the exchanger and by assuming plug-flow. Our aim is to demonstrate that thermal denaturation-aggregation of whey proteins can be more realistically represented by taking into account the different dynamical and thermal histories associated with fluid parcels which progress more or less quickly, far or close to the heating wall, inside the processing unit. A numerical approach is proposed for evaluating the thermal denaturation-aggregation of whey proteins, combining computational fluid dynamics and the population balance equation. The approach is illustrated by the evolution of a suspension of beta-lactoglobulin. Fluid flow and heat transfer are solved through the finite element method in the Eulerian frame, while product transformation is evaluated along representative Lagrangian trajectories. Outlet bulk results show that the product reaches a higher level of transformation than in assuming plug-flow and radially-independent temperature.