The importance of the mixing environment during precipitation has been examined in two systems. In the first, isoelectric fractionation of the two major proteins of soy was characterized. Two extremes in the speed of acid addition (rapid, with no mixing, and slow, via acid dialysis, with complete mixing) are compared to determine their effects on the properties of the precipitate. Total protein yield, fraction composition, and aggregate microstructure did not depend significantly on the method of acid addition. Particle size distribution and hindered settling behavior did differ and were explained using a model of aggregate strength;In the second system, lysozyme was recovered from egg white by continuous precipitation with polyacrylic acid (MW 4 x 10('6)). Precipitator residence time and shear rate had significant effects on the size distribution of the precipitate, but no clear effects on the composition. Precipitate mean size increased with higher shear, indicating growth phenomena predominating over breakage. Also, an enhancement to the growth rate at small sizes was noted. The Camp number successfully characterized the interaction of shear rate and residence time on the particle size. Resolubilization of the precipitate and recovery of the lysozyme produced high yield;A population balance model has been used to characterize this polyelectrolyte precipitation. The particle size distributions of aggregates formed have been modeled. The models account for aggregate growth (by both shear-driven and Brownian-like collisions), breakage (by hydrodynamic shear or aggregate-aggregate collisions), and birth (by the breakage of larger aggregates into a few daughter fragments). The kinetic constants show dependencies on shear rate and residence time that are not theoretically predicted. The model constants also show a dominance of growth over breakage, supporting qualitative interpretation of the particle size distributions. A mechanism for growth-rate enhancement, caused by polymer extensions from the particle surfaces, produced slightly improved model performance. A collisional breakage mechanism is supported.