The effect of pH on the precipitation of soy protein between pH 4 and 6.8 was investigated. Precipitation between pH 5.7 and 6.8 gave a binary liquid-liquid separation in which a protein-rich secondary liquid phase containing between 20 - 30 wt % protein of 80 - 99% purity in glycinin formed as droplets of 1 - 10 mu m. The droplets could be coalesced upon centrifugation to form a homogeneous bottom liquid layer. The high-protein concentration phase was stable (did not precipitate or crystallize) for up to several months when stored at 4 degrees C. Stable high-protein liquid phases have not previously been reported. In the pH region for which liquid-liquid separation occurred (pH 6.15 to 5.35), the droplet-size distribution was controlled by nucleation and growth. No drop breakage, and little or no drop coalescence was observed. Mixer speed and geometry did not have a large effect on the size of the protein-rich droplets in suspension. Nucleation occurred in two separate bursts at pH 6.15 and pH 5.85, followed by growth of the new nuclei as pH was decreased. Thus, the drop-size distribution was monomodal between 6.15 and 5.85, and bimodal between pH 5.85 and 5.35. At pH 5.6 and below, protein precipitated as amorphous material which intertwined with and bound primary particles into aggregates. These agglomerates grew with decreasing pH, particularly below pH 5.35, as more protein precipitated with amorphous material acting as a binder. The addition of NaCl prevented the liquid-liquid separation at higher pH. Instead solid precipitates of a different morphology were formed at low-pH only. (c) 2007 American Institute of Chemical Engineers.