Protein separation and characterisation are crucial challenges faced by industrial biologists and engineers working with proteins. The key aim is to extract the valuable protein from a broth containing a mixture of proteins in the most efficient and economic way. In this work salt precipitation (crystallisation) has been investigated as a purification technique for a binary protein mixture. For the bulk recovery of a valuable protein, a thorough understanding of the interactive effect of another protein on the solubility of the desired protein in a mixture is essential for successful crystallisation.

Tetragonal crystals of lysozyme were used to estimate the solubilities in the pH range 4.0 to 5.4 in salt concentrations from 0.2 to 0.5 M at 20^oC. To estimate the effect of other protein, lysozyme solubilities were also measured in presence of ovalbumin. A steep drop in solubility was observed in presence of ovalbumin in pH range 4 to 4.5 at low salt concentration (< 0.3 M). Formation of ovalbumin-lysozyme complexes were also observed at pH 4 in low salt concentration. Minimum effect on lysozyme solubility was observed at high pH (> 5.0) and salt concentration (> 0.3 M). To estimate the interaction between lysozyme and ovalbumin in salt solutions cross interaction chromatography (CIC) was used. Strong cross interaction was observed at low salt concentration (< 0.3 M). This work shows the effect of ovalbumin on lysozyme solubility and correlates the second virial cross coefficient (B₂₃) with the solubility of lysozyme in a binary mixture.