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Supercritical fluid processing of proteins: lysozyme precipitation from aqueous solution

Aqueous solutions of hen egg lysozyme (3% w/v) were dispersed and precipitated by a homogenous mixture of supercritical carbon dioxide–ethanol using the Solution Enhanced Dispersion by Supercritical fluid (SEDS) process. The effects of different working conditions, such as temperature, pressure and the flow rates of the solution and ethanol, on the particle‐formation process were studied. The morphology, particle size and size distribution and biological activity of the protein were determined. The precipitates were examined with high‐sensitivity differential scanning calorimetry (HSDSC) and high‐performance cation‐exchange chromatography. Particle size measurements showed the precipitates to be aggregates with primary particles of size 1–5 μm. The similarity of HSDSC data for unprocessed and processed samples indicated that the different physical forces that stabilise the native form of lysozyme are unchanged after SEDS processing. From FT‐Raman spectroscopic studies secondary structural changes were observed in certain SEDS‐produced lysozyme, with most processed samples displaying a slightly more disordered secondary structure than the unprocessed sample. However, SEDS samples produced at 200 bar and 40 °C exhibited negligible disturbance. Thus the SEDS process utilising aqueous solution was able to bring about size reduction of lysozyme with minimal loss of biological activity.