Investigation of structural changes of β-casein and lysozyme at the gas–liquid interface during foam fractionation

Purification and separation of proteins play a major role in biotechnology. Nowadays, alternatives to multistep operations suffering from low product yields and high costs are investigated closely amidst which one of the promising options is foam fractionation. The molecular behavior at the gas–liquid interface plays an important role in the formation and stabilization of enriched foam. This study for the first time correlates the physico-chemical parameters to the molecular structure in view of protein enrichment during foam fractionation of the two relatively different proteins lysozyme and β-casein employing biophysical techniques such as circular dichroism (CD) spectroscopy and infrared reflection absorption spectroscopy (IRRAS). In case of lysozyme, high enrichment was achieved at pH < pI in contrast to current opinion. This is due to partial unfolding and aggregation of the lysozyme molecules under favorable foaming conditions that resulted with high enrichment of foamed protein. Under these favorable conditions, CD spectra and IRRA spectra show that the unfolding of lysozyme is partially irreversible. However, the unfavorable foaming conditions, giving low enrichment, promote only minor structural changes and these changes are fully reversible. In case of β-casein, no pronounced unfolding can be observed using CD spectroscopy and IRRAS. The β-casein molecules adsorb and purely reorient at the gas–liquid interface, depending on favorable or unfavorable conditions.

► Design of experiments was used to describe influence of factors on foam fractionation and their interaction.
► Samples resulting minima and maxima enrichment were investigated with circular dichroism to detect the changes after foam fractionation.
► Infrared reflection absorption spectroscopy was used to monitor structural changes at the gas–liquid interface during protein adsorption.
► It was shown that physico-chemical conditions have influence on the molecular structure of protein at the gas–liquid interface.