Janus-like napins for switchable protein foam stability

Protein foams occupy two extremes for industrial relevance: they must exhibit long stability in applications such as cappuccino, yet become highly undesirable during processes like protein extraction, where persistent foams hinder efficiency. Ideally, foam stability should be a tuneable property of protein systems. Such control becomes possible when the delicate balance of interprotein attractive and repulsive forces can be precisely modulated. Rapeseed proteins, napins, are suitable proteins to prove this, due to their defined amphiphilic Janus-like nanostructure and their, for a protein, low zeta-potential of maximum −15 mV that can be varied through the pH of the environment. Through ellipsometry, atomic force microscopy and interfacial shear rheology it was shown that napins at 0 mV form viscoelastic monolayers on the interfacial surface. Increasing their zeta-potential to −15 mV enhanced the repulsion between the protein particles and created a less dense monolayer, resulting in a liquid-like surface structure. Thin film balance analyses showed that the critical film pressure of uncharged napins was twice as high compared to when their surface charge was −15 mV. Due to this difference in the critical pressure, coalescence caused a rapid foam collapse at −15 mV, while at 0 mV napin foams were highly stable, comparable to egg protein foams. We demonstrated that the foaming properties of particle-like proteins, such as napins, are highly sensitive to electrostatic repulsion, unlocking broad opportunities to leverage these proteins in advanced industrial and materials systems.