Protein gel shrinkage during solvent exchange: quantification of gel compaction, mass transfer and compressive strength

Abstract

In this work, we experimentally investigate into one step solvent exchange of cylindrical shaped whey protein isolate (WPI) gels, where the pore fluid water inside the pores is replaced by ethanol. Solvent exchange has three major impacts on macroscopic gel properties, leading to an increase in compressive strength, shrinkage of the gel cylinders diameter and consequently a change in the spatial protein distribution. 1D-Raman spectroscopy was employed for in-situ investigation of pore fluid composition and protein density distribution, together with a conventional camera to track the diameter change of the gel during solvent exchange. Compressive strength of the gel cylinders is measured ex-situ using a universal testing machine. For gels with three different protein concentrations, we report comparable shrinkage of the diameter to about 85% of the initial size, with most shrinkage taking place during the first stages of solvent exchange, when ethanol is detected in the outermost layers only. Further shrinkage is prevented by a sharp increase in mechanical stiffness of the gels. Solvent exchange leads to an inhomogeneous protein density distribution for most gels, with a compaction towards the radial center. We showed a complex interaction of solvent exchange kinetics and pore fluid dependent stiffness of the gel matrix controls the final size and density distribution of the gel.