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

Martin P Dirauf,Anja Hajnal,Pavel Gurikov,Andreas S Braeuer

doi:10.1016/j.foodhyd.2021.106916

Martin P Dirauf, Anja Hajnal + Show 2 more

Open Access

https://doi.org/10.1016/j.foodhyd.2021.106916

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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.

Highlights

Biopolymer aerogels are open structured mesoporous solids that feature ultra-low density, high porosity and specific surface area, low heat conductivity, biocompatibility and biodegradability (Fricke & Tillotson, 1997; Soorbaghi et al, 2019)
We analyzed the one step solvent exchange of whey protein isolate (WPI) hydrogels in pure ethanol using a combination of Raman spectroscopy, photography and compressive strength test
Raman spectroscopy revealed the composi tion profiles through the gel, conventional photography accessed the shrinkage of the central diameter of the gel and the compressive strength measurements gave new insights into shrinkage phenomena accompa nying solvent exchange of protein gels

Summary

Introduction

Biopolymer aerogels (bio-aerogels for short) are open structured mesoporous solids that feature ultra-low density, high porosity and specific surface area, low heat conductivity, biocompatibility and biodegradability (Fricke & Tillotson, 1997; Soorbaghi et al, 2019). The emer gence of capillary forces is avoided by removing the pore liquid in a single-fluid phase process For this purpose the pore-liquid water is first exchanged by an organic solvent which at moderate pressures and mild temperatures features complete miscibility with the drying agent carbon dioxide (CO2) (García-Gonzalez, Camino-Rey, Alnaief, Zetzl & Smir nova, 2012). A volumetric shrinkage of more than 90 % has been reported when algi nate hydrogels were exposed to pure ethanol in a single-stage solvent exchange process This is mainly attributed to changing the solvent-matrix interactions when the pore-liquid gets less-polar during solvent exchange (Subrahmanyam, Gurikov, Dieringer, Sun, & Smir nova, 2015). This is usually achieved by a stepwise gradual solvent exchange (García-Gonzalez et al, 2011)

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