Whey protein-based aerogels: Structural insights, zinc-carrier properties, and zinc bioavailability in Caco-2 cells

Due to their large surface area and modifiable structures, aerogels derived from biopolymers like whey protein isolate (WPI) are gaining attention as micronutrient carriers. However, their ability to incorporate essential trace elements like zinc, and how these ions affect aerogel structure and zinc bioavailability, remain poorly understood. This study aimed at characterizing the influence of zinc ions on the structural and morphological properties of WPI-based aerogels along with their ability for zinc delivery. Initially, hydrogels were prepared with WPI and zinc chloride at pH 1 and pH 3 and compared with formulations containing calcium chloride or sodium chloride. The hydrogels were then converted into aerogels via solvent exchange and supercritical drying. Analytical characterization included liquid chromatography-mass spectrometry for protein component profiling, flame atomic absorption spectrometry for metal retention, Fourier-transform infrared spectroscopy for secondary structure analysis, Brunauer-Emmett-Teller surface area measurements, scanning electron microscopy for morphological assessment, digestibility and zinc uptake in Caco-2 enterocytes via Zinpyr-1 fluorescence. Results showed a relative increase of β-lactoglobulin and a substantial ion loss during the hydrogel-to-alcogel transition – up to 70 % for calcium and 52 % for zinc – mainly due to solvent exchange, which affects protein-ion interactions. Fourier-transform infrared spectroscopy revealed increased intermolecular β-sheet formation and hydrogen bonding in the presence of divalent ions, suggesting enhanced protein-metal coordination. At pH 1, the aerogels exhibited pronounced porosity, a large surface area and enhanced enzymatic digestibility. In contrast, aerogels formed at pH 3 appeared denser, less digestible and retained higher amounts of metal ions. Zinc release from undigested aerogels was limited, likely due to matrix binding, whereas trypsin digestion slightly increased the zinc but without clear evidence for increased bioavailability compared to inorganic zinc. These findings show that pH and ion type influence the structure and function of WPI aerogels, highlighting their potential for targeted, digestible metal ion delivery. • Aerogels prepared at pH 3 retain more zinc and calcium ions than at pH 1 • Divalent ions promote intermolecular β-sheet and hydrogen bonds in the proteins • Aerogels formed at pH 1 display higher porosity, whereas those at pH 3 are denser • Increased density of aerogels prepared at pH 3 reduces susceptibility to digestion • Zinc uptake in Caco-2 cells is similar from aerogel carrier and inorganic ZnCl 2