Structural characterisation and sorption capability of whey protein aerogels obtained by freeze-drying or supercritical drying

Whey protein isolate (WPI) hydrogels were converted into aerogels by freeze-drying (FD) or supercritical drying (SCD). SCD resulted in denser aerogels, with a network of closely-associated WPI microgel aggregates, and pore size <1 μm in diameter. In the FD aerogels, individual aggregates were fused and formed a self-supporting protein matrix with larger pores (2–5 μm in diameter). Both aerogels showed a glass transition at 161 ± 4 °C and maintained their original porosity at room temperature at an equilibrium relative humidity <80%. The kinetics of both water and oil uptake were slower in the SCD aerogel than in the FD aerogel. While water absorption caused aerogel swelling and destructuring, the oil-loaded aerogels retained their integrity and adsorbed up to 75% (w/w) oil. The oil-loaded aerogels showed differences in firmness and oil holding capacity (OHC) depending on the drying procedure applied: SCD samples presented a higher firmness (~30 N) and OHC (~96%) than the FD samples (~19 N, OHC ~ 45%). This study demonstrates that the drying technique selected can substantially alter the functional attributes of whey protein aerogels.