Interactions of cellulose cryogels and aerogels with water and oil: Structure-function relationships

Food-grade porous materials, aerogels and so-called cryogels, were prepared from cellulose hydrogels obtained from solutions at increasing cellulose concentration (3, 4, 5%, w/w) by supercritical-CO2-drying (SCD) and freeze-drying (FD), respectively. The structure depended on the applied drying technique, with aerogels showing a denser network with pores <200 nm in diameter, a specific surface area of 370-380 m2g-1, and a porosity of 92-94%. Cryogels presented larger pores (2-5 mu m diameter), much lower specific surface area (around 30 m2g-1), and higher porosity (95-96%). Water vapor adsorption by aerogels and cryogels was higher than that of neat microcrystalline cellulose. The absorption of water and oil was investigated as a function of time and at equilibrium. While water was almost immediately absorbed by both aerogels and cryogels, a much longer time was needed to reach oil absorption equilibrium. Moreover, aerogels required a longer absorption time than cryogels. Material morphology governed the kinetics of absorption; the absorption at equilibrium was directly dependent on material pore volume rather than on its morphology or material-fluid affinity. As a result, due to their lower pore volume, aerogels absorbed a lower amount of water or oil (4-8 gfluid/gdry matter) than cryogels (8-12 gfluid/ gdry matter). All samples showed high fluid holding capacity (>96%). Water absorption caused a firmness decrease, but the firmness of oil-filled materials was the same as that of the unloaded ones. This study demonstrates that food-grade cellulose aerogels and cryogels can be structurally designed by varying cellulose concentration and drying techniques to obtain controlled food fluid loading.