Effect of coffee physical structure on volatile release

The mechanisms of volatile release from soluble coffee powders with different roasting degrees were studied. The presence of volatiles in the headspace during coffee humidification was analyzed by gas chromatography. Small amounts of volatiles were observed at low water activities (a_w), independently from the roasting degree; as the a_w increased headspace volatiles rose and then decreased as the moisture further increased. The changes in a_w and volatiles went along with the changes of coffee structure from a free-flowing powder to a sticky viscous fluid. The mechanism of volatiles release was controlled by both kinetic and thermodynamic factors. The former prevailed at low a_w in glassy systems; while the latter became important at high a_w when, due to increased mobility, equilibrium conditions were approached. Modified state diagrams were used to predict the critical temperature and a_w at which structural collapse and volatile release occurred. As far as equilibrium condition was achieved, coffee volatiles were partially re-adsorbed in the liquid phase.