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 (aw), independently from the roasting
degree; as the aw increased headspace volatiles rose and
then decreased as the moisture further increased. The
changes in aw 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 aw in glassy systems; while
the latter became important at high aw when, due to increased
mobility, equilibrium conditions were approached.
Modified state diagrams were used to predict
the critical temperature and aw at which structural collapse
and volatile release occurred. As far as equilibrium
condition was achieved, coffee volatiles were partially readsorbed
in the liquid phase.
