evaporation of water, which is 43 kJ?mol^1 at 40 8 C. Difficulty of removal
of the last bit of water is thus not due to strong ‘‘binding’’ but to very slow
diffusion, as discussed above. Note that the sorption enthalpy has always to
be supplied, whether water is removed while remaining liquid (as in reversed
osmosis), or by evaporation. In the latter case, also the enthalpy of
evaporation has to be supplied.
Because of the finiteDHs, the water activity of a product increases with
increasing temperature (at a given water content), relatively more so for a
higherDHs, which implies a loweraw. This is expressed in the relation of
Clausius–Clapeyron:
dðlnawÞ
dð 1 =TÞ¼
DHs
Rð 8 : 7 ÞFigure 8.5b gives an example of the temperature dependence ofaw. It is seen
that a given water content is reached at a higher water activity for a higher
temperature. This is an additional reason why drying is easier at a high
temperature.
Hygroscopicity. A dry material will take up water from the
surrounding air if the latter has a higheraw(relative humidity) than the
material. The rate of water uptake will be faster for a greater difference
FIGURE8.5 (a) Sorption enthalpyDHsðkJ?mol^1 Þas a function of water activity
aw, for potato starch. Also the range obtained ataw¼0.5 for various materials is
given. (b) Desorption isotherms—w^0 in g water per g dry matter versus water
activity—of (dried) potato at three temperatures (indicated).