Net isosteric heats of sorption
The net isosteric heats of sorption for different moisture
contents as influenced by drying method are shown in
Fig. 3. The net isosteric heat of sorption decreased with an
increase in moisture content. A steep slope of the curve is
observed at low moisture content. This is probably due to
the existence of highly active polar sites on the surface of
the food material which are coved with water molecules
forming a mono-molecular layer at low moisture content
(Tsami, 1991). The heat of sorption is higher at low
moisture content and this indicates strong moisture–food
content interaction in the dried product (Shivhare et al.,
2004).
Freeze-dried mushroom shows the highest heat of
sorption at low water content, followed by vacuum-dried
and hot-air dried mushroom. This means that the energy
required for drying mushroom to a safe water activity will
be higher for freeze-dried than for vacuum-dried and hotair
dried mushroom. Similar results were reported for
Shiitake mushroom (Ko et al., 1999) and ginger powder
(Shin et al., 2003). The net isosteric heat of sorption of
water in Inonotus mushroom can be expressed mathematically
as power function of moisture content as follows:
Hotair drying : Qst ¼ 140:003M0:2394 e ; R2 ¼ 0:9810;
(4)
Vacuum drying : Qst ¼ 108:208M0:2024 e ; R2 ¼ 0:9893;
(5)
Freeze drying : Qst ¼ 108:868M0:1973 e ; R2 ¼ 0:9922:
(6)
These mathematical relationships may be used to calculate
the heat of sorption of Inonotus mushroom for various
moisture content depending on the drying method.
4. Conclusion
The EMC of the mushroom increased with water activity
at selected temperatures regardless of drying method used,
following the form of a Type II isotherm. At constant
water activity, the EMC values decreased with an increase
in temperature. The Oswin model described well the
adsorption isotherms of mushroom in the range of
20–50 1C and 0.11–0.93 water activity. The monolayer
moisture content (m0) decreased as temperature increased
and the value for freeze-dried sample was higher than that
of vacuum-dried and hot-air dried sample. The net isosteric
heat of sorption decreased with increase in moisture
content of mushroom and can be well predicted by power
functions of moisture content.