How to use the DRYIT semi-continuous tray dryer
Practical Action
The implications of this difference are in packaging and storage requirements. All foods will
come to equilibrium with their environment. For example very dry foods will absorb water in a
humid climate until the system is balanced and no more can either be absorbed by the food.
Or likewise the air will absorb water from a moist food in a dry climate. This phenomenon is
similar to the movement of water to the surface of a food during drying.
The key factor is whether the moisture content at equilibrium with the environment is greater
than the critical moisture content of the food. Drying will, in most cases, lower the moisture
content of the food to below the critical moisture content. However once dried the food must
be stored and packed under conditions which maintain the moisture content below the
critical moisture content and this depends on the environment. For example in a very humid
climate, foods with low critical moisture contents such as dried herbs need to be packaged in
materials which have very good water vapour resistant properties such as polypropylene.
The rate of drying
Another important concept in drying is the rate of drying. Earlier we talked about the rate of
evaporation and the movement of water from inside the food to the surface where it gets
picked up by the moving air as water vapour. Two basic rates of drying are observed during
the drying process:
• Constant drying rate
• Falling drying rate
Constant drying rate (figure 4)
This takes place in the early stages of drying. At this stage there is a high availability of water
on and very near to the surface and hence it can move easily and quickly to the surface where
it evaporates. More water then moves to the surface. During the early stages it can therefore
be assumed that the surface is constantly wet due to the constant relatively large volumes of
water that are moving to the surface. The rate of evaporation is therefore constant. And
therefore the rate of drying is also constant. See points B - C on the drying curves.
Figure 4: typical drying curve
Figure 5: typical drying rate curve
Falling drying rate (Figure 5)
As the food dries the movement of water to the surface for evaporation decreases. This is
because there is less water and components such as sugars and starches, which release water
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