Agriculture Reference
In-Depth Information
During microwave drying water vapor flow from the interior of the cossettes is formed,
so this vapor flow leads to the formation of porous (puff) tissue that can later support
rehydration. During microwave drying, there are two opposing effects. The first overheating
of the sample because of the large amount of energy absorbed, while the other is the
formation of porous tissue structure caused by the buildup of moisture extraction from the
pulp interior. So the combination of these two effects produces a minimum in the values of
the sugar beet pulp equilibrium moisture content of.
The rise in microwave power leads to an increase in the rehydration initial rate due to the
formation of porous structure of the tissue fibers during drying at higher powers. On the other
hand, the same effect has the rise in rehydration temperature, i.e., with increasing rehydration
temperature rehydration initial rate increases due to reduced water viscosity at higher
temperatures (Figure 7.). Compared to untreated pulp, lower values of rehydration initial rate
can be explained by the presence of air in the more porous structure of the treated fiber
created during microwave drying.
The equilibrium moisture content increases with rehydration temperature, while the
influence of microwave power is more complex and has the same trend as for untreated
fibers. With increase in microwave power from 150 W to 250 W equilibrium moisture
content decreases. This is probably connected to increase in temperature due to intense
heating of the samples during microwave drying. Further microwave power increase causes
additional equilibrium moisture content rise, due to fact that at this power level increase of
temperature is not so important because high flux of steam from the sample is responsible to
changes in the tissue structure making it more porous.
Figure 8. Effects of rehydration temperature and microwave drying power on equilibrium moisture
content of treated sugar beet pulp.
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