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3.1.3 Dehydration 101: Four Phases Of Hot Air Dehydration




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This article is from the Food Preserving FAQ, by Eric Decker ericnospam@getcomputing.com with numerous contributions by others.

3.1.3 Dehydration 101: Four Phases Of Hot Air Dehydration

First Phase. (Raising the Core Temperature) In the first phase of raising
the core temperature, the product is warmed as fast as possible without
case hardening to within 10 to 20 degrees of the process air temperature.
In the counterflow configuration the wet fruit is placed in the cool end
and is subjected to very wet air that has lost 20 degrees or more by
passing through the Tunnel. This wet air transfers heat very fast and as
the car moves forward in the dryer, the process air temperature rises and
the humidity drops. This accelerates the transition to the second phase. In
the Parallel flow configuration the wet car is placed in the hot end and
the product is immediately subjected to the high temperatures and low
humidity of the high pressure end. Rather than pulling the product when it
is dry (counterflow), parallel flow requires that, at in less than two
hours, another car must be placed in the hot end to prevent the previous
car from case hardening. Thus the wet product drives the dehydration rather
than the dry product. As each car is placed in the high pressure end, a
charge of wet, cool air, bathes all of the cars behind it for a few
minutes. This dehydration and rehydration cycle continues throughout the
process.

Second Phase (Rapid dehydration). In the second phase moisture content of
the product is in near free fall. To maximize production, moisture inside
the dryer needs to be controlled. As a general rule of thumb, moisture
content of the process air, when drying most products, measured at the high
pressure end, should be 17% to 19%. After the air passes through the dryer,
measured at the cool end, the relative humidity should be 35% to 50%.
Remember each product is different and should be treated as such.

Third Phase (Transition). Transition is the critical phase, from the point
of view of damaging the product. The high rate of moisture release
experienced in the second phase slows down to a crawl. Most of the free
water has been driven off. Capillary action at the cell level now provides
the majority of the free water being driven off. The evaporative cooling
that has kept the core temperature of the product well below the process
air temperature, slows as well. Case hardening, cooking and caramelizing
are all very possible as the product passes through the transition phase.

Fourth Phase (bake out). The final phase is characterized by a slow
reduction in the product moisture content. This phase is normally the
longest,and depending upon the target moisture content, may include over
1/2 of the dwell time. Caramelization is still a threat in the last phase,
as well.

 

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