Dehydration 101:
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Dehydration 101 (A primer for new Tray Dryer
Operators.)
Revised: 2/2/00
OVERVIEW:
“Regardless
of how brilliant the design, or how skilled the fabricators might be, it is the
operators of a Tray Dryer that will make it a success, or failure.”
The following information is offered as a starting point
from which you will be able to jump-start your introduction into the fascinating
technology of dehydration. Throughout this lesson, every effort has been made
to follow the K.I.S. principle (Keep it Simple). The intent is to provide a
basic understanding of dehydration, but without the scientific jargon.
This paper is broken down into the physics of dehydration, recognition of the four phases of dehydration, maximizing production and finally, how to trouble shoot the process when you are having problems.
Dehydration is more an art form than an exact science. As your personal experience grows, don’t be afraid to experiment. There will always be more than one way to dry a specific product. Your challenge is to find that special mix of temperature, air velocity, relative humidity and dwell time that maximizes both production and product quality.
HISTORY:
In its simplest form, dehydration technology is thousands of years old. Dried meat on sticks and corn dried in the sun are two examples of early man’s ingenuity.
After 1900, the need for technology to accelerate
dehydration and remove the dependence on sun dried processing became acute.
These factors triggered the invention of the “Natural Draft” dehydrator. This
design incorporated a fire near the bottom of a hillside. Stacks of wooden
trays were filled with product and placed in racks. An exhaust vent in the upper portion of the roof allowed the smoke
and hot gasses to escape with the water vapors. As the fire heated the air, it was carried upward providing the
critical airflow and low humidity necessary for dehydration.
The Natural Draft Dryer is generally accepted as the first
commercial dryer and instituted the use of wood frame trays and artificial
heat. Unfortunately over time, most burned down and today there are no known
surviving examples.
Ten years later, the Natural Draft Dryer gave way to a mix
of crude dryers that incorporated small fans. Finally, between 1910 and 1920,
Mr. L.N. Miller invented a box-like dryer, with artificial heat produced by
oil, a large fan capable of high air velocity, humidity shutters and bleeder
vents. This was the predominant design through the 1940’s and spawned many
variations.
In the 1960’s, a group of scientists at the University of California at Davis developed the now common overhead return Tunnel Dryer. Variations of this design are now in use throughout the USA and overseas. Commercial Dehydrator Systems, Inc. now carries on the tradition of L. N. Miller’s dryers and the technology from UC Davis, which will keep dehydration alive into the centuries to come.
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 the product, to within 10 to 20 degrees of the
process air temperature. In the counter
flow 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 length of
the Tunnel. This wet air transfers heat very fast and as the car moves forward
in the dryer, the process air temperature rises and 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 (counter flow), parallel flow requires that 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 re-hydration cycle continues throughout the process.
Second
Phase (Rapid
Dehydration) In the second phase the moisture content of the product is in near
free fall. To maximize production, moisture inside the dryer needs to be
controlled. As a rule the 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 the relative humidity at the cool end should be
35% to 50%. Remember, each product is different and should be treated as such.
Third
Phase (Transition) Transition is the most critical phase, in
regards to possible damage to the product. The high rate of moisture release experienced
in the second phase slows down to a crawl. Most of the water in the product is
gone. Capillary action at the cellular 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 caramelization 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 the dwell time. Carmelization is
still a threat in the last phase, as well.
DEFINITIONS:
(Hopefully Mr. Webster will
forgive the following abuses)
Batch Drying: Of the three ways to use the Tray
Dryer, “Batch Drying” is the simplest and least commonly used. Batch drying
refers to the loading of the tray dryer with all of the product-laden trays and
cars at one time and drying the lot, without moving the cars within the dryer.
While some products react well to this procedure, most do not. The loss of the
even and consistent dehydration quality motivates most operators to investigate
other protocols. The problem with batch drying is in the lack of uniformity of
the environment the product is exposed to. Since the leading edge of the
leading car “sees” a much different environment than that of the trailing car,
significant differences in moisture content will occur within the product. It
is like drying the same product in two different dryers, each set at a
different temperature.
Bound water: Water found in most products comes in two forms, free
water and bound water. For our purposes, bound water is locked up or “bound”
with salt, sugars, or proteins, and as such, are not available for use by
bacteria or mold spores for propagation. Bound water is not normally a concern
in dehydration. See “free water”.
Caramelization: Normally associated with fruit and
vegetables with significant sugar content. Caramelizing is simply the burning
of sugars. Caramelizing is normally associated with running the dryer too hot
with too much air velocity. Tearing open a sample and smelling a “Camp fire”
scent is the classic test. For most purposes a caramelized product is ruined,
with no way to salvage it for human consumption.
Case Hardening: Like caramelizing, case hardening
is caused by too much temperature, too much air velocity and too little
humidity. Symptoms include a virtual halt in dehydration and a tough
leather-like outer skin. Increasing the humidity is the key to salvaging the
product. The product can normally be salvaged by massive re-hydration.
** NOTE: Fire hoses have been used to wet and soften the skin in an effort to kick-start dehydration again. Once softened, dehydration begins almost immediately.
Cooked: As with
caramelization above, your product has been forever changed into something
else. (Will not re-hydrate back into the original form.) No amount of
re-hydration will help. The oils and sugars inside the product have changed and
will not keep. The rancidity clock is ticking and refrigerated storage is the
only alternative.
Cool End: The cool end
of the dryer refers to the end that encloses the fresh air inlet, combustion
air inlet, and the return air gap (in the air deck). Sometimes called the low-pressure
end, this part of the dryer brings in fresh air, mixes in the return air and
exhausts the saturated air. The fan bulkhead separates the “Cool End” from the “Hot
End”.
Counter flow: Counter flow refers to the
direction of the airflow within the dryer. The fresh (wet) product laden cars
enter the dryer through the cool (low pressure) end doors and are stepped
forward periodically as the cars loaded with dry product are removed from the
dry (hot) end of the tunnel. When dry cars are removed, an entire row moves
forward, and a new row of wet cars enters the dryer. With each step forward the
product “sees” a new drying environment; always dryer and hotter. Counter flow
dehydration is normally associated with a lower process air temperature and higher
quality dried products. Drying is accomplished from the inside out, and case
hardening is rare.
Dehydration: The process of driving free water from products like
fruits, vegetables and nuts at an accelerated rate without damage to the
product. The purpose of dehydration is to stabilize the product at a low
moisture content at an accelerated rate.
Drying Personality Just as people are unique, so are
many products that can be dried in a tray or tunnel dryer. A carrot will
respond to dehydration in a radically different manor than a prune. This personality
causes the product to respond to dehydration in a unique manner, unlike any
other product. The variables inside the dryer that you have some control over
are: temperatures, air velocity, relative humidity and dwell time. Constant
monitoring and timely reaction to changing conditions in the product and/or the
environment will ensure quality dehydration.
Hot End: The Hot End or high-pressure end begins at the fan
wall and extends the length of the air deck, down through the air deck gap and
back through the first few cars on the ground level. Distinguished by high
static pressure and high process air temperatures, the hot end is where the dry
product exits from the dryer when drying in the “counter flow” configuration.
Parallel Air Flow: Parallel airflow is a drying
system that maximizes production. The “wet” cars enter the dryer from the hot
end. The hot process air passes through the trays in the same direction, as the
cars are moving inside the Tunnel Dryer. Parallel airflow is used when
production requirements outweigh quality concerns. The process air temperatures
are high, sometimes nearly 200 degrees (F). The hot air from the fan reaches
the fresh product first. To counter the potential for case hardening, another
car full of fruit is placed upstream of the first car at a specifically timed
interval. The cooling action of the moisture driven off the upstream car
re-hydrates the original car slightly, thus averting case hardening. The timing
of the introduction of the upstream car is critical, which means the last car
(wet end) comes out of the dryer, whether it is ready or not. This is the cause
of the quality issue. Parallel Flow is an adaptation of the original Counter-Flow
methodology. See Counter-flow air flow.
Stewing: Just as it sounds, the product is not drying,
normally from too much humidity inside the dryer. Add fresh air. The product is
salvageable only when “Stewing” is discovered early. See Cooking.
Tray Loading: The depth of the product on a tray
is driven by drying personality and production considerations. To achieve even
drying, the tray loading must be consistent and uniform. Heavy loading on one
side and light on the other will result in the heavy side not drying, and the
light side over drying. This is often seen where the trays belly in the center.