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3.1.4 Dehydration 101: Definitions


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

3.1.4 Dehydration 101: Definitions

(Hopefully Mr. Webster will forgive the following abuses)

Batch Drying: Of the three ways to use the Tray Dryer, Batch Drying is
simplest, and least commonly used. Batch drying refers to the loading 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 dyer. While some react
well to this procedure, most don't The loss in the even and consistent
dehydration motivates most operators to investigate other protocols. The
problem with batch drying is in the 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 edge of the trailing
car, significant differences in moisture content will occur with in the
product. It is like drying the same product in two different dryers, each
dryer 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.

Caramelizing: 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 and 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 relative 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: I have seen fire hoses used to wet and re-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 forever been 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
fresh air, mixes in the return air and exhausts the saturated air. The fan
bulkhead separates the Cool End from the Hot End.

Counterflow: Counterflow refers to the direction of the air flow 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 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 new row of wet
cars enter 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, so it can be stored without refrigeration, remain free of
microbial action and can be re-hydrated to nearly the original form,
appearance, taste and nutritive value.

Drying Personality Just as people are unique, so are the 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 unique
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: temperature, air velocity, relative humidity
and dwell time. Constant monitoring and timely reaction to changing
conditions in the product and/or in the environment will insure quality

Hot End: The Hot End or high pressure end begins at the fan wall and
extends across the air deck down through the air deck gap and extends 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

Parallel Air Flow: Parallel air flow 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 air flow is used when production
requirements out weigh 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 the first car at a
specifically timed interval. The cooling action of 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

Stewing: Just like 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 the tray is driven by drying
personality and production considerations. To achieve even drying the tray
loading must be consistent and uniform. Heavy on one side and light on the
other will result in the heavy side not drying, and the light side over
drying. Often seen where trays belly in the center.


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