Dehydration 101—A primer for new Tray Dryer Operators.
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.
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.
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 one half the dwell time. Carmelization is still a threat in the last phase, as well.
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 ° (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.