Dehydration 101

Dehydration 101

A History of Dehydration – Why do we dry food?

In its simplest form, dehydration technology is thousands of years old. Many early civilizations dried their meat on a stick, or smoked Jerky to preserve it. They also dried corn (aka maize) in the sun.

In the early 1900’s, following the industrial revolution, the need for technology to accelerate dehydration became acute. This triggered the invention of the “Natural Draft” dehydrator. This crude 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. It instituted the use of wood frame trays and the use of artificial heat. Unfortunately over time, most of these Natural Draft dryers burned down, and there are no known surviving examples of them in existence today.

Next came a series of ever improving dryers that incorporated small fans. Between 1910 and 1920, Mr. L.N. Miller of Miller Dehydrating Co (now Commercial Dehydrator Systems, Inc.) invented a box-like dryer, with artificial heat produced by oil, and later electricity, using 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 (UC Davis) developed the now common Overhead Return “Tunnel Dryer”. Variations of this design are still in use throughout the USA and overseas. Commercial Dehydrator Systems, Inc. proudly carries on the tradition of L. N. Miller’s dryers and the technology from UC Davis, keeping dehydration alive into the centuries to come.

 

Four Phases of Dehydration – Knowing where you are at in the dry cycle

FIRST PHASE: Raising the Core Temperature – In this first phase, the product is being warmed as fast as possible, to within 10 to 20 degrees of the process air temperature. In this phase we want to be careful not to apply too much heat too quickly. Some humidity will build up during this phase, but it is not yet releasing from inside the product. So there is danger of cooking, stewing, or case hardening if heated to fast.

SECOND PHASE: Rapid Dehydration – In the second phase the product is now releasing moisture content at a near free fall rate. This is where the majority of the moisture comes out readily. To maximize production, humidity inside the dryer during this phase needs to be controlled. As a general rule – the humidity of the process air inside the chamber at the hot end needs to be kept at between 12 to 18% to encourage dehydration but still protect the outside layer. As the air passes through the dryer the relative humidity at the cool end will gather, and can be as high as between 35 to 50% if a lot of moisture is coming off the product. However, each product is different and should be treated as such.

THIRD PHASE: Transition – The third phase, Transition is the most critical phase to watch, in regards to possible damage to the product. The high rate of moisture release during the second phase has slowed down to a crawl in phase three. Most of the water in the product is now 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 is less readily available. Case hardening, cooking, and even caramelization are all possible dangers 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 is the longest, slowest part of the process typically, and depending upon the target moisture content, may be well over half of the dwell time. Every 1% of moisture removed now can take exponentially longer to eke out of the product during this final phase. Recall also, that caramelization can still happen during this final phase. Keep an eye on your product, test it often as it gets near to done, and document your finished dwell times, so that you do not overdry or damage the product at the end.

 
Over the years, we have found some great articles for reference. Here we have published the saved list to help you and your team grow in your knowledge of dehydration. Many sources below you may have to search online. If you have any trouble, please contact your salesperson. We saved PDF versions and links that we can send your way!

Bear in mind, that when it comes to dehydration, there is no age. Research on this topic comes in and out of fashion every few decades. All will offer something of value to you as you dig deeper – no matter how long ago it was published!

While our understanding and application of dehydration processes has evolved over the years – some info reference may be out of date. SO if there is a question, such as some of the pre-treatment methods or lethality steps – then please do verify within your industry before putting to use. But when it comes to the basic laws of science and dehydration, to our knowledge those have not changed.

Written resources by Darren A. Traub over the years, found at Process Heating. Including topics on Dehydration, Dryers, Industrial Thermal Drying, Drying requirements, and more here: www.processheating.com/authors/2140-darren-traub – or see the following articles direct:

The Drying Curve – www.process-heating.com/articles/86586-the-drying-curve-part-1

The Psychrometric Chart – www.process-heating.com/articles/83740-the-psychrometric-chart-part-1

Big Air – www.process-heating.com/articles/86609-big-air-part-1-the-psychrometric-chart-unraveled

So Goes the Flow – www.process-heating.com/articles/86672-so-goes-the-flow-part-1

The Engineering ToolBox is also a great resource of articles, including:
Dry Bulb, Wet Bulb and Dew Point Temperatures. Engineering ToolBox, (2004).
www.engineeringtoolbox.com/dry-wet-bulb-dew-point-air-d_682.html

Drying Heat-Sensitive Products – October 13, 2000 by Bill Butler Clark, P.E., Wenger
www.process-heating.com/articles/86725-drying-heat-sensitive-products

 
 
 
 

Tips for Drying Fruits & Vegetables

  • By Commercial Dehydrators
  • 06 Jul, 2018
The following guidelines for drying fruits and vegetables are provided as a simple way to start out in dehydration. There will be hundreds of ways to prepare your own product and make it unique! Test and record your results to improve as you grow in skill.

Bannanas

Bananas make a great snack. They are portable and have a creamy texture that hits the spot. Dehydrating bananas is great way to preserve them year round. Banana chips make a sweet treat that can be packed in lunch boxes, added to trail mix, or just eaten plain.

For a sweet snack, choose ripe bananas. Ripe fruit will have a higher sugar content, and be full in flavor. Here, we want to walk the line between ripe and not browned or mushy. Save those for your banana bread instead!

Peel and slice your banana into approximately equal sized coins in thickness. Lay out a single layer (no overlapping) onto your drying tray for the most even finished results. Where cut fruit overlaps, moisture will be trapped and pieces can become stuck together. Making it harder to release.

With all fruit, be careful not to set your dehydrator temperature too high, as we want to dry the fruit – not cook, stew, or burn it. Remember that yummy sugar content also makes the product subject to possible caramelization (sugars burnt) in the process. We want the product to be able to dry to a nice chewy snack or crunchy chip, if we dry it longer.

Blueberries

Blueberries are useful in a wide variety of snacks and recipes. They are available to the consumer as fresh, frozen, and dried products. 

Due to the wide variety of uses, your target moisture content may be different:

– When used to flavor Ice Cream or  Jellies and Jams – the target Moisture Content (MC) can be as much as 50%. 

– For trail mixes – the target MC will be in the 15% to 20% range. 

– When used in pancake/muffin mixes (where the dried fruit and flour mix are not separate), the MC must be at 5% to 7% to prevent clumping. 

To maximize product quality, and minimize loss, many choose to sugar infuse Blueberries prior to dehydration. This packs them full of flavor.

NOTE: In most cases Blueberries are harvested for fresh consumption, allowing for final ripening during freight, distribution, and on the grocery shelf. Be aware that frozen Blueberries may not have the fresh ripeness, they may be tart or bitter (fully ripe fruit is best suited for dehydration).

For small and medium volume production, tray drying Blueberries is the most cost effective method. We generally recommend the process temperature to be at or around 165°(F), with at least 500 feet per minute of airflow, and Relative Humidity (RH) of 17% to 19% during the drying cycle. The most popular methods for tray drying Blueberries include Counter-Flow, and Batch-to-Dry method. Parallel-Flow is problematic with case hardening as the primary issue, and Classic Batch results in uneven product moisture content from rack to rack.

Sizing the fruit prior to dehydration will greatly improve the consistency and overall quality of results.

Please remember your Dryer is not a “magic machine“; and just like computers: garbage in = garbage out. High quality dried fruit must start out as high quality fresh fruit, harvested at the peak of ripeness. Yes, we can augment the sugar content, and extend the availability of the fruit by freezing, but ultimately quality begets quality, and quality sells!

One last note: If you dry frozen fruit, and then attempt fresh fruit, you will notice that the fresh fruit takes longer to dry. The shortened dwell time for the frozen product, is due to break-up of the skin and access into the fruit. When cells in the fruit freeze, the cell membranes burst, allowing “free water” to escape at a much faster rate than fresh fruit will allow.

The very last note: Blueberries are typically dried whole, with the skin intact. However, some progressive operators are now passing the fresh fruit through a set of rollers, set just tight enough to split the skin (not crush the fruit), and thus are achieving significantly reduced dwell times. The fruit must be sized prior to this operation.

Limes

Dehydrated lime is made by drying matured fresh limes. The color of dehydrated lime varies from yellow to black depending on the drying method. Dehydrated lime is used in cooking and also crushed and mixed with water for a refreshing drink with very high Vitamin C content.

Onions

When you first hear the term dehydrated onion, you would probably wonder, who in the world would want to use a dehydrated onion? For one thing, whenever an onion is required in a recipe, what you always do is take a fresh onion and chop away. This is also what your mother did, as well as your grandmother. There is nobody you know who uses dehydrated onions in cooking. Furthermore, you do not even know what a dehydrated onion looks like… or so you think.

Practically everybody has eaten something with dehydrated onion in it—maybe as often as everyday, for some people. Unless you are a vegetarian, I’m sure you have probably eaten a McDonald’s hamburger at least once in your life. Tiny pieces of dehydrated onions are in this burger mix that America consumes by the millions each year.

Of course, chances are you have never seen what these dehydrated onions look like outside of the burger. And even if you do know what they look like, you probably would not want to eat it or use it in your cooking. They are not very aesthetically appealing, although they are a quite useful ingredient in many fast foods and instant foods.

 

Speaking of instant foods, dehydrated onions are very much in demand by manufacturers of instant soups. Dehydrated soups are very popular among young people today primarily because of their convenience.

All onions for processing are grown from specific varieties best suited for dehydration. Specific strains of the Creole Onion, Southport Globe Onion, and the Hybrid Southport Globe were developed by the dehydration industry. They are white in color and process a higher solid content which yields more flavorful and pungent onion.

Onion dehydration involves the use of a continuous operation, belt conveyor using fairly low temperature hot air. The heat originally was generated from steam coils, but now natural gas is more popular. Typical processing plants will handle thousands of pounds (or kilograms) of raw product per hour (single line), reducing the moisture from around 80% to less than 5% finished.

Papayas

Papaya is becoming increasingly a popular fruit on world markets.

Papaya is full of Vitamin A and makes an unusual fruity alternative to apricots and apples. Dehydrate your papaya for a healthy treat you can take along anywhere.

It can be ground to a powder and sold to food processors world wide, as an ingredient in juice drinks, for example.

Prunes

History: The commercial dehydration of Prunes has evolved from the early days at turn of the twentieth century – when drying small amounts of fruit on wooden trays, laid out in the sun was common. Today’s modern high tech and high volume systems are a far cry from those humble beginnings.

The first attempts at building a commercial Prune dryer (not requiring a sunny day), centered on the “Natural Draft” dryer. This design required a small fire be built at the bottom of an inclined shaft, and trays loaded with fresh fruit were placed in the flue where the hot gases and smoke passed up through the trays. The natural tendency of the warm air to rise provided the air velocity, and the heat from the fire raised the core temperature of the fruit, which helped to accelerate the drying process. Even at this early date, the operators knew enough to move the trays ever closer to the fire as the product dried, to prevent case hardening. From 1910-1920 this was “state of the art” dehydration technology.

Prunes: Early on, we referred to the fruit on the tree as Plums, and when it was dried, we called this wrinkled, oversized raisin a Prune. Today there are many varieties – from baseball sized fruit to the tiny “Breakfast fruit,” all requiring dehydration to stabilize the fruit that will suffer degradation within hours of picking. Dehydration allows for the ambient air storage of Prunes for months, if not years.

Prunes are dried whole, normally with a quick water rinse, to remove dust and rudimentary debris removal helps keep the dried product at high standards. The skin of the fruit is remains intact and the pit remains inside the fruit. While most processors dry the fruit without sizing (field run), there is a move to pre-size the fruit into two or more sizes. Drying each size separately. Thus improving the quality and uniformity of the dried product, while cutting the time and expense in the operator. This effort insures the small fruit are not over dried and lost, while the large fruit are dried to standard and without re-drying or equilibration (mixing the dried fruit so as to achieve desired moisture content) needed.

Tray drying is the norm for commercial and industrial level processors, but drying on continuous flow belt dryers will slowly replace the tray dryers in the future due to the high labor costs and the continuing costs of wood trays.

When tray drying prunes, there are two procedures currently in use; Counter-Flow and Parallel-Flow for large production applications. The only known application of Parallel-Flow dehydration is in the California Prune Industry. While Parallel-Flow is acknowledged to be faster, it engenders quality issues that have limited the viability of the fruit and customer satisfaction of the end product. The Counter-Flow method takes more time in the dryer, but generally turns out a product that is less affected by case hardening, is dry uniformly from pit to skin, and is softer to the touch. For smaller volume applications, Prunes can be dried in Batch, Batch to Dry, and Batch Rotation methods.

In all but the Parallel-Flow method, the process air temperature should be 165°F, with a Relative Humidity (RH) measured at the hot end at about 17% to 19%. Fruit on the trays should be 1 to 1½ fruit deep on standard wooden trays. The fruit needs to be placed on the tray uniformly. Dwell times vary based on fruit size and sugar content, but will generally range from 20-24 hours. The fruit is removed only when it is at the correct moisture content. In the Parallel-Flow process air temperatures (PAT) run from 185-190 °F, and dwell times are mandated by a time schedule required due to the fruits propensity to case harden. Pulling the fruit to a schedule, not when it is at the correct moisture content, requires the re-dry and equilibration procedures.

Raisins

Raisin dehydration in heated air dryers was first devised in the early part of the twentieth century as a means of salvaging rain-damaged raisins. It was also used for a short period to preserve wine grapes during the Prohibition era after ratification of the Eighteenth Amendment. In 1925 the light colored golden bleached raisin was developed, and it required dehydration. Annual production of this new product was about 25,000 tons until World War II. During the war, dehydration was used extensively to ensure a stable supply of product. Many of the earliest dehydrators had no fans and used natural draft to slowly move air past fruit placed on a slotted floor or through fruit placed on trays. In the early 1920s commercial dehydrator firms began marketing a number of types of forced-air dehydrators and these quickly became the industry standard. The main differences between the designs were the location of the air re circulation duct, the type of heating system, and the type of fan, but they operated in the same general fashion as the truck-in-tunnel dehydrators now in use.

Red Peppers

Bell peppers will re-hydrate to double in size in cool water. They can be used in any recipe where raw peppers are required. Drained, they can be sauteed, added to fresh salsa, or salads (although the size of the dices may be too small for a green salad). They do not have the crispness associated with fresh but flavor is there.

Cooked into casseroles, soups, meatloaf, or other main dishes, you will find they are identical to cooked fresh peppers.

Strawberries

Food Dehydration is the oldest form of food preservation. In the beginning, people used a lot of salt to preserve foods, and they dried their food in the sun or on stove tops.
Strawberries are one of the world’s most popular berries. They are most available April through July, but can be enjoyed year round by preserving them. Unlike frozen strawberries, dried strawberries can be stored at room temperature and do not rely on electricity to maintain their quality.