DEHYDRATED FOODS

Drying Food

More fruits and vegetables are preserved by drying than by any other methods. Grains, beans, peas, nuts, fruits, all those foods are preserved by drying. Certain grains and fruits depend on the warm wind to dry naturally in the fields, others are commercially dried in machine and the process of artificial drying has taken the meaning of “dehydration."

Evidence shows that vegetables and fruits were dried from the earliest times. The Romans were particularly fond of any dried fruit they could make; American Indians used the heat from fire to dry foods. In the Middle Ages purposely built “still houses” were created to dry fruits, vegetables and herbs in areas that did not have enough strong sunlight for drying. A fire was used to create the heat needed to dry foods and in some cases smoking them as well. Meat was dried in such houses by burning small fire, and the product was dried with “smoke” what gave the meaning to “cold smoking.”

Artificial drying (dehydrating) took off in about 1795 when French inventors Chalet and Mason designed the first hot air food dehydrator. A short time later in 1810, another Frenchman, Nicholas Appert received from Napoleon Bonaparte the reward of 12,000 francs for inventing canning process. Those two new technologies, dehydrating and canning, started a new era in food preservation. Although scientists of the day were not able to explain those new technologies, nevertheless the processes worked well and not the nature, but the man was finally in full control.

The water content of food is usually very high, typically 80% to 95% for various fruits and vegetables and 50% to 75% for various meats. The water content of thoroughly dried foods is around 10-20%. There are three types of drying:

Sun drying

Solar drying

Dehydration

Freeze drying.

Freeze drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate (evaporate) directly from the solid phase to the gas phase. Freeze-drying does not usually cause shrinkage or toughening of the material and flavors, smells and nutritional content generally remain unchanged, making the process popular for preserving food. This is a purely commercial method requiring substantial investment in equipment.

Principles of Food Drying

Drying can be defined as removing water from the food to air. In solar drying the air escapes to the atmosphere, in a dehydrator, the moist air is sucked out of the dryer by a fan. Water in food must be heated before it evaporates and that requires energy. In solar drying the sun provides the heat, in a dehydrator the electricity heats up the air and the fan moves it around.

As the temperature rises, the vapour pressure of the water increases until water molecules break free and evaporate from the surface of the food. As this surface water is removed, it is replaced with water from deeper within the food. Air conveys heat to the food and air removes moisture vapour from the dehydrating food.

Bound and Unbound Water

In foods water is present as free water and bound water. The unbound (free) water is within the pores and spaces between plant cells and is easy to remove. Bound water is locked within cells and much more energy must be applied to remove it.

Adding salt. Adding salt contributes positively to removal of the moisture. Sprinkling salt over sliced tomatoes immediately draws the moisture out. In meats, for example jerky, it removes moisture and provides safety against growth of bacteria. Even if salt is not sprinkled over meat, it is still there, as most jerkies are marinated and the marinade contains salty liquids like soy sauce.

Adding sugar. Sugar draws easily moisture from the fruit and binds free water. Fruits for jams are often mixed with sugar and left for a few hours to draw the juice out. Then, they can be cooked for shorter time.

Applying salt and sugar will not free the bound water.

Successful Drying

The drying process depends on:

Applying enough heat to remove moisture, without cooking the food.

Having an adequate air circulation to carry off the moisture.

Factors that Influence Drying

The greater the surface area and the more porous the surface - the faster the drying rate.

The greater the velocity of air flowing over food - the faster the drying rate.

The higher the temperature of air - the faster the drying rate.

The lower humidity of air - the faster the drying rate.

A thicker food requires more time to dry out. Take for example Chorizo sausage which like other sausages is drying from inside out. The length of the sausage does not affect the drying time, but the diameter does. A larger diameter sausage needs more time to dry out.

Fruit is sliced thin to shorten time and make drying process safer. Fruit leathers are spread to 1/8 inch thickness, beef jerky is sliced to ¼ inch thickness, tomatoes and other fruits are sliced thin. Some fruit, for example plums, apricots, raisings are dried whole, other fruit like mangos or papayas are dried as chunks. Adding salt to sweet fruits is not practiced as it will affect their flavor, however, fruits are acidic and acidity keeps bacteria in check.

Native Indians did not know salt so meats were dried in air or over fire, however those meats were thinly sliced. Eskimos dried meat and fish in air at very low temperatures, what helped was the windy weather. Low temperatures inhibited growth of bacteria.

Case Hardening

Well balanced drying takes place when the amount of evaporated water from the surface is replaced by an equal amount of water from inside.

The equilibrium state: diffusion rate = evaporation rate

When moisture from the surface of food is being removed more rapidly than water can diffuse from the moist interior of the food, a hardening of the outer surface maydevelop. This usually happens at low humidity and fast air flow. This hardened surface will retard the free diffusion of the interior water. As a result, moist interior becomes susceptible to microbial spoilage. This is a common problem for traditionally made slow dried salami. The problem is prevented by controlling the humidity and the temperature of the circulating air, and the speed of the air flow. For this reason home food drying instructions start at a low setting. To slow down the removal of moisture at the beginning of fermentation, meats are usually fermented at very high humidity setting (90-95%) with very slow air flow. Foods are more exposed to case hardening at the beginning of the drying process when they contain all moisture.

Water Activity

We know that bacteria need water and any moist food is at risk of being spoiled. Can we draw a conclusion that the wetter the food the faster it will spoil? Not so, there is more to it... A potato starch may contain 20% of moisture and be stable, but sugar with 4% moisture content may spoil.

It is the availability of unbound (free) water, not the actual water content, that determines perish ability.

In 1953, the new term water activity Aw was created to better define relationship between unbound water and food spoilage. Water activity is an indication of how tightly water is "bound" inside of a product. It does not say how much water is there, but how much is available to support the growth of microorganisms. Adding salt or sugar "binds" some of this free water inside of the product and lowers the amount of available water to bacteria. This is why we add so much sugar to jams, to immobilise free water. Sugar steals available water and holds it so tight that bacteria cannot use it. The following statement describes the importance of water activity (Aw).

Below certain Aw levels, microbes cannot grow.

"A potentially hazardous food does not include... a food with a WATER ACTIVITY value of 0.85 or less."

All we need to do is to test dry food for Aw in order to determine its safety.

There are a number of devices that can measure water activity.

AquaLab Pawkit water activity meter.

Sun Drying

Open air sun drying has been used for thousands of years. Warm temperatures, low humidity and prevailing winds are ideal conditions for sun drying. Sun drying is reserved to areas with low to moderate humidity. Such conditions are present in the USA in California and in Europe in Spain and Italy. The Gulf of Mexico states in the South of the USA are too humid for sun drying food on a commercial scale. Cooler night air is not capable to hold so much humidity as the hot air during the day and the moisture condenses. The food will acquire back the moisture and will not dry out. The solution will be to cover food or bring it inside, unfortunately, in both cases the drying process will be too expensive to compete with other areas. In the San Joaquin Valley in California, raisins are dried day and night without being moved.

In humid climate the drying will be slow and mold may appear. Open air sun drying is at the mercy of elements which we cannot control. In wet weather the product can either be covered or taken inside, however, such simple methods are usually reserved for countries with poor electrical infrastructure. Sun drying is labour intensive and requires plenty of space.

Screens placed on blocks allow for better air flow around the food. The blocks may be placed over a sheet of aluminum to reflect the sun rays and increase the drying temperature. The best screens are stainless steel or food grade plastic.

Drying food may attract insets and birds, so consider adding a protective cover over the food. This may be another screen or a sheet of cheesecloth.

Solar Drying

Solar drying uses sun energy but is performed in a solar dryer designed and built for this purpose.

Solar food dryer is built to dry food using solar energy. Sunlight enters an enclosed chamber (sun collector) where it strikes a dark surface converting its energy to heat. The heat dries the food. Airflow is accomplished by a natural draft (warm air rises) which can be controlled by adjusting vents. The same principle is used for controlling draft in a traditional smokehouse.

The sun collector and the drying chamber can be combined in one unit or separated. When the collector is free standing, the heat it generates is directed intothe drying chamber. Solar dehydrators may have backup electric heating to provideheat when the sun is not around.

Solar drying is an improved “open air sun drying” method which offers significant benefits:

Fast drying.

Higher drying temperatures.

Offers airflow and temperature control.

Food is protected from elements.

The following drawing depicts one tray solar dryer.

The inside of the dryer is lined up with a black absorbing material or painted black. Cold air flows in at the bottom but the sun rays will heat up the drying chamber and the air in it. Warm air rises up and leaves the chamber. This natural air flow depends on the difference in height (pressure) between fresh air intake and hot air exhaust. The bigger the difference the faster the air flow.

The following drawing is a step up design of the previous arrangement.

During the day the solar dryer works on sun energy. The warming chamber supplies warm air to the drying chamber which is loaded with three removable trays. At night time there is no significant drying, besides, there is a danger of condensation build up as cold night air cannot hold moisture. This will happen in hot and humid areas like Louisiana or Florida. The optional heating element can be switched on and the drying will continue through the night. The heating element can be placed in the warming chamber instead.

There is a pipe stack through which escapes warm and moist air. The higher the pipe, the bigger the difference in pressure between fresh air inlet and warm air exhaust. This results in a stronger air flow. By making air intake and pipe outlet adjustable, the air flow and humidity can be controlled during drying. Thermometer and humidity sensor can be installed for more precise control.

Both devices, solar dryer and meat smokehouse employ similar design features:

Fresh air intake

Smoke (hot air) exhaust

Air flow

Insulated frame

Heating element

Food Dehydrators

Food dehydrators offer fast drying which results in the immediate removal of moisture, this in turn inhibits bacterial growth. Bacteria do not get enough time to multiply as with each passing minute they find worse conditions to grow (less moisture). Dehydrators are good for drying thinly sliced fruits and vegetables, small fruits like grapes or pitted plums and thinly sliced meat. Dehydration obviously is a more expensive method (electricity cost), however, it can be controlled and is independent of weather changes.

Dehydrators give us control over the drying process:

Temperature – fully adjustable.

Ventilation and removal of moisture – provided by a fan.

There is no humidity control in small dehydrators, most of drying is done inside of the house where high humidity is not present.

Home-food dehydrators fall into two categories:

Those with stackable trays.

Those constructed of a rigid box with removable shelves.

There are models with base-mounted fans that move hot air vertically; others have a rear-mounted fan for moving air horizontally. Some models rely on convection drying, with no fan at all.

What Dehydrator to Buy?

Look for the following features:

1. Temperature control. Get the unit that allows you to set temperature by degree, and not just low, medium or high setting. All dehydrators will dry vegetables and fruits, but if you want to dry jerky the unit must be capable of maintaining 160° F (72° C).

Do not relay on the dehydrator’s temperature setting. Get a dehydrator with temperature control. You can determine the true drying temperature of your dehydrator using a dial-stem thermometer, then compensate the dehydrator’s temperature control for any difference. Switch the dehydrator on to its maximum setting and record the temperature. For safely drying jerky at home, the dehydrator must be able to maintain the temperature of 155° F (68° C). Some dehydrators come with factory preset temperature that can’t be controlled and they cannot be adjusted.

2. The timer. Look for dehydrator with the timer, but keep in mind that the timer does not decide when the food is dried, this is your job. You may have moist apples and drier ones and the drying times will be different. You may have different foods on different trays and the drying times will be different. Different brand dehydrators dry foods in different time. A dehydrator is just a tool, you are the cook.

It does not matter whether dehydrator comes with a mechanical or digital controls, but you need a timer. Most dehydrators are not equipped with an On/Off switch.

3. The size. Nine tray dehydrator may look impressive, but do you have enough counter space for it? A smaller 4-tray Nesco dehydrator will fit everywhere and you can add more trays to it later.

4. Accessories. Make sure you can get non-sticks trays and mats for fruit roll-ups and finer mesh screens for small diameter items.

Stackable Trays Dehydrator

Nesco makes round and rectangular stackable tray dehydrators. FD-1040 Gardenmaster Digital Pro Food Dehydrator comes with the adjustable 90-160° F degree digital control. This four-tray unit can be expanded with accessory Add-A-Trays®, up to 20 trays (20 sq. ft.) Drying pressure adjusts automatically to the number of trays.Power head unit sits on top of trays and contains controls, the heater and the fan. The fan blows hot air downwards and there is no possibility of liquids dripping into the heater chamber. Patented Converga-Flow® forces heated air down the exterior pressurised chamber, then horizontally across each individual tray, converging in the centre, for fast, even and nutritious drying. No flavor mixing! No need for tray rotation.

Some dehydrators, for example L'Equip 528, have the fan and the heater mounted in the base, and blow heated air up through the trays.

Stackable trays dehydrators may stack up to 30 inches tall, but they consume little counter space. They offer inexpensive way to get started with food dehydrating. Older models suffered from one limitation: the trays closest to the heater and fan dried much faster than those further away and the tray rotation was needed. Recent models provide equal air distribution in all areas and the trays do not need to be rotated.

Box-and-Shelf Dehydrators

Box and shelf dehydrators are made by different companies, Excalibur has been produced since 1973, so it the best known brand. It is, however, very expensive. In Excalibur, the heat source and fan at the back of the shelves instead of beneath them. Hot air blowing across the shelves eliminates the need for tray rotation, although the company advocates the tray reversal as the rear of the tray gets most of the heat.

Living Foods dehydrators

Living Foods dehydrators do not have fans, but rely instead on convection drying. Heat, generated by a heating element mounted at the base of the box, rises through the trays. They provide silent operation and less use of electricity but the drying process is longer. If they are designed like a traditional smokehouse, they can operate on gas burner or even wood, however, it is advisable to have a separate heating chamber. This chamber can be connected to a drying box with a pipe, exactly like a smoke generator is connected to a smokehouse.

Need a Really Big Dehydrator?

If you need a very large-capacity dehydrator, you have to build one yourself.

The smokehouse design principles can be directly applied to drying foods as meats are smoked below 140° F (60° C). You can smoke/dry meat jerky in a smokehouse, something that cannot be accomplish even in the most expensive dehydrators.

Air Dried Meats and Sausages

Preserving thinly sliced meat by drying was practiced in many areas of the world and the technology was basically the same. Even Eskimos who lived in the coldest climate on earth were preserving seal or whale meat by drying it in the open air. The best known air dried meats are:

Hams

Pork butts

Jerky

Pemmican

Biltong

Sausages

We all know that meat would spoil if not kept under refrigeration so the concept of drying meat seems to be very difficult and dangerous. And it is, unless proper steps are taken. This is not rocket science and we could do it thousands of years ago when computers were not around and the word meat science was not invented yet, we should be able to produce air dried meats without a sweat today. It is necessary to know the basics well, the good news is that the basics like many other things depend to a great extent on common sense and logic. Air dried, fermented and cold smoked meats are intimately related and follow the same guidelines. A reader is referred to the Chapter on Fermented Meats which includes a lot of information on drying and safety.

Take for example traditionally made Hungarian salami. The sausage is made without sugar so we may say that there is no fermentation. The small account of glycogen (sugar) that meat contains and any resulting fermentation can be considered negligible. The whole process of making this sausage depends on drying. The Spanish will call it chorizo and in Lithuania they will cold smoke sausages and after drying it will be called Cold Smoked Sausage. Of course it is more complicated to dry a huge piece of meat such as a ham than a thinly cut strip of meat that will become jerky, nevertheless the rules of the game remain basically the same. You can not just hang meat and let it dry as it will spoil. Even when left in a refrigerator it will spoil although at a slower rate. You have to prevent bacteria from growing and that is accomplished by:

Applying salt. This prevents bacteria from growing and removes moisture. The less moisture remains, the harder it is for bacteria to grow.

Adding nitrite/Nitrate. This prevents pathogenic bacteria, notably Clostridium botulinum from growing.

At the beginning of the process keeping meat at refrigerator temperature. This prevents bacteria from growing.

Drying in the air. Once the meat has lost some moisture it is safer to continue drying in the air at 50-54° F (10-12° C). We don't want to dry meat in the "danger zone" 60-140° F (16-60° C) when bacteria find favorable conditions to grow. When drying is accomplished in a dehydrator (jerky) the temperature should be 145° F (63° C) which is above the danger zone.

This leads to a common question "what if I don't want to use nitrates"? A commercial producer must conform to the Government standards and he has to use them. It is possible to obtain permission from the Food Inspection and Safety service to make products without nitrite, but this is a very costly and involved process. Needless to say such an establishment will be under great scrutiny of meat inspectors and most processors don't want to be bothered with such a waiver.

It is not expected that an Eskimo will cure seal or whale meat with nitrites. They would rather pound their meat pieces until thin, and dry them in windy and cold conditions that most of us will find impossible to duplicate. It also goes without saying that it takes longer to remove moisture from the inside of a 18 lb. ham than a 2 oz. strip of meat. More Nitrate can be safely applied to a meat that will dry for a year as the Nitrates dissipate slowly in time. For this reason you can apply 625 ppm of sodium nitrite to a dry product and only 156 ppm to a regular smoked sausage. The salt is applied at around 6% to a country ham which needs plenty of protection, about 3% for a dry fermented sausage, 2.5% for semi-dry fermented sausage and only 1.8% for a smoked one. At home you can do whatever pleases you, although we don't see why someone would not want to make products in a safe way.