Food dehydrating 101

That may sound like a stupid question, but the answer might be more complicated than you imagine. Understanding the "hows and whys" behind food dehydration will help you make better decisions in how you treat individual ingredients.

On a molecular level, sugar is strongly attracted to water.

Zooming down to the molecular level, sugar has electronic properties that attract water, and in other words, it is hygroscopic. Sugar is so attracted to water that it can actually pull water molecules straight from humidity in the air. That is why table sugar often clumps up. In this case, the sugar molecules crowd around the water molecules that are pulled from the air, causing them to stick to each other.

Dehydration preserves food because sugar attracts water, and this will suck the water out of nearby bacteria.

When you dehydrate fruits and vegetables, sugars make up a large portion of the remaining mass of the dehydrated food. These sugars strongly attract any water that happens to be passing by. Bacteria resemble little bags of water. If a bacterium lands on a piece of dehydrated food, the sugars in the food strongly attract the water molecules in the bacterium. The sugar sucks the water right out of the bacterium, killing it in the process. This very principle is why jelly and ketchup – both high in sugar, can outlast everything else in the refrigerator.

The sugar concentration of a food determines how dry it needs to be in order to stay preserved.

In fact, the less sugar something has, the drier it needs to be in order to stay preserved. For example, dried pineapple will greatly outlast something less sugary like green beans dried to the same degree. It’s imperative to ensure vegetables are dry to a crisp, whereas sweet fruits can still retain some chewiness before reaching the same level of preservation.

What foods are worth dehydrating?

Not all foods are worth the effort and cost of dehydrating. Unless you're growing your own food, it's going to be hard to beat the price and convenience of anything Costco sells.

Below I've assembled three lists of ingredients commonly used in backpacking meals. The first is ingredients that are generally not worth the time and effort, the next list is the "sometimes" ingredients, and the last list of ingredients are usually always worth the effort. These lists assume that you are not growing your own food. I have graded ingredients based on a blend of cost as well as how well the ingredients stand up to the dehydration and rehydration process. With these lists in hand, I tend to watch for deals or for particularly high quality produce to come through my favorite produce stand.

Depending on the ingredient, dehydrated food can last many months when stored properly. Dehydrated food will last the longest when oxygen is removed and the temperature is low. In other words, vacuum-pack your dried food and store it in the refrigerator. If you can keep moisture from getting into the dried food, most ingredients will stay preserved for many months.

If you're sensitive to sulfur dioxide, be aware that it's used as an antioxidant preservative in most conventionally dried fruit and vegetables.

If you're sensitive to sulfites, you might need to avoid the sulfites hidden in dried fruit. This means that most commercially dried fruit is off-limits. Unless you've got a health food store nearby with a stock of non-sulfured dried fruits, drying all your fruit yourself may be your only option.

Sulfur dioxide is a powerful antioxidant and antimicrobial and is considered safe for human consumption by the FDA as well as consumer watchdog groups (such as the Center for Science in the Public Interest). It is commonly used in red wine and is found on almost all conventionally dried fruits and vegetables, and greatly increases their shelf life. The only reason to avoid it is if you are allergic to it.

Some individuals, particularly those with asthma, may have sensitive reactions to sulfur dioxide. Sulfite sensitivity is one of the most commonly listed food allergies, and the FDA estimates that as much as 1 percent of the population may be sensitive.

If you have adverse reactions to sulfites, you might have luck finding sulfite-free fruit at some natural food co-ops and health food stores. Otherwise, you might consider buying a larger capacity food dehydrator for dehydrating all of your ingredients to ensure that sulfites stay out of your food.

When dried, most produce loses from 60 percent to more than 90 percent of its weight when fresh. Take this into account when comparing prices between dried and fresh food.

For example, tomatoes typically lose about 95% of their mass when dried. If you need dehydrated tomatoes and they are on sale fresh for $2.00 a pound, you are still getting a much better bargain if you buy sun-dried tomatoes for $12 a pound (and this doesn’t even account for the amount of work you’ve been spared).

Dried ingredients to buy commercially dehydrated:

Assuming that you aren't growing your own food, and that you don't have sulfite allergies, most of the time the following ingredients are not worth the effort and energy costs of dehydrating yourself. Check larger grocery stores with a bulk bin section for these items. Most of fruits and vegetables on this list dehydrate quite easily, so don't hesitate to dry them yourself if you already have the fresh produce.

• apples
• apricots
• beef jerky
• bouillon
• cranberries
• dates
• eggs
• figs
• fruit leathers
• garlic (can be bought as powder)
• granola
• imitation beef or chicken
• milk
• most herbs/spices -- check out Latino and Asian grocery stores for the best deals.
• noodles
• onions
• plums (prunes)
• potatoes (can be bought as dried hash-browns, flakes or slices)
• raisins
• salmon jerky
• salted fish
• seasoning mixes
• sauce mixes
• shrimp
• squid
• sun-dried tomatoes
• textured vegetable protein (TVP)

Dried ingredients that are sometimes worth dehydrating yourself:

These ingredients can often be found commercially dried for reasonable prices, but this is not always the case. You might sometimes find a good enough deal on fresh produce to make it worth the effort of dehydrating them yourself.

• tomatoes
• pineapple -- can sometimes be had very cheaply from fruit markets
• mushrooms -- expensive dried, but you can sometimes find good deals, especially in Asian markets on Shitake mushrooms
• berries can be cheaper to dehydrate yourself

Ingredients that are almost always worth dehydrating yourself:

And finally after all the drum rolling, we get to the list of ingredients that are almost always worth the effort of drying yourself. If you're dehydrating food to make your own backpacking meals, many of these ingredients can bring your meals in the backcountry to life.

Vegetables (and fruits most people think of as vegetables)

• artichoke hearts
• beans of all kinds (make sure they are fully cooked!)
• beets
• bell peppers
• cabbage
• carrots
• celery
• corn
• cucumber
• eggplant
• hot peppers
• kale, other greens
• leeks
• mushrooms
• onions (only if you want larger chunks -- onion flakes are cheap)
• parsnips
• peas
• potatoes - diced or cubed only (can be bought frozen), dried flakes, slices, and shredded is readily available pre-dried
• rutabaga
• spinach
• squash (Summer and winter)
• sugar snap peas
• sweet potatoes and yams
• tomato sauce, tomato paste (make fruit leather)

Fruits to dehydrate:

• bananas (most commercial banana chips are deep-friend and coated in corn syrup)
• cantaloupes
• cherries
• honey-dew melons
• kiwi
• peaches (puree and make a fruit leather)
• pears (puree and make a fruit leather)
• strawberries
• watermelon

Meats and proteins that dehydrate well:

If you're making jerky, go for lean cuts of beef. The less fat, the better it will preserve. If you plan to rehydrate dried meats for a backpacking meal, choose only ground or minced meats. Dried chicken breast chunks or beef will almost never rehydrate adequately. Again, lean cuts will not spoil as quickly. Even if you're a diehard carnivore, you might find yourself happier in the field with the results you get from well-seasoned TVP in your dinners than from rehydrated meats. Jerky tends to be the most palatable form of dried meats, but it doesn't really have a place in most entrées.

• ground beef
• ground sausage
• sliced lunch meat
• bacon
• Canadian bacon
• tofu
• tempeh

All about food dehydrators, sun-drying, and dehydrating with conventional ovens

Most conventional food dehydrators are energy expensive.

Apart from the fresh food costs, the greatest expense in dehydrating food is from the energy required to generate heat. Many counter-top dehydrators use 500-1000 Watt heat elements and this will certainly be felt in your electric bill if you regularly run the dehydrator. If you're on a flex-rate utility plan, you can save a lot of money by running your dehydrator during non-peak usage times.

Considering the cost of electricity, it's worth looking into cheaper alternatives if you live in a warmer climate.

Live in the a warm climate? Consider sun-drying your food.

If you are living in a dry, arid climate like the American Southwest, sun-drying fruits and vegetables may be feasible. The University of Georgia Cooperative Extension Program recommends sun-drying of foods if the temperature is a minimum of 86° F, and the humidity is below 60 percent. If you live in a humid climate, however, sun-drying is likely out of the question, as mold can grow very quickly in warm, moist environments.

Conventional ovens can be used for food dehydration, but it can sometimes be a challenge to get good results.

Because internal air is not circulated in most consumer-grade convection ovens, dehydration can take two to three times as long as it does in a dedicated dehydrator, and this will consume much more energy. Furthermore, most conventional oven are not designed to have fine temperature control in the 95° F to 160° F range, (“warm” is open to interpretation) and the temperatures fluctuate widely throughout the intervals in which the heating unit is either on or off. Using an oven thermometer is a must to ensure the temperature is within the target range. If you have a pizza stone, add it to the oven. This will hold heat and help buffer the temperature fluctuations of a low-end electric oven.

Drying racks are typically a must if using a conventional oven for food dehydrating.

The slats on most oven racks are too wide to work for most food drying. You will need special racks or screens so you don’t lose food through the slats as the water evaporates and the food shrinks. Unless you have access to inexpensive food-safe metal screens or racks, the cost of purchasing drying racks can approach the cost of a dedicated counter-top food dehydrator complete with racks.

This brings us to the conclusion that unless you live in a warm, dry climate where you can sun dry your food, it makes sense to buy or build a dedicated food dehydrator. The energy expensive consumer units aren't the only option though. If you're a DIY kind of person, you can build your own.

DIY custom-built food dehydrators are easy to build and can be solar-powered.

Before addressing conventional food dehydrators, one particularly interesting option for do-it-yourself types is to build a custom dehydrator powered by a solar furnace. Doesn't the term "solar furnace" sound exciting?

Various plans for solar furnace dehydrators can be found around the internet and in back-issues of homestead magazines like Mother Earth News. Most solar furnace designs rely on re-purposed aluminum beverage cans, plywood and Plexiglas. Hot air from the furnace can be blown into a cupboard built to hold drying racks. This type of design will outlast almost any conventional dehydrator and takes just a few watts of energy to run the fans.

The major downside to solar furnaces is that your food dehydrating plans are at the mercy of the weather, and that you'll need to start most batches early in the day. Controlling the temperature will be also be a challenge unless a thermostat that controls fan speed (or some other temperature control mechanism) is included in the design. Obviously, you will need good access to direct sunlight for your solar furnace.

Other popular DIY food dehydrator designs employ simple incandescent light bulbs as heating elements.

Using incandescent lights as a heat source makes temperature regulation easier, as you can control the temperature by simply adjusting the wattage of the light bulbs. This is essentially a shout-out to the Easy-Bake ovens of our youth.

I won't go into DIY food dehydrators any further here. The internet is awash in instructions and designs, and I'm sure you can find something that works for you if you have the inclination.

Counter-top food dehydrators are affordable, readily available, and work well.

Most people will be best served by small countertop food dehydrators. These can easily be purchased from department stores or Amazon.com. Prices range from $40 to $150 for the popular models, more for commercial dehydrators. Most inexpensive counter-top food dehydrators have stacking trays and top-mounted heating elements and fans to force heated air into the drying area. Others are cabinets with removable trays and rear or bottom-mounted fans and heating elements.

Nesco is the biggest name in consumer-grade dehydrators and they make several models that will be adequate for most hikers and campers who only need to pull them out a few times each year. Their capacity can be increased by purchasing additional trays.

Racks and trays for dehydrating

If you are building your own dehydrator, sun-drying, or using an oven, you will need adequate drying racks. Ideally, drying racks have slats that are fine enough to stop drying food from falling through as it shrivels.

Avoid metal trays that aren't stainless steel -- food-safe plastic trays are the best for dehydrating food.

Some people use wire screen (hardware cloth) for trays, not realizing that the galvanized metal is not food-safe. Aluminum, copper, zinc, and other metals will react with many ingredients and can leave dangerous deposits in your food. Food-safe plastic, stainless steel, or Teflon-coated trays are your best bet.

Use nylon mesh for containing loose foods like corn and from preventing the food from sticking to the drying racks

For dehydrating food such as peas and corn, polypropylene or nylon mesh netting, spread over a cake rack works well. Laying the mesh between the food and the drying racks also prevents the food from sticking as it dries, and makes it much easier to clean the racks.

Use food-safe nylon or polypropylene only

Rolls of food-grade, reusable polypropylene mesh netting can be found from various meat processing and commercial food equipment suppliers. Food-safe polypropylene and nylon mesh is safe for use at temperatures well above the temperatures at which dehydrators operate. For easy cleanup, they can be run through the washing machine when you’re finished. You may also find adequate mesh material at the fabric store, but exercise caution with any materials that are not explicitly deemed food-safe, as you may end up unintentionally ingesting traces of harmful chemicals.

Equipment for drying sauces and purees

Sauces, pastes and purees can be dried into leathers resembling fruit leathers. Fruit leathers are a good use of mediocre fruit because you can add honey and a little fruit juice to make them taste better.

Sauces need to be spread thinly and evenly, usually no more than 1/8” to 1/4" thick, on plastic-wrap lined trays or Teflon baking sheets. After the sauce is firm enough to handle (usually a few hours in the dehydrator), peel this from the plastic wrap and turn it over to ensure even drying.

Do not use aluminum foil or waxed paper for sauces.

Aluminum foil will react with acids and salts, and the paraffin on the waxed paper can melt and be displaced by the liquid, soaking the paper and ruining your sauce. Placing the sauce on plastic wrap or directly on a Teflon cookie sheet is the best. If you place sauce directly on Teflon, take care of your Teflon and only use a soft-tipped rubber spatula to peel the sauce away when it has dried into a leather.

Useful equipment for dehydrating food

Beyond a food dehydrator and drying racks, the following tools will make drying easier.

• apple corer
• colander
• food processor
• fruit/vegetable peeler
• mesh (food-safe nylon or polypropylene)
• oven thermometer
• scrub brush for vegetables
• sharp knives
• thick scrub brush for cleaning drying racks
• vacuum sealer (optional, but nice)

Prepping ingredients for the dehydrator

When I first start drying my own food, I had a few funky batches because I never bothered to do my homework. I realized that beyond temperature and timing control, it was crucial to avoid contamination as well as use only fresh, high-quality ingredients.

Freshness is paramount when drying foods like vegetables or fruits. Older produce is more likely to go bad WHILE you're dehydrating. Conditions in an operating food dehydrator might be tough for survival of bacteria or mold, but just beneath the surface of the fruit or vegetable it is a different story. Here it is moist, the temperatures will slowly transition up through optimum growth temperatures for bacteria and molds. As the surface of the food dries and has its cellular structure damaged, air can leak into warm, moist pockets, providing a nearly ideal (albeit ephemeral) aerobic environment for contaminating microorganisms to thrive.

Select only the freshest, highest-quality fruits and vegetables. Drying will not improve or mask their quality.

If microbial contaminants have already had a head start with the neglected fruit atop the fridge (in other words, the fruit is approaching over-ripeness), these critters can explode in population growth between the time you prep the food and when food is dehydrated to a point that it's no longer a hospitable environment.

Maintain good hygiene: Always thoroughly wash produce, your hands and equipment.

Microorganisms exist all around us that can metabolize and spoil food. If you've ever worked with sterile fields in microbiology, you know how easy it is to introduce contaminants even with remarkable precaution.

Bacteria will already be present on the surface of produce and on equipment and work surfaces in your kitchen. Like all critters, bacteria and mold will grow exponentially until the essential resources run out. The more organisms you can eliminate on the food, the smaller the final population will be once the food is dehydrated to a point that the bacteria can no longer grow. Simply put, the cleaner you can be, the fewer contaminants will be introduced in your food, and the less "funky," the final product will taste.

Sanitize your equipment with white vinegar

White vinegar is excellent for sanitation as the acetic acid it contains is acidic enough to kill most contaminating microorganisms. It also is excellent at cutting grease. Use undiluted white vinegar in a spray bottle, and apply this to counter-tops, drying racks, and wooden cutting boards. Wipe it off with a clean, dry cloth, and repeat this until the surface is clean. As the vinegar evaporates, it will leave behind the acetic acid solids, further protecting the surface. White vinegar is the one of the best methods for cleaning wooden surfaces like cutting boards, as soap and water will damage the wood fibers.

Sanitizing with bleach

Sanitizing bleach mix is the industry standard for safe food handling. This can be used on your drying racks, knives, and work surfaces.

• Only use cool water when mixing and using bleach for sanitation. Using hot water will cause the chlorine to evaporate. Chlorine gas is highly toxic and damages your respiratory tract.
• Never mix bleach with any other cleaning products.
• Aim for a bleach concentration of 100 ppm. This is about 1/2 tablespoon of bleach per gallon of water.
• Wipe down surfaces with a dishrag soaked in the sanitizing solution, and let the surface dry before using it.

Personal hygiene

Keep your nails trimmed, and wash your hands thoroughly, making sure to scrub under your nails before handling produce.

Washing vegetables and fruits before drying

Most conventional fruits and vegetables are coated both in pesticides and mineral oils or waxes to keep them fresh longer. This isn't healthy to ingest, and it is recommended to scrub fruits and vegetables with a vegetable scrub brush and cold water to remove this. Soap can also help remove wax coatings, but it also can be dangerous to ingest and you should be very careful to rinse your vegetables well. Most natural food stores sell fruit and vegetable washes that are preferable to dish-washing detergent, as they're typically made out of safer ingredients.

Recommended method for washing and prepping fruits and vegetables for drying

1. Sanitize all of your equipment with bleach or vinegar, and wash your hands.
2. Do not wash vegetables and fruits until you are ready to use them.
3. Rinse under tepid potable water, and use your hands or a vegetable scrub brush to remove dirt and debris.
4. Avoid using water more than 10° F cooler than the produce. Doing so will create a pressure difference between the produce and the outside, causing microorganisms to potentially get sucked into pores in the produce.
5. Optionally use an FDA-approved produce detergent as directed.
6. Peel the vegetables or fruit if it has a thick, waxy coating (as what is commonly seen with apples or cucumbers).
7. After peeling, rinse produce once more to remove any remaining residues.
8. Slice or prepare individual fruits or vegetables as recommended in recipes here. Remove any brown or bruised portions.
9. Blanch, or pre-treat produce as described here before dehydrating

Avoid using hydrogen peroxide or bleach for sanitizing produce

Vegetables and fruits disinfected in dilute hydrogen peroxide or bleach solution will be less likely to immediately make you sick from pathogens and will typically last longer than untreated produce. This can also pose serious health risks, however, and is best avoided.

Why? Both bleach and hydrogen peroxide are powerful oxidizing agents and kill microorganisms by breaking and reforming chemical bonds (covalent chemical reactions) with the molecules that make up the microbe. Not only will this occur on the microbes, but also (much more) on the surface of the fruits and vegetables. This will form countless complex organic compounds, some of which will inevitably be unsafe for ingestion.

The FDA warns against the uses of bleach and hydrogen peroxide on the surface of fruits vegetables. They recognize the potential for the formation of dangerous organochlorine compounds, which can build up in body tissue and act as carcinogens.

Even though bleach sanitizing is popular with some practitioners of food-drying, I cannot recommend doing this. Instead you should carefully wash with water as directed below, or if you are intent on a chemical wash, only purchase FDA-approved non-oxidative produce washes.

For those of you that don't mind going to the oncologist or Jesus sooner, here's my advice on using bleach: Make sure to only use bleach sold as "food-grade" and never use a concentration greater than 200 ppm (about 1 tablespoon of 5.25% sodium hydrochlorite -- common household bleach -- to one gallon of cool water). Thoroughly rinse the vegetables as any remaining bleach will be strongly evident in the taste, and will ruin the food.

Where to go from here

But where's the part about actually drying the food? Every ingredient you dehydrate will be a bit different and it's very much worth the effort to Google a few food dehydrator recipes for an ingredient before you go to the effort of drying it yourself. The important details are: a) What's the best way to cut and prepare the ingredient? b) What's a good temperature? c) How long? and d) What pre-treatment is most appropriate for a given ingredient? With these details in hand, we can employ proper technique to wash and prepare our fruit, and will likely get the highest quality results possible for our efforts.