Food Drying 101

Drying food is a simple process, with a few basic rules and requirements that assure safety and quality. It doesn’t require a lot of high tech equipment, though having a dehydrator does allow for better quality of some foods, because it’s easier to regulate temperature and drying time. This page will take you through the basics of drying with a dehydrator, with a mention of oven and sun drying. It’s pretty thorough, so get yourself a cuppa and settle in!

How Does Drying Preserve Food?

Microorganisms and enzymes that spoil food need moisture to be active. Drying food deprives them of that moisture. However, it does not kill them, so care in preparation, drying and packaging is still necessary to avoid microorganism growth. Enzymes that might cause flavor and color changes can be deactivated with certain pre-treatments. Aside from a few specific safety protocols, food drying is very flexible, and allows you freedom to experiment and customize. You decide about the size of food pieces, combinations, pretreatments, and packaging. I find it enjoyable, and from time to time I make a cool discovery that surprises me.

Nutritional Value of Dried Fruits and Vegetables

Fresh produce provides calories, fiber, vitamins and minerals. Dried produce can be expected to have the following changes:

Calories: No change. Calorie content will be higher per unit of weight since the water content is gone.
Fiber: No change.
Vitamins: The vitamins most often found in fruits and veggies are A, C, and the B vitamins. If veggies are blanched (below), vitamin A is better maintained. Loss of vitamin C can occur depending on choice of pretreatment. Speed of drying and absence of sunlight in drying and storage, as well as decreasing temperature as complete dryness approaches, will help maintain vitamin C level. Only moderate loss of vitamin B occurs.
Minerals: If produce is soaked, some minerals may be lost, but no data is available. No minerals are lost in the drying process.

Yield

Yield of dried produce is directly related to how much water was in the original product. For example, 25 lb of apples yields about 4 lb dried, and 25 lb onions yields about 3 lb dried. Expect your produce to shrink substantially (that’s what we want in our pack!).

Overview of Drying Methods:

Dehydrator Drying  Using a food dehydrator produces the best quality dried food because it’s possible to regulate air flow, temperature and time. Those three factors must work in concert for really great dried food. A good food dehydrator doesn’t take much in the way of electricity (my Excalibur is said to use around 8 cents an hour), and runs quietly in the background. More on choosing and using a dehydrator here.

Oven Drying  Oven drying is a good alternative to dehydrator drying, thought it can take about twice as long as a dehydrator to dry foods, costs more money, and tends to produce food that is more brittle, darker and a bit less flavorful than food dried in a dehydrator. It also ties up your oven for anywhere from 4 to 36 hours at a time, so planning is key. I used to use my oven a lot to dry food, then decided to upgrade to a dehydrator. It’s been worth every cent! And I get make casseroles at the same time (I’m big into one-pot meals!). Before drying with your oven, test the temperature with an oven thermometer for one hour. Prop the door open a bit (you’ll do this when drying your food in it). The temp should maintain a temperature between 130F and 150F. If it wavers a lot, your food may not come out so great. Too hot = food cooks. Too cold = food spoils before drying.

Air Drying  Air drying can be useful for herbs and some peppers, especially in a dry climate (peppers are paleo-approved, but not AIP!). The material is tied in bunches and hung out of the sun to dry, or placed on a tray and set out to dry. I have dried herbs on hot summer days here in Seattle this way, but I would not try it here with any other foods. Too much risk of mold growth. In Arizona, however…

Sun Drying and Solar Drying  Drying food in the sun is the oldest method of food drying, and can be simple and reliable especially in a dry climate. Solar drying is done using a specially constructed box that heats the food with passive solar. I live in the Pacific Northwest. To dry food in the sun here is not even an option but for a few weeks a year. If you live in a dry state and want to explore sun drying options, there are resources online that can tell you how to build a sun setup that keeps out bugs and critters. NOTE: Sun, air and solar drying are not recommended for meats, fish and bird; higher temperatures are required for these items, and it’s hard to regulate that with these methods.

Selecting Produce for Drying

Fruit should be firm, fully ripe, and heavy for its size. Handle it gently and process it immediately, as bruising will make for faster spoilage. Old or bruised fruit will produce lower quality dried product, but sometimes it’s fine to use for making fruit leather. Caution: Sometimes older, bruised fruits will spoil before the drying process is over, because the enzymatic and bacterial action is so far along by that time. It’s your call; if you have doubt, throw that sucker in a smoothie and eat it up right now.

Veggies selected for drying should be fresh and in peak condition. Avoid young veggies – their color and flavor tend to be weaker. Older veggies tend to be more woody and tough. Dry as soon as possible after harvest.

Enzymatic Changes and Time Frame

Enzymatic changes happen quickly once produce is harvested. Be efficient in both preparation and drying (for example, don’t dry one item with another that needs a different temperature, because this will mean one of them takes much longer to dry than it ought to). Drying should be done as quickly as possible for each item, at a temp that won’t negatively affect texture, color and flavor (refer to my charts for drying fruits and drying vegetables).

Preparing Fruits and Veggies For Drying

Fruits  Wash all fruits in cold water, but don’t soak unless using a pretreatment, because extended soaking can lead to nutrient loss and waterlog the fruit, which leads to longer drying time. Remove stems and peels where appropriate. Peels are fine on some fruits such as apples and peaches – your choice. Core the fruit or remove the pit, cut into uniform sections or slices, and trim away soft spots or funky stuff. Use a sharp knife – you’ll bruise the fruit less.

Vegetables  Wash in cold water just before drying. Do not soak. Towel-drying helps reduce drying time. Depending on the veggie, peel and trim, then cut, slice or shred into uniform pieces. Veggies with peels tend to be tougher once dried, but some are fine that way. Remove fibrous or woody sections. A mandoline slicer works like a dream for veggies you want to slice in thin, uniform pieces, such as sweet potato or beet chips.

Pretreatment Overview

Pretreating food is not essential, but it can improve quality, especially of vegetables. I don’t generally use pretreatments, since I don’t care if my produce changes color, though I do see the benefit of blanching vegetables (easier rehydration). Five reasons to pretreat:

1. Preserve color and flavor
2. Minimize nutrient loss
3. Stop enzyme action that decomposes the food
4. Ensure more even drying
5. Extend shelf life

Pretreatment options for fruits:

• Ascorbic acid/citric acid dips
• Salt solution dip
• Syrup blanching
• Honey dip
• Crazing

Pretreatment options for vegetables:

• Steam blanching
• Water blanching

Pretreating Fruits

Pretreating Vegetables

How to Dry Foods

Whichever method you use to dry your food, make sure each item has enough space around it on the tray so that air can flow. The space will grow as the items shrink, so it doesn’t have to be a lot. No stacking of pieces.

Dehydrator Method: Different foods can be dried at the same time, as long as they all require the same temperature. If you are drying foods that take different drying times, monitor them so you can pull items as they dry. Dry strong-smelling foods such as onions separately, since they can impart a flavor on other foods. It really goes without saying, but don’t dry foods together that need different temperatures. Monitor the drying process. If necessary, rotate trays for even drying. Shredded, grated or finely cut foods may need stirring. In general, larger pieces don’t need turning because air flows through the tray screen. The less you open the door, the more constant the temperature remains.

Temperature Settings for Fruits and Vegetables

If temp is too low, the product will sour. If the temp is too high, there is a danger of moisture being removed too quickly; this results in a hard skin forming on the outside of the item (“case hardening”), preventing the evaporation of remaining moisture, which is a mold and spoilage risk. To speed up overall drying time dramatically, set the temp high at 160°F for the first 2-3 hours, then turn it down to 135°F for fruit or 125°F for veggies. During the initial hours, the food temp won’t exceed 118°F because of the high initial moisture content. Neither will the air temp in the dryer actually reach 160°F (it takes some time to build up) so there’s not a risk of damaging foods that need a low drying temp. This procedure can cut drying time in half for some items!

If you can’t be around to monitor temps, just set the dryer at 135°F for fruit or 125°F for veggies and expect longer drying times. NOTE: This technique is said to help preserve helpful enzymes that raw food advocates rave about. They state that any food that goes above 140°F in temperature loses its vital enzymes, losing its value as a food source. I know, there seems to be a contradiction about temperature, enzyme deactivation, and all that. The way I understand it, drying deactivates the enzymes that spoil food (or.. help us digest it, for after all, isn’t digestion in the same category?) but doesn’t kill them. I like the idea of live enzymes in my food. Graaaawr! For meat temperatures and safety information, go here.

Drying Time

Many factors affect drying time, including size of food pieces, relative humidity, moisture content of the foods, pretreatment method, dryer type, temperature, and amount of air movement in the drying environment. The fruit drying chart and vegetable drying chart reflect averages for drying time; you’ll find a rhythm with your own kitchen and food preferences. Generally, fruit can take from 3 to 36 hours, and vegetables from 3 to 16 hours. You will be the judge of when a food is dry.

Gauging Food Dryness

To check for dryness, remove a piece of food from the dehydrator, and allow it to cool completely. Then check for dryness. If you are uncertain, place a few items of the food in a closed plastic bag or jar; if you see condensation, it’s not dry yet. When in doubt, dry the food more. The drying process accelerates toward the end of the process, so check more frequently as it gets close.

Fruits will feel softer and less dry when warm in the dryer. They are dry when they are pliable and leatherlike, with no pockets of moisture, and no obvious moisture when broken in half. When a few pieces are squeezed together, they should fall apart when released. High sugar fruits such as figs and cherries will be slightly sticky. Fruit leather can be peeled from the drying surface.

Veggies are sufficiently dry when they are brittle or leathery. For example, zucchini may be leathery while peas will crunch. Certain veggies like plantains, sweet potatoes and yams may retain a bit of chewiness, especially if coated in coconut oil – err on the side of dry. Herbs are dry when they are brittle. If you want to geek out and test for dryness test based on water weight, here’s a link to the Utah State University cooperative extension website where they discuss that technique.

Conditioning Fruit

Pieces of fruit are not always uniformly dry after coming out of the dehydrator. Conditioning is a process that checks for excess moisture and distributes remaining moisture before final storage. To condition, loosely pack cooled fruit in a large glass or plastic container to about two thirds full. Seal tightly and store in a dry, room temp, well-ventilated place. Shake the containers every day for two days up to a week. If there is evidence of moisture on the lid, return the food to the dryer. The turning of the food assures even redistribution of remaining moisture and reduces chances for spoilage, especially from mold. Because veggies dry to a nearly waterless state, conditioning them is not necessary.

Rehydrating Fruits and Vegetables

Though many of the foods on this blog will be eaten in dried form, some will need rehydrating before consumption.

Fruits: Cover in boiling water and let sit for 5 minutes. Drain.

Vegetables: Use cold water. Start with 1/5 – 2 cups water per cup of veggies, and add more as needed. Let sit for .5 – 2 hours, depending on the veggie and the size of the pieces. Root veggies take the longest. Rehydrated veggies should plump up to the same size they were fresh. Veggies added to soup or stew can just be thrown in without rehydration.

Packaging and Storage

Proper packaging and storage is crucial. Once you have determined the food is properly dried, package it immediately for storage. It protects your food from oxygen, moisture, light, microorganisms, and pests (including your own sticky little fingers when you’re craving carbs! More than once I’ve finished my hiking snacks, without even leaving the house!). During storage at room temperature, the most common type of spoilage is mold growth. The cooler the area, the less chance mold will grow. Many people store dried foods in the fridge or freezer, which also keeps quality high. A dark place is ideal because light reduces vitamin A and C content of fruits and vegetables.

Containers: An ideal container for dried food is:

• Clean
• Dry
• Non-toxic
• Lightweight
• Moisture resistant
• Airtight
• Protective against light
• Easily opened and closed
• Durable
• Low-cost

Use glass, plastic (non-BPA), metal (never galvanized steel), or re-closable mylar prepper-type bags like these ones. Containers should be as full as possible without crushing the contents. Try to store foods in amounts that require the container to only be opened once. This avoids re-closing a package with newly introduced mold spores or moisture. One good method is to store foods in sealed plastic bags, within a larger, light-proof container with a tight-fitting lid. It’s fine to store different foods in the same container, but keep in mind that some foods may flavor others.

Vacuum Packing Dried Produce: One option is to vacuum pack your dried fruits and veggies.
Fruit: Fill canning jars with dried fruit. With lid lightly screwed on, place jars in oven at 325F for 15 minutes. Tighten lid when jars are removed. Test the lids on the fruit after it has cooled to see that you have a seal.
Vegetables: Never use this method unless you are certain the veggies are dry, either by drying to the brittle stage or by the geeky calculation method linked to above. For veggies, dry to 90% solids level.

Labeling: Label each package with the type of food, pretreatment step (especially if you did use the sulfiting – asthmatics are at risk) and date. Labeling helps avoid unnecessary opening of packages to check the ingredients. It’s not a perfect world; just do your best. Properly dried foods will keep for a year if sealed in proper packaging and stored in a dark, cool, dry place. I play it safe by storing foods I know I won’t use for a while, in my freezer.

Tips for Top Nutrition and Flavor

• Don’t overblanch
• Dry produce as quickly as possible without raising temps above 15oF initially or 140F for the remaining drying time
• Dry herbs, coconut and mushrooms (non-AIP) at lower temps
• Do not overload dehydrator
• Keep food on trays well-spaced with no overlapping
• If possible, dry when relative humidity is low
• Do a proper check to be sure foods are really dry
• Condition dried fruits
• Store foods in packaging that does not admit moisture, oxygen or light
• Store foods in a cool, dark place
• Store foods in amounts that can easily be used at one time, to avoid repeated opening of container

RESOURCES:

Brennand, Charlotte P. “Home Drying of Food.” Utah State University Cooperative Extension website, (1994). http://extension.usu.edu/files/publications/publication/FN-330.pdf

Swanson, Marilyn A., Sandra M. McCurdy. “Drying fruits and vegetables.” Pacific Northwest Extension Publications (University of Idaho, Oregon State University, Washington State University), (2003). http://extension.oregonstate.edu/lane/sites/default/files/images/pnw0397.pdf

Sant, Laura L., Carol Hampton, Sandy M. McCurdy. “Making jerky at home safely.” Pacific Northwest Extension Publications (University of Idaho, Oregon State University, Washington State University), (2012). http://extension.oregonstate.edu/fch/sites/default/files/documents/pnw_632_makingjerkyathome.pdf