Your school garden has fulfilled its promise of plenty, and the harvest has begun. Perhaps you're celebrating abundance by feasting on your delicious produce. But what happens when the harvest and feast are over?... When the plant's life cycle ends, or frost or heat preclude the garden's ability to bear, and the leftovers from your cornucopia are ready to eat NOW?
This is a question our ancestors faced and answered by developing methods to preserve food. Wherever circumstances and urges have taken us — into a winter in early North America or on a space shuttle flight — we've had to eat! By exploring preservation methods in the school garden, both ancient and modern, students can appreciate the climatic and survival challenges faced people in different places and eras. Older students can examine the chemistry and economics of different types of food preservation. What's more, the class can use preservation techniques to turn the school garden harvest into creative gifts and/or fundraising products.
The oldest method of food preservation is drying. The edible seeds of many plants (grasses, beans, and sunflowers, for instance) naturally dry as a part of their life cycles. Many of these items can be dried in the classroom; drying items from the school garden in the classroom contribute academically and are also a source of beauty. Ancient peoples, through trial and error, discovered that other foods, when dried, remained edible for longer. Though we now know that dehydration works because microbes and enzymes that cause spoilage and decay can't do their jobs without adequate moisture, the basic requirements for drying are the same as ever. It takes dry air passing over food to dehydrate it to the point where it will not spoil in storage.
Consider enabling your students to appreciate one of the original "mothers of invention" by delving into one of the food drying methods described below. You might choose a "low-tech" method, such as air drying herbs in the classroom, or engage students in actually constructing a food dehydrator (or using a commercial one).
This type of project can also be a springboard for exploring the history, chemistry, economics, and culinary possibilities for other types of food preservation, such as pickling and canning.
- For Air-Dried Herbs: Scissors, rubber bands, large paper bags
- For Sun-dried Fruit: Knife, cutting board, non-metal tray, cheesecloth, oven
- For Solar Food Dehydrator: Black construction paper, glue, scissors and/or box knife, wooden dowels (at least 3/8" diameter), plastic needlepoint mesh (12" x 18", available at craft stores) or rigid plastic grid (check hardware stores), tape, cellophane wrap, meat or candy thermometer. Choose a sturdy cardboard box approximately 24" long x 18" deep x 18" tall
- For Preparing Produce for Drying: Cutting board; knife; plates; large pots and bowls; kitchen colander or sieve; plastic freezer bags, jars, or plastic containers with tight fitting lids; hotplate, steamer, or stove (optional)
Simple Drying Methods
Primitive dehydration methods — air and sun drying — are still effective and easy ways to preserve herbs, seeds, and some fruits. The drier and sunnier your climate, the better. These methods offer a simple way to introduce students to the principles of food drying.
1. Have students pick healthy, good-quality stems of garden herbs. Harvest them just before flowering for best flavor. If necessary, wash herbs and allow them to air-dry before bundling. Bind the ends of six to eight stems with a rubber band.
2. Tie a brown paper bag around them to shield them from light. Leaves that touch the inside of the bag may stick and dry poorly, so make sure there's plenty of room.
3. Poke holes in the bag for air circulation, and hang it in a warm, dry room. After herbs dry in 1 to 2 weeks, remove the dried leaves from the stems and use them for cooking, teas, and sachets. Your younger student growers might explore such questions as, How do the flavors and aromas of fresh and dried herbs compare? How might each type affect the flavor of spaghetti sauce?
If your daytime temperatures reach 90° F by noon, and the humidity is less than 60 percent, you can try drying blueberries, raspberries, and other local fruits in the sun.
1. Rinse and thoroughly drain the berries. Spread them on a towel to absorb as much of the rinse water as possible, then spread them one berry deep on clean tray.
2. Position the tray in direct sun and cover it with cheesecloth. Secure the cheesecloth under the edges of the tray so it will not blow off. Turn fruit once each day. Bring the tray in at night and keep it in a cool, dry place. Berries are considered dry when they are hard and rattle when shaken on the tray. This should take about 4 days.
Because insects can come in contact with sun dried foods, it's a good idea to pasteurize your fruit before you store it. See pasteurizing in the Safety Caution! section below. Store berries in containers with tight-fitting lids. Enjoy them in the classroom and consider packaging and sending some home as gifts. Have students compare the appearance, texture, and flavor of the dried berries with their fresh counterparts.
Constructing a Simple Food Dehydrator
It can take 3 to 4 days of hot, dry weather (85° F and humidity below 60 percent) to dehydrate a juicy vegetable like a tomato.* If you live in an area with cool and/or humid summers, you're better off finding a design that generates extra heat and air flow or purchasing a commercial dehydrator. Try to involve your class as much as is reasonable in the planning and building process.
1. Measure 8 inches up from the bottom of your cardboard box and draw a level line across the front. Draw a line along the sides from your first line up to the back of the box and then cut along them as shown. Glue pieces of black construction paper to the inside surfaces.
2. Cut a hatch about 6 inches square in the back for access and use a piece of tape to stabilize the door as illustrated. To enable air flow, poke vent holes in the sides and bottom.
3. Cut your dowels slightly longer than the length of the box. Make holes about 8 inches up from the bottom in the ends of the box to fit the dowels tightly, and slide them through. Wash and dry the needlepoint mesh or plastic grid, cut it to fit if necessary, and set it on the dowels.
4. Tape the thermometer to the inside wall of the dehydrator. Cover the top angled face of the box with plastic food wrap and tape it in place.
5. Place your prepared fruit, vegetable, or herbs onto the mesh tray via the back hatch, and close it securely to prevent insects from entering. Prop up the dehydrator on boards or other supports so air can enter through the holes in the bottom. (If insects find their way into the dryer, you can cut a piece of cheesecloth or window screen and tape it over the holes on the outside of the box.)
6. Face the dehydrator into the sun, turning it several times a day to capture the most direct rays. (This offers a great opportunity for lessons on the sun's angles and Earth's movements.) Move the dehydrator inside at night.
7. Have students keep track of the inside temperature and stir the food occasionally to speed drying. They should observe the produce daily and note changes in characteristics such as color, texture, and odor.
* Temperatures inside a food dryer should be at least 95° F for drying herbs and edible flowers and 120° F to 140° F for drying vegetables and fruits.
Bacteria and mold can grow in foods that are incompletely dehydrated. Err on the side of caution and over dry the food rather than leave too much moisture in it.
Conditioning. To prevent mold growth during storage, you must condition food first. This equalizes the moisture content throughout the pieces of dried food. Pockets of moisture in stored foods are sites where toxic molds and bacteria can grow.
To condition dried food, put it in a clear container on a cool, shady shelf for one week. Check container daily; if moisture condenses on the inside, the food is not fully dehydrated. Return it to the dehydrator for another round, or heat pasteurize it.
Pasteurizing. You can use either of these methods to pasteurize dried food:
1. Deep Freeze. Place food in freezer bags, and store for 48 hours in a freezer set at 0°F.
2. Heat Treatment. Preheat oven to 160°F. Spread produce no more than 1 inch deep on a cookie sheet and heat in oven for 30 minutes.
How does this simple design hasten the drying of food? An angled face catches the sun's rays more efficiently than a flat surface. The black interior surface absorbs and intensifies heat inside the box, drying the air and, along with the holes, creating air flow. As the air warms and rises, it pulls the heavier, cool air in through the lower vent holes. Challenge students to envision (and perhaps create) other designs for a simple dehydrator that provides the same types of conditions. (They might modify a cold frame, for instance.)
Preparing Produce for Drying
Whichever method you use, you'll need to prepare your school garden harvest for drying. A food's moisture content and chemical makeup affect how it should be prepared. (For instance, dipping certain types of fruit slices in ascorbic acid or concentrated lemon juice before drying reduces the browning that exposure to air can cause.)
Rinse produce and towel it dry before cutting it. Refer to the chart below to find out how to prepare and pretreat various fruits and vegetables. Some vegetables should be blanched (partially cooked) prior to dehydration. This destroys enzymes that diminish color and flavor of the stored food.
Boil one gallon of water for each pound of vegetables to be blanched. (Alternately, you can use a steamer or electric rice cooker to blanch the vegetables.) Have a large bowl of very cold water handy. Slip the sliced vegetables into boiling water for the suggested time; drain and cool immediately in the bowl of cold water. Allow them to fully cool before patting the slices dry and placing them in the dehydrator.
Storing and Using the Dried Harvest
If your class doesn't polish off your dried produce right away, you'll want to safely store your treats. Determine that the food is fully dry using the dryness tests described in the chart below. Next, condition the food as described in the Safety Caution section above. Pack it in jars with tight lids or freezer bags. Press the air out of the bags, or suck it out with a straw, and seal them. You should consume the dried harvest within a year.
Reconstitute vegetables by soaking a cupful in 1 1/2 to 2 cups of water until they plump up. Bring fruits back to life by pouring boiling water over them and letting them soak for 5 minutes before draining them. To take advantage of all the nutrients and vitamins, use the soaking water in your recipe!
Preparing the Harvest for Drying
|Apples||Peel, quarter, and core; slice into pieces 1/8" to 1/4" thick||Mix ascorbic acid solution (2 1/2 teaspoons per cup of water) and dip slices in it||Soft and leathery; can't squeeze juice from cut slice|
|Beans||Cut in short pieces||Steam 3 minutes OR boil 2 minutes||Very dry, brittle|
|Cantaloupe||Choose ripe melon; wash, peel, and remove seeds; slice 1/4" to 1/2" thick||None needed||Leathery and pliable; no pockets of moisture|
|Carrots||Slice 1/8" thick||None needed||Tough or brittle|
|Corn||(See "pretreatment")||Blanch whole ear for 4 minutes; cut from cob||Brittle|
|Onions||Slice 1/8" to 1/4" thick||None needed||Very brittle|
|Peas||Shell||Steam 4 minutes OR boil 3 minutes||Hard, wrinkled, and green|
|Peppers||Slice 1/4" to 1/2" thick||None needed||Tough to brittle|
|Summer Squash||Slice 1/4" thick||None needed||Leathery to brittle|
|Tomatoes (paste or drying varieties best)||Dip in boiling water to loosen skins; peel and slice 1/2" thick||None needed||Crisp|
|Winter Squash||Cut into chunks; slice into 1/8" pieces||Steam 3 minutes OR boil 1 minute||Tough to brittle|
Consider asking the class to compare selected fresh fruits or vegetables with their dried counterparts (grapes and raisins or fresh tomatoes and sun-dried, for instance). Encourage them to describe their items' respective appearances, aromas, and flavors. If students find that the items' weights vary, ask them to consider the possible explanations.
To dig deeper, ask, What other changes in our produce might have occurred in the drying process beyond the color, texture, and other qualities we could observe? How might we find out? Their research may reveal that the nutrient content changes during processing. Vitamins A and C are destroyed in drying. Although blanching reduces these losses, it leaches water-soluble B-complex and some vitamin C into the water. Calorie content by weight is greater in dried foods because they are more concentrated; fresh produce contains more water. (Students may have noticed that some of the dried produce in grocery stores is brighter than their school-dried goods. That's because many commercially dried fruits have been treated with sulfur.)