 |  | Preserving Food by Drying: a Math-Science Teaching Manual (Peace Corps, 1980, 218 p.) | ||
 |  | Part 2. Food dryers | ||
|  | (introduction...) | ||
| | Chapter 3. Preparing for food drying | ||
| | Chapter 4. Building food dryers | ||
|
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This part develops and applies the concepts learned in Part I. It contains activities concerned with food drying, and activities constructing food dryers.
Figure
Note: Other subjects for related work are agriculture, math and trigonometry.
Chapter 3 begins with simple food drying to determine the water content of foods. Volume, weight and surface area are measured, expanding on the food drying activity in Chapter 1. Heat, air movement and the angle of sunlight, which were introduced in Chapter 2, are explored further as students build and test small models of food dryers. As the students improve the effectiveness of their small models by modifying them, they will determine which of their designs work most effectively.
In Chapter 4, various designs of dryers are presented with some building suggestions. These dryers cover a range of designs using a variety of materials. They can be modified a number of ways to suit the local materials and climate, in accordance with what the students have learned with their small dryers. However, students may build a large dryer they design themselves, instead of depending on the designs presented.
Some trigonometry is required to build one of the dryers. The math class could work on this aspect while the science class works on another aspect or another dryer. The two classes could then build the large dryer(s) collaboratively. If this is not possible, you can make the necessary calculations and give the figures and dimensions to students interested in this dryer.
Dryers using heat from a source other than the sun are also introduced. If electricity is available, a very inexpensive model can be built, encouraging the science student-s to apply concepts in an alternative situation. Also, the students can test and compare the results of drying foods with solar dryers to the results with alternative dryers (coal, paraffin) in order to determine the advantages of various dryers.
Both small and large dryers can be built with tools the students have made. If the students have not made tools or do not have access to tools, you may want to precede some of the activities with tool building. If each group of students has a set of tools, the problem-solving activities can run more smoothly.
Getting the Community Interested
To increase the likelihood of the methods and skills being generalized, try to involve the community as you and the students gather materials for the activities. (Some items may be donated also. Are dry mild, bursar wheat, and other dry staples available in the local area?) Would a local restaurant owner give food to the science class in exchange for dried food during the season of shortage, thus improving the menu and saving money on purchases of expensive imported items? Would farmers give their extra perishable crops in exchange for having some of it dried? Are there international agencies or businesses in the area? Would they donate food or money for food for the project? What could the science class offer them?
Presenting the food drying project as a potential community service increases the likelihood of assistance, and provides a link to the community for increased education.
Advantages of Food Drying and Solar Dryers
Preserving food by drying allows for nutritious meals even in the season of shortage. Besides this obvious advantage, there are other reasons for preserving food by drying.
1. Drying preserves the vitamin, mineral, protein and fiber content of food more than methods which expose it to extremes in temperature. (See pp.162,174)
2. Dry food is often more flavorful than fresh food because even after rehydration it is somewhat smaller than its original size. The flavor is therefore more concentrated. Eaten dry, the flavor and nutrients are much more concentrated.
3. Drying costs less than freezing and canning.
4. Dry foods require very little storage space.
5. Dry foods when kept dry will remain edible indefinited. Mold and bacteria can grow only where there is water.
Solar dryers offer further advantages:
1. Solar dryers can maintain high enough temperatures to eliminate the possibility of mold or bacterial spoilage. (See p. 122) Spoilage is common with open drying methods, especially in humid climates. Also, if the solar dryer can reach a temperature of 55° C, further pasteurization is not needed. This saves time and effort. (See pp. 125, 171-2)
2. Solar energy is free and readily available.
3. Food in solar dryers is protected from dirt and animals.
4. Any insects already in the food will be killed or driven off by the high temperature in the dryer.
5. The rise in temperature inside the dryer reduces the relative humidity. The moisture in the food is carried off by the heated air moving around the food. As a result, food can be dried faster in a solar dryer than outside.
Objectives
The students will learn to identify and manipulate the variables involved in food drying:
1. Heat
2. Moisture content
3. Volume
4. Weight
5. Area
6. Humidity
7. Air movement
8. Angle of sunlight
This will be done through discussion, activities and the building and testing of small and large food dryers.
Activity II-1 WATER CONTENT OF FOODS
(See introduction to II-2, p.91 ). In Chapter 1, the students began a preliminary investigation of the water content of food. In this activity, students weigh various foods wet and dry and calculate the water content. The students should apply the ideas learned in the evaporation activities to reduce the time needed to complete the activity. They will begin also to collect information about food types to use later when the small and large food dryers are built and tested.
Each sub-group of students should have a balance to use for its studies. As discussed in Appendix G, it is also a useful learning experience to develop one's own system of measures.
By comparing the wet weight and dry weight, the students can calculate the percentage of water in foods.
DIAGRAM 32
Formula:
(Final dry weight)/(First wet weight)x100 =Food; nutrients and
fiber and some water = F
100% - F = Water content; removed by drying = W
Example:
(1 bottle)/(4 bottlecaps)x100 = 25% Food content
100% - 25%
= 75% Water content
The students also can compare the volume of food pieces wet and dry. For some food items, this would be an approximation. For example, mango slices can be put in a straight sided jar and the volume of a cylinder can be calculated for the space the wet slices occupy and the space the dry pieces occupy. For other food items which can be cut in geometric shapes, such as potato or melon, the volume can be calculated directly. Using grid sheets may facilitate this measurement.
Further Study
If math is to be emphasized, encourage your students to calculate the volumes of their food pieces using the following formulas:
|
Cube |
V = S³ |
Cylinder |
V = pR²h |
| |
S = One dimension |
|
p = 3 |
| | |
|
r=radius |
|
RectangularPrism |
V = 1dw | |
h=height |
| |
l = lenght |
Sphere |
V = 4r³/3 |
| |
d=depth | |
|
| |
w=width | |
|
As a guideline, dried fruit and vegetables have l/6 - 1/3 the volume and around 10 - 20% the water of their original fresh state.
Have soap and basins available and encourage everyone to wash their hands before handling food. Have the students clean the food also before preparing it for drying. Encourage sanitary conditions during the activities. Some discussion of the reasons for sanitation may be developed as the students work on their experiments. Note also the pre-drying procedures outlined in Chapter 8.
Materials
Vegetable: potato, cassava, carrot, celery
Fruit: banana, mango, melon
Cereal or seed: bean, rice, sorghum
Protein: fish, cheese
(Use food items available. Although it is useful to test a variety of foods to compare the water contents and drying times, it is not necessary to have one of- each food to test)
Razor or similar cutting tool
Water
Basin, bowl or bucket
Equal arm balance or other device for comparing weights
Bottlecaps, tins or similar items to be used as weights
One small dish or piece of pasteboard for each piece of food to be dried
1 cm grid sheet
Straight sided jar to calculate approximate volume
String
Ruler
Soap
Cloth: cheesecloth, gauze, nylon tulle
Each sub-group should have one set of materials. Weighing equipment and basins can be shared. Direct the students as follows or hand out work sheets.
After washing your hands, take one food sample. Check to see that your sub-group has a variety of food samples. Cut the clean food sample so that you can measure and dry it. Food for sampling should be gathered when ripe. Pieces should be selected which have no cracks, bruises or rotten spots. Bad spots should be cut out.
First, measure the cut piece. What is its area? What is its volume? Record this information.
Next, select a dish or pasteboard piece, weigh it and record the weight. Place the cut food piece on the dish and weigh them together. Record the result.
Place the food piece and board or dish in a warm place and cover it with a piece of cloth. Note the time and location. Leave it for a number of hours or until the next day. Then weigh it (food and dish) and record the weight. Wait again for some time and weigh it and record weight. Continue to wait, weigh, and record until there is no change in the weight. What is happening? Why is there no more change in weight?
Next, measure the food piece. What is its area? What is its volume? How does this volume compare with the first volume? What fraction or percentage of the first volume is the final volume?
Compare the first weight of the food with the final weight of
the food.
What fraction or percentage of the first weight is the final
weight?
How much water has evaporated from the food?
What percentage is
the water content of the food sample?
How does this food compare with the
other foods?
How long did it take to reach a final weight?
How does the
time compare with the other foods?
How long did it take to reach a final
weight?
How does the time compare with the other foods?
With the same food
dried in a different location?
Report your findings to the rest of the students. Some of the results can be presented in the form of a graph.
Figure
Activity II-2 SMALL SOLAR FOOD DRYERS
In this activity, students begin to apply the concepts learned and information gathered in Part I. This activity links with the last activity of Part I, Activity I-ll, where the students discuss and design food dryers. You can do this activity before, during or after Activity II-1 depending on the areas of student interest and what is most convenient for you.
Materials
Tins and pieces of metal from tins
Black paint, soot or black cloth
Wood
Pasteboard or wood boxes
Paste or cement or other adhesive
Thermometers
(Angle measuring device)
Equal arm balances or other devices for weighing
Razor blades, knives, glass cutter
Aluminum foil or aluminum paint
Cotton gauze, nylon tulle, mosquito netting
Nails
Glass or plastic
One type of food to slice for drying tests, preferably one that does not spoil readily.
Soap
Basins, bowls or buckets
Rulers
Hammers, saws, chisels, drill, etc.
Each sub-group should have access to enough materials to build, test and modify their food dryers until they are satisfied with the results. This activity, as others, can be conducted outside, especially if the construction noise would be distracting to other classes. The activity should continue for a number of class periods until the students have determined which models work most efficiently and effectively in the local environment. Encourage the use and modification of the designs the students developed in Activity I-11.
Have all the materials in a central location to facilitate the building. Ask the students to build a small device to dry food. Let the students know that after they have built and tested the small models, the class will build one or more large dryers. Ask the students to find out which design will dry food the fastest.
Let the groups work independently. While the students are building the small models, move from group to group to ask questions. Ask questions about the design and how it takes these variables into consideration?
How is the air around the food heated?
What way will the air move through?
What would be the best location for that model?
Will moisture gather inside?
What would happen to the device on a windy day?
What surface color will work best?
Where will the food be placed?
Can the model's angle be changed?
Some models built by students are diagrammed below:
DIAGRAM 33
Have one type of food available for cutting and drying. As each sub-group finishes building its small model, including trays or shelves (p.96) for the food, have these students prepare some food for drying. Encourage the students to wash their hands before handling the food. Ask the students to keep records on their experiments.
How did you cut the food?
What size were the pieces?
What time of day did you begin to dry the food pieces?
What was the weather like?
Where did you place the dryer?
How was the dryer placed?
What did the food pieces weigh wet? What area? Volume?
How much did the food change as it dried?
Did the drying go fast or slow at first? Or was the drying rate consistent the whole time?
How long did the drying take?
What was the dry weight? Area? Volume?
Ask the students to observe all the models being built and discuss their designs and ideas between the groups.
After the drying tests, have each group report on their design and the results to the class. From this discussion should come ideas for modification of some of the dryer models. These will then be tested. Eventually, the students should agree on which designs work best. Then have the students work on the designs of one or more large dryers. At this point, there may be some collaboration with the math and woodworking classes. Collaborating on different aspects of the same problem or project involves using time allocated to different subjects which are often taught by different teachers. When circumstances favor this arrangement, learning outcomes are often enhanced.
SUMMARY
The students now know what has to be done to food to dry it. They also know how to make the many decisions necessary to build an effective dryer, and how to test and adjust until good results are gotten.
The next activity will be building large dryers. Build the designs the students have determined are best suited for the local environment and materials available, or have them adapt a design from Chapter 4. Alternate heat sources for drying can be introduced, if desired, while the building is going on or after the students have built and tested the sun heated dryers (e.g. paraffin, coal).
The dryers presented here are samples to use after the students have experimented with small models. Some may not be appropriate for your local situation. All of them can be modified considerably. Some designs can be used with the sun's heat or with alternate heat sources. Some of the dryers can be adapted for indoor use which may be useful if there is a long wet season. All of the dryers can reach temperatures around 45 degrees centigrade and can therefore also be used to incubate yogurt, sour cream and other cultured milk products. Another modification to consider: Can a dryer be used as a chick incubator?
Food drying trays can be built with a variety of materials. (See "Food Preservation Resource Packet",) The general design is a wooden or metal frame with cloth, netting, woven wood strips or woven bamboo attached. There must be holes for ventilation. Woven wood, bamboo or baling wire trays are sturdier and can be washed and reused more than trays made with cloth. Rubbing the trays with a small amount of cooking oil keeps the food pieces from sticking and makes cleaning easier. Don't use a material such as aluminum, copper or fiberglass which can contaminate the food. Generally each 20 square decimeters of tray accommodates about one kilogram of raw food. It is best to keep the size of the trays small enough to carry easily. This way, food can be washed and cut and placed on the trays indoors and then carried outdoors and put in the dryer. Also, the food laden trays can be moved easily indoors at night or if it should rain.
SOLAR DRYERS
Three very different dryers are described in this section. Each dryer has certain advantages. The students have weighed the factors affecting dryer operation in the local environment as they built and tested the model dryers. They can therefore develop a design appropriate to the locality and the materials available, using the designs shown here as a resource for additional ideas. All the dryers are heated by solar radiation and operate with convection currents of air. The food is enclosed and is protected from dirt and pests. All the dryers reach temperatures in the optimum drying range: 40°C. to 60°C. Some dryers can reach this range even on overcast days in the colder climates.
Site
Location of the dryer should be chosen based on needs, i.e. exposure to maximum sun; little wind, enough for ventilation but not cooling.
OIL DRUM SOLAR DRYER
As the sunlight changes direction throughout the day, this dryer with its curved surface has about the same area available to collect solar energy. Once the dryer has warmed up, the heated air moves across the food inside at a fairly constant temperature as long as the sun continues to shine on it.
The sun heats the drum. The air outside the drum is warmed by touching the drum, before it flows into the drum. Two layers of plastic insulate this air from the cooler air outside. This minimizes unnecessary loss of heat.
DIAGRAM 34
Inside the drum the heated air flows past the food and gains moisture. It also gains some additional heat by touching the inside of the drum. Then it leaves the dryer through the vent at the top of each wooden end. However, the food should not receive much radiation from the drum because the food may get too hot and cook.
DIAGRAM 35
A black surface absorbs radiation best, and is also the best surface to radiate heat.
A light colored surface absorbs radiation poorly because most of it is reflected away. A light colored surface is also the poorest surface to radiate heat.*
This is the reason why the inside of the drum is supposed to be painted a light color. In your locality, it may not matter, but if you don't paint it and the food cooks instead of drying, you should then paint it.
Materials
Oil drum
Chisel and hammer or welding torch
Clear plastic
Cloth for pad under drum
Hinges for door
Paint thinner
Tacks or very small nails
Nails or pegs
Saw
Trays (see the first page of Chapter 4)
Wood for frame
Black paint or something similar
Aluminum paint or whitewash
Paint brush
Net or wire to cover vents
Pegs, wood strips and small blocks
Hammer
DIAGRAM 36
Oil Drum Solar Dryer Construction
(See diagram) Read through completely.
1. Cut the top and bottom (the ends) out of the oil drum, using a hammer and chisel or a welding torch. Cut a rectangular piece from each area on the side of the drum between the ridges as shown above.
2. Paint or coat the outside black on all surfaces that sunlight will reach. Paint the inside with aluminum paint or whitewash.
3. Measure and cut pieces of wood to make the ends, the bottom, and the sides.
4. Make the ends. Near the top of each end is one vent opening, which should be covered with netting or cloth. In one end is a door which is large enough so that trays can be put in the dryer easily. Hinge the door.
5. To support the drum, attach blocks of wood to the inside surface of each wooden end.
6. To provide a place to fasten the inner layer of plastic, attach curved strips of wood, or small blocks of wood, (at least 2 cm wide) to the inside the drum to hold the trays.
8. Nail the cloth paid onto the bottom. Its purpose is to prevent air from flowing between the drum and the bottom.
9. Attach the ends to the bottom, leaving sufficient space above the ground for air to enter freely. The bottom should be a short distance below the blocks or strips that will support the drum, the cloth pad to fit firmly against the drum.
10. Put the drum on the blocks.
11. On the bottom air vent side, beginning where the bottom is attached to the ends, attach a layer of plastic to the inside of the curved strips or blocks on the ends. Let it hang down on the side that is not cut away for the air vent. Pull the plastic smooth and attach it to the edge of the bottom. Nail the side piece on this side only to the bottom. These nails pass through the plastic you have just attached to the bottom. Also nail it to the ends.
12. Place the other side piece in position. The inner plastic is now between the side and the bottom. Nail this side to the ends only. (Most of the bottom is cut away on this side for the air intake).
13. Pull the plastic smooth and attach it to the side.
14. Attach the outer plastic sheet to the ends and to the sides. Plug any leaks.
15. Position the dryer to take best advantage of the sunlight for as many hours of the day as possible.
MUD WALL SOLAR DRYER
This dryer was developed in Tanzania. It is built in a permanent location so care must be taken to determine the best direction to position the dryer, and to locate it where it will not be disturbed. It could be built on the school grounds or somewhere in the town or village so that community residents could use it. Except for not being movable, this dryer has all the advantages of other solar dryers, making it more efficient and effective than open air drying and much cheaper than drying with alternative heat sources. It also costs very little to build, and uses building skills that most people have.
DIAGRAM 37
Materials
Plastic sleeve - all dimensions for the dryer must match the plastic sleeve so that the cover fits on the walls. One standard size has a circumference of about 240 cm and can be gotten in any length.
Nails
String
Bamboo tubes the width of the dryer (cut ventilation holes)
Lime or cement or cow dung
Hammer
Knife
Measuring tape or meter stick
Clay soil or termite hill soil or some similar material
Wood or bamboo poles as long as the dryer
Boards to cut for end pieces for sleeve top
Reeds, strips or twigs
Charcoal powder
Wood posts about 50 cm long
Sand paper or something similar
Saw
Hoe
Mud Wall Dryer Construction
1. The wood frame for the plastic sleeve is built first so that the other construction can be matched to the plastic sleeve roof.
A. Cut the end boards in triangular shapes that will fit in the plastic sleeve.
B. Cut out holes at the corners and fit and nail the poles into these holes.
DIAGRAM 38
C. Sand down all the rough edges on the frame so that the sleeve will fit over the frame without tearing.
D. Slip the plastic sleeve over the frame. Roll the excess plastic on the ends around wood poles. Draw the rolls up tightly against the ends and tie them securely.
2. The dryer walls are built next to match the dimensions of the plastic sleeve roof.
A. Clear and level the site for the dryer.
B. Mark out the dimensions using pegs driven in the earth and string drawn between the pegs. The corner angles should be 90°.
The plastic sleeve roof should overlap the outer edge of the walls by 3 to 5 cm so rain will not run off the roof onto the walls.
C. Dig holes for the poles at the corners and at 25 cm intervals. The holes should be about 25 cm deep.
D. Put the poles in the holes so that they are aligned and all the same height, which should be about 25 cm above ground level.
E. Tie or nail twigs or reeds connecting the poles at two or three levels above the ground. This makes a sturdy frame.
F. Mix the clay or similar substance to make a smooth mud. Pack this inside the spaces between the linking reeds. Smooth the mud completely over the inside and outside of the frame to make smooth walls.
G. Insert the bamboo tube vents from side wall to side wall while the mud is wet. Set them at 50 cm intervals. When the dryer is finished, the food trays can sit on the bamboo tubes above the ground to assure good air circulation.
H. Around the top edge of the wall, carve out half circles for exhaust vents. Make sure the rest of the top edge of the wall is smooth and level so that the plastic sleeve roof will sit securely on it.
I. After the mud has dried completely, plaster over the outside walls with lime, cement or cow dung.
J. Mix charcoal powder with mud or some other sticky substance and plaster the inside walls and the bottom of the dryer with this black paste.
3. Build trays to hold the food. The trays should be wide enough to be supported by the bamboo tubes. Instructions for making trays are on the first page of Chapter 4. After the trays with food have been placed in the dryer, the roof is set on top and can be tied down to pegs to secure it against the wind.
COLD FRAME CABINET SOLAR DRYER
This rectangular shaped dryer has a roof angled to catch the optimum solar radiation. The students experimented with this angle in Part I. Generally, it can be approximated if one knows the latitude of the area.
|
Latitude |
Roof Angle with Horizontal |
|
0° |
0° |
|
10° |
0° |
|
20° |
10° |
|
30° |
15° |
|
40° |
25° |
|
50° |
35° |
The double layer glass or plastic framed roof is hinged and the food trays are put in the dryer from the top.
The cabinet should be at least three times as long as it is wide to minimize shading from the sides.
The double cabinet walls and bottom are insulated with about 5 cm of thickness between inside and outside walls. Wood shavings, sawdust, dry organic matter, animal hairs and other similar things can be used for insulation. The double walls and bottom are sealed tightly after insulating.
The inside is coated with black to absorb heat.
The dryer is set on legs. There are holes along the front edge of the bottom, and along the top of the side and back walls. Convection currents or air move in through the bottom, pass by the food, and out the top.
Braces are attached to the inside of the dryer to put the trays on.
DIAGRAM 39
If math is to be emphasized, trigonometry can be used to calculate the dimensions of the end pieces once the optimum angle with the horizontal has been determined.
DIAGRAM 40
ABC-is a right triangle. Angle x is known; it is the angle for optimum solar heating. Angle z is 90°.
Angle y = 90° - Angle x
The framed glass or plastic size is known. Therefore, the hypotenuse (C) would be known.
Since:
Sin y = B/C
Cos y = A/C
Then:
C sin y = B
C cos y = A
Also, since A² + B² = C², any side dimension can be calculated from the equation. For example, if B = 16 and C = 18, then,
|
16² + |
A² = 18² |
| |
A² = 18² - 16² |
| |
A² = 68 |
| |
A = 8.25 |
As with the mud wall solar dryer, it is good to start with glass or plastic and frame the double layer. Then with the known dimensions of the lid, the other dimensions can be calculated. The glass or plastic may be available only in certain widths. The length of the lid, and therefore the cabinet, needs to be three times the width or more.
Materials
Glass or clear plastic
Nails
Wood pieces for legs and frame and braces for trays
Hammer
Black paint or charcoal paste
Hinges
Wood boards for inside and outside walls and bottom
Saw
Insulating material such as sawdust, animal hair rags, lint, rice husks
Wood strips or tape to cover the edge of the insulating material
Drill
Short pieces of bamboo or other tubes to keep insulation from plugging the vent holes in the sides and bottom
As you consider these designs for dryers, keep in mind that the students are likely to think of useful modifications of materials as well as design, in producing a dryer suitable for your locality.
After the large dryers are ready, proceed with the lessons on treatment and preparation of food for drying in Part III and begin preserving usable quantities of food by drying.
ALTERNATE DRYERS
CABINET DRYER
This dryer is the most complicated design and requires more woodworking skill than the other dryers. It can be adapted for use with several heat sources and so, can be used indoors during the wet season. This may be viewed as an advantage. It can use electricity from a hot plate or similar source, or a paraffin heater, or a coal burner, or a solar plate collector similar to the device built in Activity I-llB, to supply warm air. The heat source is below the cabinet and convection causes the heated air to move through the trays carrying the moisture from the food out through the top. It is possible to collaborate with the woodworking class to build this dryer. Note : Discuss dangers of paraffin and coal .
DIAGRAM 41
DIAGRAM 42
Materials
Wood
Hammer
Wood strips about 2 cm by 4 cm to hold shelves
Wood pieces about 4 cm by 4 cm for the frame
Hinges and latches for the doors
Saw
Ruler
Wide wood pieces for sides, back, doors and top (or bamboo or reed mats)
Trays that can slide into the cabinet
Nails
You can adjust the dimensions to fit the materials available. Generally, the cabinet dryer is about 2(1/2) to 3 times as tall as it is wide. Also, if other dryers are being built, it is a good idea to make a standard size tray that will fit all the dryers; this consideration also affects the choice of dimensions. The trays should be less deep than the dryer to allow for circulation of the warmed air so that the moisture is lifted from the food and carried out.
The walls of the cabinet can be constructed with wide pieces of wood or can be built with a number of thin strips fit tightly together and sealed or lined with plastic. Split bamboo strips, woven bamboo or reed mats can be used as walls if lined well with plastic and nailed to a wood frame.
As with other dryers, check the temperature with a thermometer placed on a tray or hanging from a tray. Adjust the heat source or the sliding top as indicated by the temperature.
SOLAR CABINET DRYER
The cabinet dryer can also be used outdoors, since the food is protected from dirt, insects, rodents and birds. By attaching a solar plate collector to the base, the air can be warmed using the sun's heat. The device used in Activity I-11B is a simple solar plate collector. Solar radiation passes through the clear glass or plastic and is absorbed by the black surface. Air moving through the collector is heated and moves up out of the collector by convection. The solar plate collector shown below uses black coated corrugated metal to absorb the radiation. The heat is then transferred to the air inside which is touching the metal. This collector has a double panel of glass or plastic to insulate the warm air inside. If only one thickness of glass or plastic is used, the air inside can lose much heat by touching the transparent material which is cooled by the air outside. In this collector, the air passes on both sides of the heated metal, as shown in the diagram.
The solar plate collector is attached to the cabinet with pegs or screws. The angle of the collector should be the angle that resulted in the best heating in the experiments in Part I. If you make its connection to the cabinet flexible, the collector can be raised or 'lowered at the base to adjust for the optimum angle.
DIAGRAM 43
LIGHT BULB DRYER
If crops mature and spoil during a cloudy time of year in your locality, and electricity is available or can be generated, this dryer may be useful.
It is simple and inexpensive to build. It could also be kept in the classroom and used for pasteurizing food which has been dried in the sun in solar dryers that do not reach pasteurizing temperatures. This small dryer reaches a range of 55° to 60° C. after heating for a period of time. (This dryer was adapted from an Organic Gardening & Farming magazine design.)
Materials
50 watt light bulb
Cardboard or wood to make two boxes
Aluminum foil or aluminum paint
Hammer
Metal sheet cut to fit top of box or a cookie sheet that fits the box
Light cricket, base, cord and plug
Small nails
Knife or scissors
Screwdriver
Saw
Chisel
Black paint and brush or black cloth and cement or a smoky flame for coating the bottom of the metal sheet with soot.
1. Paint the bottom of the metal sheet black, or cement black cloth to the bottom or cover the bottom with soot.
2. Line the inside of box with aluminum paint. One could use other shiny substances to line the box; the object is to create reflective surfaces to focus the light and heat on the black bottom of the metal sheet which absorbs the heat.
3. Notch the box or cut a small opening at the base of the box to pass the electrical cord through to the electricity source.
4. Set the bulb, socket and base in the bottom of the box.
5. Make holes in the metal sheet and place it on top of the box.
6. Make a pasteboard or wood box with no bottom which is almost as large as the tray. It should have holes along the lower edges of three sides. The top should be adjustable to control the flow of air leaving the box.
DIAGRAM 44
One 50 watt bulb is adequate for 15 to 20 square decimeters of tray. If a larger area is set up for drying, use six 50 watt bulbs for each square meter of area. the depth can vary considerably; with the bulb placed 20 to 25 centimeters below the sheet, the dryer will heat from 20° to 30° C within an hour. After four hours, the temperature will be between 55° and 60° C.
Rub the sheet (tray) lightly with oil before putting food slices on it. Each 20 square decimeters of tray space holds about one kilogram of raw food. Drying time averages from 8 to 12 hours. This may vary with the relative humidity. Place the top box on the dryer. If the dryer is used in a breezy location, make sure that the top box is placed over the tray so the side with no holes is facing the breeze to prevent the flow of air from being too rapid, Have the students experiment to see how controlling the flow of air over the tray affects the drying rate. If there is concern about insects or dirt, cheesecloth can be draped over the upper box, covering the air holes.
The cost of using this dryer can be calculated using the following formula:
(watts/1000 X energy prices)/(square decimeters of drying space) = cost per hour per square decimeter of drying area
For example, in the United States, electricity costs about 5 cents a kilowatt hour, so using this dryer for 12 hours costs about 3 cents. This dryer activity can be tied to other electricity studies. Encourage your students to compare this dryer's effectiveness and cost with other dryers they build.
SUMMARY
The students now have the training and, if they have built one or more large dryers, the equipment to preserve significant amounts of food by drying.
Part III provides the students with sufficient background in nutrition to realize that including a variety of foods in one's diet is worth making an effort. There are also reference tables and practical guides for designing good diets.
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