(Peace Corps, 1984, 175 p.)

Nutrition and solar drying[edit | edit source]


Solar drying can help maintain and improve health by providing nutrients that might not be available during times of the year when certain foods are either scarce or expensive. However, in order to retain the most food value possible, it is necessary to follow guidelines for preparation, pre-treatment, drying and storage. In this session, the participants examine the nutritional value of some foods that will be dried, and focus on the importance of retaining vitamins during the solar drying process.


To identify the nutritional value of a variety of commonly eaten foods.

To examine factors which contribute to vitamin loss or retention during the process of food preservation and storage.


Putting Food By, pp. 392-403.
Village Technology in Eastern Africa
Child Nutrition in Developing Countries
Solar Food Dryer Plans, pp. 24-26


"Nutrition and Health Considerations in Solar Drying" Handout 4A
"Temperature Factors for Food Preservation and Storage" Handout 45


Examples of a variety of locally available fruits, vegetables and herbs that will be dried later in the training.


Talk with participants who have knowledge about nutrition, and involve them in presenting the session.

Base your selection of food to be dried on the list of commonly eaten foods developed in Session 2.


  1. (10 minutes) Warm-up Activity and Introduction

Ask each participant to select one of the examples of foods and herbs you have brought to the session. In turn, each person is to describe something nutritious and delicious about the food chosen. For example: "Carrots are good for the eyes, and are delicious in soup."

When all participants have had a turn, summarize the nutritional qualities of the items described.

  1. (15 minutes) Presentation of Nutrition Information

Focus on:

  • characteristics of Vitamins A, C and 9 (the ones most likely to be lost during food preservation and storage.)
    - the conditions which would contribute to either their loss or retention

Point out that the "how" of retaining vitamins will be covered more thoroughly in later sessions.


An effective way to present the information is to use a method appropriate to community education, such as a flannel board, puppets, posters or demonstration. If some of the participants have experience in teaching about nutrition, involve them in the presentation.

  1. (5 minutes) Summary

Discuss any questions which may arise concerning vitamin loss and retention.
Distribute Handouts 4A and 49 as reference material.


Solar drying can contribute to maintaining good health, by providing important nutrients that might not be available to people during times of the year when certain foods are either scarce or expensive. However, as in any method of preserving food, there are precautions that should be taken to make sure that the food that is eventually eaten is safe to eat and of the highest possible quality.

Remember that the best foods are those that are unprocessed, fresh, local and organically grown. That is the ideal. When such foods are solar dried, they are nearly as nutritious (and can be as tasty) as in their original form. But: Remember that solar drying will not improve upon the quality of the food you've started out with. It's got to be good and clean from the start.

Food that is solar dried will retain a high proportion of its original food value, if the process is done correctly. Much of the food value will be lost if the food is exposed to too much heat, light, air and water depending on the original vitamin content. Inasmuch as solar drying depends upon heat, light and air, it is important to keep in mind some basic guidelines when preparing, as well as drying the food.

Solar drying is not a process of sterilizing food. If there were potentially harmful micro-organisms to begin with, they have a good chance of surviving the drying process. Therefore, it's important to remember: if it's not good enough to out in your mouth fresh, it's not good enough to eat when dry. The corollary to that rule of thumb is: if the equipment, utensils, trays and storage containers you use are not of food-grade quality, then neither will the food be that they touch. If you understand the WHY, then you'll understand the HON. (Please refer to Putting Food By for other nuggets of wisdom.)

Following are some tidbits about the WHY of health, hygiene and nutrition relating to solar food drying:

There are four basic causes of food spoilage: bacteria, molds, yeast and enzymes. Not all bacteria, molds or yeast are harmful: in fact, some are beneficial (as are the bacteria in yoghurt, the mold in bleu cheese, the yeast in bread or beer). As for enzymes, they are necessary for the process of ripening and maturing.

Of concern are the micro-organisms that make foods dangerous to eat. For example:

Bacteria: There is a relatively small percentage of bacteria which are pathogenic; that is, capable of causing illness. Some are extremely resistant and will survive, and even flourish in the same heat that will kill mold and yeast. The spores of such bacteria (the way they reproduce) require extreme heat and pressure to be killed. One type of particularly resistant bacteria is called Salmonella, which can survive even in frozen foods. It dies at around 60ºC (140ºF), but only if the temperature is maintained for at least 20 minutes. Salmonella is brought to us via insects, rodents, animal and human excrete and all the-things they have come in contact with, for example water and earth.

Another bacteria responsible for much food-related illness is Stapholococcus aureus. It comes from food that is prone to decomposition and has not been refrigerated. The only way of killing the Staph bacteria and the toxins they produce is to boil the food for at least an hour, or hold it at around 116ºC (240ºF) for 30 minutes. If there is any doubt about it, boil the food or dispose of it.

There is another bacteria, which although not usually a problem in solar drying, does deserve a paragraph of its own: That is C. Botulinum, the bacteria that cause botulism. These bacteria live without oxygen, so they are usually found in canned food. However, on rare occasions, they are found in other kinds of preserved Food, in the earth, and when temperatures are between 21ºC (70ºF) and 43ºC (110ºF). They are not usually found when there is less than 35% moisture content in food.

There is another type of botulism bacteria (type E) which is occasionally found in seafood. Make sure that when you are either canning, or eating canned food or seafood to watch for the following tell-tale signs:

  • leaking or seeping liquid from the container or can; inflated can; mold outside or inside; bubbles in the contents; cloudy liquid; spongy or dry looking food; the color is odd; slimy or soft consistency; foul smelling. If there is any doubt, destroy the food. Don't give it to an animal, or leave it exposed where a child or pet might get to it. Dispose of it carefully. Cooking will only destroy the bacteria, but not the spores. Foods with less acidity are the most susceptible to botulism, because the bacteria survive in an alkaline environment. Something can seem to be alright, or borderline, but if you have a doubt, throw it out. Better to waste a bit of food than suffer the consequences.

Mold and yeast: These micro-organisms are in the same family, and will be put in the same category for now. They both reproduce with spores, (microscopic seeds). They appear on food when there is sufficient oxygen and moisture. The danger is that they can produce mycotoxins and aflotoxins, both of which are dangerous and should not be eaten, ever. Don't eat moldy food, except if the mold was purposely put there, as is done with certain cheeses to give a distinctive flavor. (By the way, penicillin is made from a type of mold, so don't discount the usefulness of all molds!) Mold eats the natural acid in foods, and leaves it vulnerable to bacterial invasion. Acid foods don't mold very often, although it does happen. Mold will grow when the temperature is higher than 0ºC (32ºF). It will live in colder temperatures, but it won't flourish. The growth accelerates between 10ºC (50ºF) and 30ºC (88ºF), and will slow down and die from around 60ºC (140ºF) to 88ºC (190ºF).

Generally, some percentage of the vitamins in the food are lost, either in the preparation (blanching, sulfuring), the processing (depending on the kind of dryer used, the amount of light and heat that enter), and the form of storage used. It is possible, however, to retain a certain amount of

Vitamin C when ascorbic acid is added. When drying food, more Vitamin C will be retained if blanching is not done. Apparently, sulphuring helps retain Vitamin C and A (although it destroys the 5 Vitamins). Insofar as exact amounts of vitamin loss, there is no agreement, even among those who are considered (or consider themselves) experts. What is known is that Vitamins A, 9, and C are relatively unstable in the presence of air, light and heat

  • characteristics of the solar drying process. It is also known that it is better to eat some nutrients than none at all, and the necessary vitamins are often available in other foods
    - especially if there are fresh foods available. However, this is not always the case. So it is important to try to preserve as many nutrients as possible.


It is a water soluble vitamin and is destroyed by heat. It is necessary for the formation of blood vessels. This is why, when there is a Vitamin C deficiency, capillaries often burst, causing bleeding under the skin or from the gums. Vitamin C is found in most vegetables, especially those with green leaves, and in fruits, sprouts and mother's milk. The body requires around 60 mg/day. An orange has about 50 mg. Sometimes Vitamin C (ascorbic acid) it used to treat fruit before preserving it, to help prevent discoloration. In canning, up to 65% of the Vitamin C may be lost. If solar drying is done correctly, the loss may be as little as 10%, providing blanching is not done. The relative low heat of drying (compared to the heat required for canning) is responsible for the retention of the vitamin during the process.


This vitamin is available only in foods of animal origin, and is found in egg yolk, liver, milk and milk products, and the oil from certain fish. The source of Vitamin A in fruits and vegetables comes from carotene, a provitamin that is a precursor to Vitamin A. It is found in dark green or orange colored foods, such as papaya, carrot, leafy greens, mangoes, melons. When children are malnourished, they have a difficult time absorbing Vitamin A through their intestines, which makes their health status even more precarious. If a lactating mother doesn't have sufficient Vitamin A, the child will also suffer from its lack. The Vitamin is necessary to maintain good vision and healthy skin. It is fat soluble, and is retained and lost in a way similar to vitamin C. The body requires about 4-5000 i.u./day.


This is a group of vitamins whose major components are Thiamine, Riboflavin and Niacin. They are water soluble, and are lost to heat. They are necessary to utilize the carbohydrates in the body, and the required amount of the 5 Vitamins is in direct relation to the amount of carbohydrates that are eaten. That means that the more carbohydrates you eat (rice, potatoes and other staple foods in much of the world) the more 5 vitamins you need. Sulphuring can reduce the amount of C vitamins in fruits and vegetables (which don't have that much to begin with), and they are also lost in alkaline solutions. They are found in the germ of grains and cereals, in green leafy vegetables, milk, meat and legumes. If the body lacks sufficient B vitamins, problems may result in the nervous system, digestion and general mental and physical health.


These substances produce chemical changes that promote ripening and eventually, decomposition. They are relatively inactive in a cool or cold environment, and become most active between 27ºC and 49ºC (120ºF). Enzymes are controlled by heat; they begin to be destroyed at 60ºC (140ºF), unless the food is very acid, in which case the enzymes are more resistant to destruction. They can change the color, texture and taste of food. To inactivate them, the food may be blanched. They are not dangerous to your health.

Solar P02.GIF


100 ºC Blanch or steam for 20 minutes to sterilize

80 ºC Pasteurize for 10-15 minutes

60-66 ºC Desired range for indirect drying of meat and fish

57 ºC Pasteurize for one hour

38-60 ºC Good range for drying fruits and vegetables quickly, without severe loss of nutrients or color while still protecting from microorganisms and enzyme action

45 ºC Maximum temperature for brewery grain, seed grain and rice for food.

35-40 ºC Temperature range for drying herbs

35 ºC Maximum temperature for drying beans for human food

40 ºC Food can spoil on the drying racks below this temperature in a humid climate

20-30 ºC Fish will cook in this range in direct sunlight

20 ºC Maximum temperature for storing dried foods

Below 5 ºC Dried meats and fish should be stored to avoid rancidity

At 115-12C ºC, with steam under pressure, even the hardiest spores are killed in 15-20 minutes. Safe temperatures for canning lowacid foods

At 100 ºC (212 ºF) most mold and bacteria are killed rapidly

At 49-66 ºC heat resistant bacteria thrive

37 ºC - Our body temperature

28-36 ºC - Most common molds, yeast and bacteria grow best at temperatures in this range

At 0-10 ºC microorganisms begin to be active

  • 5 ºC - No mold or bacteria grow at temperatures below this, but most spores can survive

Solar P03.GIF


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Authors Eric Blazek
License CC-BY-SA-3.0
Language English (en)
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Aliases Solar and Energy Conserving Food Technologies 6
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Created April 14, 2006 by Eric Blazek
Modified December 9, 2023 by Felipe Schenone
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