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Use of solar energy for preservation of agricultural products

Solar energy has been used for time immemorial by man for his energy needs. The intensity of use of solar energy also increased as human race progressed through the initial ages when sun was put to use not only for warmth, but also for productive applications.

Drying was perhaps the most important of such productive applications. Drying of food grains, fruit and meat for storage, drying of timber and firewood, drying of earthen utensils, etc. were some such productive applications where sun’s energy was put to effective use.

We can say that food drying is a very simple ancient skill. It requires a safe place to spread the food where dry air in large quantities can pass over and beside thin pieces. Sun is often used to provide the hot dry air. Dry, clean air including dry cold air from any source will dehydrate food. Draping food over branches or spreading it on wide shallow baskets on the roof is an old widespread tradition still in use around the world. Many other arrangements have been used to support a thin spread of food pieces. Some options that have been used are to thread the pieces on a cord or a stick and hang it over a fire, wood stove or from the rafter. Or one can bundle herbs or strawflowers and suspend them from bushes or a door knob or nails in rooms with good ventilation.

But traditional sun drying methods often yield poor quality, sine the product is not protected against dust, rain and wind, or even against insects, birds, rodents and domestic animals while drying. Soiling, contamination with microorganism, of mycotoxins, and infection with disease-causing germs are the result. They assert that the drying equipment used in industrialized countries overcomes all of these problems, but unfortunately is not very well suited for use in developing countries because it requires substantial investments and a well-developed infrastructure. They further maintain that solar drying facilities combine the advantages of traditional and industrial methods, namely low investment costs and high products quality.

Dried foods are tasty, nutritious, lightweight, easy –to-prepare, and easy –to-store and use. The energy input is less than what is needed is needed to freeze or can, and the storage space is minimal compared with that needed for canning jars an freezer containers.

The nutritional value of food is only minimally affected by drying. Vitamin ‘A’ is retained during drying, however, because vitamin ‘A’ is light sensitive, food containing it should be stored in dark places. Yellow and dark green vegetables, such as peppers, carrots winter squash, and sweet potatoes have high vitamin ‘A’ content. Vitamin ‘C’ is destroyed by exposure to heat, although retreating foods with lemon, orange, or pineapple juice increase vitamin ‘C’ contact. Dried foods are high in fibre and carbohydrates and low in fat, making them healthy food choices. Dried foods that are not completely dried are susceptible to mould. Microorganism are effectively killed when the internal temperature of food reaches 145 degrees Fahrenheit (F).

Dehydration of vegetables and other food crops by traditional methods of open-air sun drying is not satisfactory, because the products deteriorate rapidly. Furthermore, traditional methods do not protect the products from contamination by dirt, debris, insects, or germs. A study demonstrates that food items dried in a solar dryer were superior to those which were sun dried when evaluated in terms of taste, colour, and mould counts. It asserts that solar drying system must be developed to utilize this energy resource to improve food preservation.

Drying is basically a heat and mass transfer process where liquid water from the surface and form inside the grains is vaporized, mixed with drying air and vapour mixture removed. The drying air provides the heat. When drying foods, the key is to remove moisture as quickly as possible at a temperature that does not seriously affect the flavour, texture and colour of the food. If the temperature is too low in the beginning, microorganisms may grow before the food is adequately dried. The food may harden on the surface. If the temperature is too high and the humidity is too low, it makes it more difficult for moisture to escape and the food does not dry properly. Although drying is a relatively simple method of food preservation, the procedure is not exact.

In industries, dryers are used for drying of powders, seeds, cloth, paper, yarn and a large number of other products. The basic objective of drying operations in industries is to remove the moisture which has come into product during the process of manufacture. This is necessary to make the product suitable for marketing and also for increasing the shelf life. A majority of industrial dryers use steam/electricity for hearting operation required for removal of moisture.

Some dryers also use fuels like kerosene and diesel for hearting the air which is to remove the moisture. Most of these dryers work at temperatures ranging between 60 and 70 C. Many dryers temperature limits are specified at even lower value so that products which are heat sensitive are not spoiled because of over heating. It is also known that most of these dryers work at efficiencies between 30 and 6 per cent in comparison to even low efficiency of open sun drying. This shows the enormous, potential available for saving of conventional fuels be conversion, wherever possible, to solar energy based drying operations.

Use of solar energy for drying in industries can also offer advantages like better quality of the product, less pollution and freedom from unreliable supply of oil, electricity and coal. In fact in many small scale industries it has been seen that quick drying is the bottle-neck operation in increasing the productivity. These industries have then to resort to open sun drying of their product which not only requires space but also increases drying time and reduces the productivity.

Drying preserves foods by removing enough moisture from food to prevent decay and spoilage. Water content of properly dried food varies from 5 to 25 percent depending on the food. Successful drying depends on:

(i) Enough heat to draw out moisture, without cooking the food;

(ii) Dry air to absorb the released moisture; and

(iii) Adequate air circulation to carry off the moisture.

Now we look in agriculture in our country where a substantial quantity of our country’s grain and fruit production is wasted because of inadequate and improper storage facilities. Growth of insects and fungus which are the two main reasons for spoilage of these materials can be prevented to a large extent by ensuring reduction of moisture contents to safe limits before storage. Extensive research has been carried out on the moisture requirements of various types of food material before they can be stored for long term. Table shows the recommended values of final moisture content of various food materials for long term and safe storage. (See Table-1)

The drying of food material is primarily a low temperature operation as higher temperatures are likely to result in destruction of nutrients and flavour of the food. Recommended values of drying temperatures for various food materials are given in the table. A study of these temperatures reveals the reason why solar energy is being considered as appropriate sources of energy for drying.

It is well known that solar energy collection is most efficient at temperature up to 70 C. While use of conventional fuels at low temperature ranges is a most inefficient way of using these high grade energy sources. Various types of dryers have been developed at a number of institutes for use, especially for agricultural products. These dryers have been used for drying of seeds, paddy, various types of fruits, chillies and a variety of other vegetable products. The capacities of these dryers range from a few kg/day to a few tones/day. (See Table-2)

Various agricultural products require reduction in their moisture contents for storage purposes. Moisture contents of Paddy are oil seeds, corn etc. are reduced for safe storage. Dehydration of vegetables and fruits is a cheap method of storing these products for off-season method use. Similarly; tea leaves, tobacco leaves and coffee also require drying.

Traditionally solar energy has been used to dry agricultural products by spreading them on the ground in open sun. This method of open sun drying is very time consuming. Also the product gets contaminated by dirt and insects. The use of solar dryers overcomes these problems to large extent.

Solar dryers make use of solar heated air for drying agricultural products. Solar dryers developed so far are of two types;


(1) Forced convective solar dryer

The solar dryer consists of solar air heater of flat plat type and a drying chamber. The air heater forms the roof and drying chamber is placed in an enclosure formed under the air heater. The air heater is connected to the drying chamber through an air blower. The blower sucks air through the air heater and blows it through the drying chamber. A schematic of the system is showing figure. The solar air heater (flat plate type) has air flow depth of 2.5 cm.

It consists of four sheets in parallel with total area of 28m2. the drying chamber has been designed for manual loading and unloading of product to be dried. Depth of grains can be up to 40cm.


(2) Natural convective solar dryer

Natural convective solar dryer consists on three different types

* Cabinet dryer

* Multi-rack dryer

* Mini multi-rack dryer


Cabinet dryer

It is a singe rack natural convective dryer. It is a wooden hot box with glass at the top. The produce to be dried is placed on a perforated tray. Holes are provided for natural convective flow of air.


Multi-rack dryer

In the cabinet dryer the dry air comes in contact with the product only once. This air has still capacity to remove more moisture if it could be passed through additional layers of product.


Mini Multi-rack dryer

A small sized multi-rack dryer in which 5 trays at 5 different levels in vertical direction can be placed, may be designed for house–hold use.

These solar dryers are very low cost even a middle class farmer can also made them at a low cost few hundred rupees and can use them for long years. Use of solar dryers for agriculture purpose in developing countries like Pakistan has a wide scope as water for irrigation is available for us for a few days on counting system and we have a limited time to save our crops after harvesting and to sowing next crops. This time shortage problem can be overcome by using low cost solar dryers because they dried the crops, which need 7 days for drying, in 7 hours.

Another factor is very important to remember (which is one of major reasons to use solar dryers) is our regional weather, which is severe and not reliable and destroys a substantial our food, vegetables and crops. If you want to become a modern cultivator and want to earn a reasonable capital by saving y our time and crops, then these dryers are not out of reach.



Harvesting corn/Fruits/vegetables etc.       Initial M.C.     Final M.C. after dehydration

(%)    (%)

Cereals & grains       16-30  10-12

Pulses 20-22  9-10

Oilseeds        12-14  7-9

Vegetables     60-80  10-12

Fruits  60-90  10-12

Tea leaves     50      8

Coffee beans 50      12

Note: M.C. stands for moisture contents




Vegetables     Optimum       Fruits  Optimum

          Temperature (co)     Temperature (Co)

Bitter Gourd   65-70  Apples 60-65

Bringer          50-55  Apricot 55-60

Cabbage        60-65  Bananas        50-55

Carrot 65-70  Grapes          65-85

Cauliflower     60-65  Peaches        60-70

Garlic  55-60  Pears  60-65

Okra (lady finger)     65-70  Beers  60-80

Onion  60-65  Pomegranates          50-60

Peas   60-65  Figs    50-60

Potatoes        60-65  Dates  60-80

Pumpkin        65-70

Spinach         65-70

Tomatoes      60-65

Turnip 50-55

Groundnut     40-45  Seed purpose – should not be more than 40oC.

Chillies 40-45  Flower perfume 45oC

Tobacco        30-35

Tea     30-35

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