5.11. Sun-drying of fruits, vegetables, spices, tubers and other perishable products in Tanzania

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- B.N. Makwaia


In many developing countries of which Tanzania is not an exception, more and more people migrate to towns and cities far removed from the natural sources of fresh foods. The economical growing of these perishable commodities is limited to certain seasons and localities. In Tanzania during the rainy season there is an abundant supply of vegetables and there is scarcity of the same in the dry season.

To meet the demand during the entire year in all areas, the commodities have to be preserved using different techniques. Several traditional methods are used to preserve those products the most common in Tanzania in sun-drying. Although the method is cheap, there are problems associated with it. These often result in a poor quality dried product' no control over the drying process; possible contamination of the product by dirt, rodents, animals, infestation by insects or moulds; and exposure of the product to rain and wind, which causes repeated wetting and redrying.

To improve this, some research and extension work has been done at Ilonga. This includes sorting, washing, blanching, sur-drying, storage' reconstitution and cooking. The equipment used were trays and mats, swing solar drier and box solar drier. These were demonstrated in local villages and promoted nationally through radio broadcasts and new papers. The foods worked on includes



The objective of Food Science and Nutrition Research Project N° 27 of 1967 at Ilonga was to develop on a scientific basis' traditional ways of preserving foods. The methods used by different tribes were investigated for improvement to have a simple, cheap method giving an acceptable, palatable and safe product for human consumption. Emphasis was on minimizing losses in leachable and volitile nutrients like Ascorbic acid, Thiamine, Niacin and Lysine. In addition, attempts wore made to produce nutritious food mixtures for babies using vegetable flour.


The selected materials were subjected to pre-drying operations, mainly sorting, cleaning and blanching. Cleaning wee with running water. Blanching of the green vegetables and tubers was with or without sodium chlorides The materials were dried using the following equipments:

  1. Trays and Mats: The materials wore arranged on trays and mats, and directly exposed to the sun. Drying time was recorded to be more than two days.
  2. Swing Solar Drier: Designed by ITIPAT Abidjan, Ivory Coast

This consists of a bamboo rack covered by a polythene hood and it lies at an angle of 30 with the ground level.

Maximum temperature obtained 65°C
Area exposed to solar radiation 5.8 m²
Average radiation per day 540 Cals.
Energy conversion efficiency factor 0.75
Light transmission through polithene 70%
Energy required for removal of water 600 Cals/gm
Maximum evaporation of water/day 26 kg
Total loading capacity 21 kg = 5.2 kg/sq.m

Drying time for the vegetables to be two days. The drier was best utilized by turning it east in the morning and west in the afternoon. It was more efficient than conventional sun drying.

3. Box Solar Drier: Built by TAMTU Arusha, Tanzania A box made up from 10 cm insulated walls with a double layer glass lid for light transmission.

Maximum temperature obtained 110°C
Area exposed to solar radiation 1.1 m²
Average radiation per day 540 Cals.
Energy conversion efficiency foot or 0.75
Light transmission through glass 90%
Energy required for removal of water 600 Cals 600 Cals/mg
Maximum evaporation of water per day 7 kg
Maximum loading capacity 8.5 kg = 7.8 kg/sq.m

The removal of water vapour from the drier and effective rapid drying was hampered by poor ventilation. Drying time was recorded to be more than two days. A change in design to include more ventilation was advised.



The materials dried on trays and mate were of a poor quality compared to those in solar driers. This wee because of the long direct exposure to the sun leading to nutrient losses and other undesirable physiological and bichemical changes.

a. Fruits :

Dried bananas, pineapples, mangoes and tomatoes were of reasonable quality. This can be exemplified by the analysis made on the several dehydrated local banana varieties as in Table 1.

Sample Variety Moisture Protein Fat Na Cl Ca P
  % % % % % %
Ndizi Ng'ombe 11.4 4.9 0.7 18.0 0.10 0.12
Kiguruwe Malindi 12.1 4.6 0.6 19.0 0.07 0.06
Mkonosi 14.7 4.5 0.6 20.0 0.06 0.05
Kitarasa 11.8 5.8 0.3 20.0 0.07 0.09
Mshare Mnyenyere 10.4 5.8 0.3 19.0 0.07 0.05
Mshare Mchonoa 11.8 5.0 0.2 18.0 0.12 0.06
Mzuzu 13.0 3.3 0.6 18.0 0.09 0.06
Ndizi Uganda 10.5 6.3 0.3 12.0 0.06 0.25
Mririo Mnamambo 10.6 5.9 0.3 13.0 0.08 0.11


b. Vegetables:

Several vegetables were dehydrated giving good products that reconstituted into acceptable products. Cassava leaves, cowpea leaves, amaranthus and sweet potato leaves dried to 9-12% moisture, deserve special mention. Thea e were packed in polythene bags and test marketed with success. Several ground mixtures were tried as soup mixes and were found palatable and acceptable.

The high protein contents of dried green vegetables may well mean a new source of protein in addition to their high minerals and vitamins and B.

c. Root Crops:

Cassava, sweet and irish potatoes, carrots and yams, dried to 5 8% moisture content, stored in polythene bags had up to 12 months shelf life without obvious spoilage. When cocked these products wore found acceptable and palatable. The protein content of some of these is shown in Table e II.



The preserved samples were packed in paper and polythene bags, earthenware pots, willow baskets, bottles and tine. Apart from the insect damages to the paper and polythene bags, the stored materials could be stored for a year without any sign of deterioration.



Reconstitution of the dehydrated foods have been successfully tried by soaking in cold water for 3,4 hours. The material was cooked in the same water, in the way of cooking fresh vegetables, and were found to be palatable and acceptable.

The dehydrated products were also ground into powder for enriching infant foods and cereal based products e.g. porrige, dumpling (ugali), biscuits, pancakes (chapati) and soups, These mixtures have a significant increase in the nutritional quality - especially on vitamins and minerals. These also were found to be palatable and acceptable.



Market trials of the dehydrated products were carried out and the materials could very easily be sold.



Traditional sun-drying methods are still in use. The little improvement made include the practice of the scientific pre-drying operations i.e. sorting, cleaning and balanching, and the use of mats and trays raised above ground level. This allows for air flow through the product bed and eliminates many sources of contamination.

Some recommendations on traditional preservation methods of foods aimed at producing better products have been pointed out by Dr. A.C. Mosha to despense to farmers as in Table III. More Work is being done on evaluating the quality of the indiginous dried products.



There is little scope in commercializing drying of fruits and vegetables. The industrial processor has to cope with scarce, expensive raw materials, high transport costs and the very modest purchasing power of the consumer. To get raw materials the processor has to compete with the fresh market, because there is no large-scale production specifically for processing. This has two important implications.

  1. The fresh market is on the whole under-supplied, so the price is high, and market gluts occur very seldom to run a factory.
  2. Fresh market supplies come from smallholders, and to get large quantities that one needs, the processor has to spend much money in collecting small amount a of produce from many growers. As a result the cost of raw materials at the factory gate remains high, even during periods of oversupply when the grower gets next to nothing for 0a produce.

There are, however two proposed dehydration projects; one is on onions at Lumuma and the second one on pepper at Malolo, both in Kilosa destrict.



Marketing of the litthle dehydrated products, especially the cowpea and bean leaves, sweet potato leaves, Cassava leaves, and amaranthus is good. By 1969, 5,750 tons of dry vegetables were consumed in the country. This figure might have doubled as now one finds dry vegetables selling moat of the local markets.



Sun-drying is of groat economic importance, particularly in the developing countries, whore better preservation of crops is imperative, and the method is accepted, requiring low capital investment and no energy resources. In Tanzania, there is a great need to improve the current methods of drying fruits, vegetables and tubers' both at village and industrial level. However, commercial sun-drying can only be possible when the shortage of raw materials can be turned into abundance.


Type of Food Traditional Preservation Methods Mistake Corrections
Vegetables: Boiling the green leaves and sun-drying them on a mat, ground or big flat stones.
Dried vegetable is stored in pots, guards, baskets or tins.
There is neither proper sorting of leaves, washing nor boiling to inactivating destructive elements.
Drying is not carried out effectively.
The ultimate colour of the dried vegetable is unattractive and the dried vegetable is often contaminated with duet and sand.
Pick young tender leaves and sort them properly Hash and boil them for 2-5 minutes in a weak solution of table salt or Soda. This ensures the retainment of colour. Dry the vegetable a clean mat or wire mesh or screen placed on a raised platform until the product becomes triable. Turn the vegetable often to ensure proper drying. Use a hot kiln fire or the sun for drying.
Fruits: Banana Peeling banana and sometimes washed. Bananas are halved and hung using strings to dry in the sun or on a fire. Final product cooked or pounded to obtain flour. The outcome is partially satisfactory as the product get a colored. Storing vessels allow moisture in which destroy the quality of the product.
Smoky taste is often detected.
Use well matured bananas. Trash and peel' as usual slice to obtain thin slices. Boil for 5 minutes in a dilute solution of table salt Sun-drying slowly or dry in a hot kiln. Store in moisture proof containers.
Cassava leaves: (Kisamvu) Leaves are pounded, boiled, strained and then dried. how legumes were boated. Straining leaves the product devoid of vitamins & minerals. See leguminous leaves.
After or pounding, boil in a weak solution of table salt for 10 minutes to inactivate the toxic principles especially if the bitter type of cassava is used. DO on as discussed with other vegetables. When using the veg etable (cassava leaves) boil for not lass than 1/2 an hour.
Roots: Cassava Peel, cut into half, remove the pith i.e. inner portion (with fibre) and dry.
For a bitter variety, soaking in water is carried out in dark places where micro-organisms grow and the roots become mouldy.
The mouldy dried roots are pounded into flour.
The colour of flour is unattractive.
Storage in containers which allow moisture to pass through.
Sweet cassava:

Use a mature root, wash peel and remove the pith then slice Wash and boil in water for 5 minutes.
Dry in a raised platform, hygienically.

The Sitter type:

Use a well matured root, adopt the local method but use clean and safe water and then dry hygienically.
Slice the peeled cassava, wash and boil for 5 minutes in a dilute solution of table salt.
Grind, and strain and thendry.
Grind to flour and cook not less the 2 an hour in order to destroy the toxic principles.

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