Some recommendations for the operation of solar-dryers
Walter E.L. Spiess
c/o Karlsruhe Institut für Technologie (KIT)
Institut für Bio- und LebensmitteltechnikKaiserstr. 12
D-76131 Karlsruhe
Germany
Donald G. Mercer
Department of Food Science,
University of Guelph
Kemptville Campus,
830 Prescott Street,
Kemptville Ontario K0G 1J0
1. General considerations
Before the decision for a dryer is formed the climatic, technical and economic
environment in which the dryer will be operated has to be analyzed. Not all dryers fit
for all produce. Fruits are in general dried for export, in such cases driers which can
handle up to 100 kg daily and more are required in order to develop a sustainable
business situation. Dried vegetables are mainly suited for local markets; in such
cases smaller dryers with a daily drying capacity up to 10 kg are sufficient. It has also
to be considered that fresh fruits and vegetable finished for drying (cutting, slicing
etc.) are subject to fast deterioration in hot climates, they should therefore be dried
during a drying cycle to a water content which allows avoiding spoilage during any
interruption of the drying process.
Some important questions are:

Which produce will mainly be dried;

Are the climatic conditions during the harvesting period favorable for drying
(sunshine duration around 6 to 8 hours)

In case fruits e.g. mango, pineapple, banana are dried for export

are the dryer capacity and the produce sources of a size to allow for a
substantial and steady market supply, furthermore
·
Is the operating team able to cope with minor technical problems and can
the team observe International quality standards e.g. CODEX STANDARD
FOR DRIED APRICOTS CODEX STAN 130-1981 Page 1 of 5.
·
Are there packaging facilities for packaging and storage facilities (dark and
possibly cool/not humid) for an intermediate storage of dried products
available?
2. The drying equipment and the drying operation
The dryer should be operated in optimal technical conditions, it should always be
positioned in full sunlight, air inlets and outlets should be free of any obstructions. Air
inlets should be protected from dust and any fumes. Trays on which the product is
positioned for drying and the whole interior of the dryer should always be clean and
free from any residues. In order to assure optimal hygienic conditions measures have
to be taken to keep especially insects and rodents out of the dryer. Special care has
to given to the transparent cover it should be clean, free of dirt and dust and tightly
stretched over the dryer frame also when the dryer is not in operation. Further points
to consider are:

It is not advisable to operate dryers when the environmental relative humidity
very high since the relative humidity of the air used for drying has a
pronounced effect on the water removal capacity of the air.

The linear velocity of the air within the drying chamber is an important
consideration for sweeping away the saturated stagnant boundary layer
around the material being dried. Without sufficient air movement, drying will be
slow which may allow time for spoilage to occur.

The temperature of the drying air should optimally be in the range of 50°C to
55°C for most fruits and vegetables. For drying herbs, 45°C is a reasonable
maximum.

Temperatures that are too high (above 60°C) may cause case hardening
which will reduce the effectiveness of the dryer and lead to future problems
with the dried material.

The dryer trays and other food contact surfaces should be made of materials
appropriate for this purpose e.g. wood or stainless steel.

When loading the fresh product on the trays it is recommended not to exceed
a thickness of about 0.5 to 0.6 cm since it allows for good removal of the water
and diffusion to the surface while giving a good thickness of finished product.
In case of products with a waxy cuticle e.g. berries it is in addition helpful to
slice the products or disrupt the outer surface otherwise.

The product should be monitored frequently during the drying process. It
should be turned at regular intervals and racks may need to be rotated and
their positions changed to enhance drying uniformity.

The direction of airflow should be across the surfaces of the material being
dried rather than from a bottom up direction through the dryer. If this cannot be
avoided, then a solar-powered fan inside the dryer may create horizontal flow
patterns (which will also address the stagnant boundary layer problems).

In cases where the material cannot be dried in a single day, appropriate
measures must be taken to handle the partially dried food for overnight
storage. This may be done by sealing off the inlets and outlets to the dryer to
prevent the ingress of humid air during the cooler night-time hours or removing
the material and storing it in another area overnight. Such measures may be
especially required when drying products with high moisture contents (such as
tomatoes with ca. 95% wet basis moisture). Those products offer additional
challenges not offered by products like apples or mangoes (ca. 84% to 85%
wet basis moistures). The drying times of products with high moisture content
will be much longer than the ones of products with moderate water content.
Relevant is also that materials with high moisture content have a lower yield
(kg of dried product per 100 kg of fresh product) compared with products of
moderate moisture content; 100 kg of tomatoes at 95% moisture will give
about 6 kg of finished dried product whereas 100 kg of fresh apples at 84%
moisture will deliver over 17 kg of dried product at 10% moisture.
3. Fruits/vegetable suited for drying
Fruits
Apples, Apricots, Banana, Dates, Figs, Lemon, Mango, Oranges, Papaya, Peaches,
Pears, Pineapple, Plumes, and many other.
Vegetable
Broccoli, Cabbage, Carrots, Celery, Chives, Corn, Garlic, Green Beans, Edible Fungi
and Mushrooms, Jalapeno, Leeks, Okra, Onions, Peas, Peppers, Potatoes, Shallots,
Spinach, Sweet Potatoes, Tomatoes, Zucchini, and many other.
Green vegetable and potatoes should be blanched before drying. Peppers and other
vegetable used as condiments are not suited for drying.
4. Drying end point.
The produces have to be dried to different water contents in order to avoid
deterioration. A good indication for a stable product is once the structure is between
a leathery and brittle; products with a sugar content should be close to leathery
where as vegetable should be close to brittle.
5. Final Remark
In case the solar dryer is part of a commercial drying operation it should be
considered to safeguard the operation by including a “classical” dryer in the train of
dryers. Under optimal conditions the “classical” dryer could be energized by biogas
generated from biodegradable waste.
IUFoST Food Security Task Force/ WELS/DGM/2012-03-02