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The drying of foods

Foods have been preserved by drying for more than 3000 years – it is one of the oldest technologies known; potatoes in the Andes, cereals in Egypt and dates and figs in the Mediterranean. At the simplest level, all that is required is the energy from the sun. At a higher level, there are several interventions that require either the concentration of the sun’s energy in a solar collector, or use of alternative energy
sources, such as the burning of fuel or biomass.

Drying is used to remove water from foods for two reasons:
  • to prevent spoilage by bacterial activity and the growth of mould
  • to reduce the weight and bulk of food for cheaper transport and storage.

It is important to distinguish between the two different reasons for drying:

Preservation drying is the drying of an agricultural product to prevent food being spoilt. There is no value being added to the product. A good example is the drying of cereals.
Image
Figure 1: Drying apricots in Northern
Pakistan

Product refinement drying involves value being added to the product, for example dried fish, fruit and vegetables.

There are many ways of drying agricultural crops including sundrying, solar drying and the use of artificial driers. The choice of drying method depends on, amongst other factors, the final end-use of the product and the prevailing environmental conditions.

Sun drying

Advantages of sun drying

Sun drying, the use of the sun’s energy to dry agricultural produce without the use of a special structure, is the most widely practised form of drying in the world because it is cheap, easy and convenient. This type of drying is often practised at the domestic level, to preserve foods – often fruits and vegetables – for consumption later in the year.

The simplest method of sun drying is to lay the produce on a suitable surface in the sun. For example, rice is often dried on the metalled roads in Bangladesh and Vietnam; large flat rocks are used to dry apricots in Northern Pakistan (Figure 1); in Nepal vegetables are dried under roof eaves and in Zambia fish are dried on hut roofs. Sun drying obviously works best in a hot dry climate with gentle breezes.

Disadvantages of sun drying

Even though sun drying requires little capital or expertise, and can produce a product of sufficiently high quality, it has many limitations:
  • A slow and irregular rate of drying: which can increase the risk of spoilage
  • A high final moisture content; which increases the risk of spoilage, especially in humid areas.
  • The product quality is variable; some of the produce will be over-dried; some under-dried.
  • Contamination with dust and dirt and infestations by insects is likely
  • Theft is hard to prevent
  • Sudden rainstorms can soak the produce
  • The produce is prone to damage from animals and birds
  • Direct sunlight will destroy vitamins and can remove flavour.
  • Large land areas are required

Solar driers

Advantages of solar drying

This method helps to speed up the drying process, especially when the weather is not ideal. Solar drying is only for the purpose of adding value to foods, rather than purely preservation for home consumption.

Solar driers operate by raising the temperature of the air to between 10°C and 30°C above room temperature. A solar drier increases the drying power of the sun and protects the crop from dust, dirt and insect attack. It is also waterproof, and the food does not therefore need to be moved when it rains.

There are three basic types of drier, each of which has many variations in design:

Tent drier

This type consists of a ridge tent framework, covered in clear plastic on the ends and the side facing the sun, and black plastic on the base and the side in shade (Figure 2). A drying rack is placed along the full length of the tent. The bottom edge of the
clear plastic is rolled around a pole, which can be raised or lowered to control the flow of air into the drier. Moist air leaves through holes in the top corners of the tent.
Image
Figure 2: Tent drier

Cabinet drier

The basic design is a rectangular box, covered with clear glass or plastic and insulated with wood shavings, sawdust, coconut fibre, dried grass or leaves, at least 5cm thick. There are holes in the base and upper parts of the box to allow fresh air to enter and moist air to leave. The inside of the cabinet is painted black to act as a solar collector. The length of the cabinet is approximately three times the width to prevent shading by the side walls.

Indirect drier

This is a modified cabinet drier in which a solar collector is attached.

Disadvantages of solar drying

Solar drying is popular with development organisations and research stations. However, there are few small-scale solar driers that are yet operating economically. Supporters of solar drying point out that it costs as little as $10 to construct a cabinet solar
drier in Bangladesh. However small-scale food processors have a deep understanding of the socio-economic environment in which they work. Solar drying has often been introduced to solve a specific technical problem without looking at the wider socioeconomic context. The small-scale processor’s reasoning can be summarized as follows:

A higher quality product does not necessarily increase its value

  • There is no incentive for producers to risk investing in a drier if people are willing to pay nearly the same amount for discoloured or damaged foods.
  • It may not be necessary to achieve export quality for sale in rural areas.
  • Local preferences may favour sun-dried products.
  • Some indications of reduced quality, such as minor mould growth, are not easily identifiable, so a lower quality product can have the same value as a properly dried product.

The amount of the crop lost is often exaggerated by the agencies working with the farmers.

  • The amount of food lost in traditional drying is often over estimated as people remember the worst cases rather than the average cases.
  • Driers are only needed in villages if the weather is unsuitable for traditional methods.
  • Other methods are available to preserve the food if it rains during harvest, for example the harvest can be delayed, small amounts of food can be dried over a kitchen fire, or mixed with dry crop etc.

Image
Figure 3: Drying herbs

Artificial driers

Advantages of artificial driers

An artificial drier can dry crops during bad weather and can provide more control of the drying process. These use fuel to increase the air temperature, and get rid of moisture using electric fans, so are mainly used by small entrepreneurs rather than at
household level. They give close control over the drying conditions and hence produce high quality products. However, they are more expensive to buy and operate than other types of driers. In some applications, where consistent product quality is essential, it is necessary to use mechanical driers.

Conclusions

  • Sun drying is often practised at the domestic level, to preserve foods – often fruits and vegetables – for consumption later in the year (Figure 3).
  • Solar drying is only for adding value to foods. For people drying food crops throughout the year, it is unlikely that the investment in any form of drier will be economically viable.
  • The decision to invest in more sophisticated forms of drier, which require an energy
source, will depend on the type of product to be dried and its potential market value.
Last edited by Miriam Hansen .
Page last modified on Friday August 13, 2010 11:32:36 GMT.
  • A practitioner's journal on household energy, stoves and poverty reduction.



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