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.
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.
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.
- 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 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:
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.
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:
- 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 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.
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.
- 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