Food dehydration is an ancient and natural means of preserving fruit and vegetables and making them available for later consumption. Modern fruit and vegetable drying systems often use forced air processes to speed up the drying process, reducing losses from decay during the drying process and increasing production rates.

Storing, transportation and packaging costs can also be reduced by drying. The moisture is slowly removed from the product until it is below 20%. In industrial production, the majority of dried fruit continues to be produced by sun-drying, whereas vegetables are more usually dried by using forced air, which is often heated.

Both continuous and batch processes are used for this purpose. Hot-air convection is most suited for drying fruit pieces such as pineapple, mango, or banana slices, as well as tea leaves and certain herbs. Desiccant drying is the best method for products like mushrooms, since they are very sensitive to heat.

Drying of Sweet Potatoes

Sweet potatoes need six to eight weeks of curing and storage to develop the texture and sweet taste that people associate with the baked product. This curing takes place at high (80-90%) humidity levels. Quality will be reduced when storage temperatures are low and the sweet centre of the sweet potatoes will become hard. High temperatures can result in sprouting and shrivelling, with poor, stringy and dry textures.

Sweet potato may also be dried as chips or processed into flours. Such processing has many advantages:

• It makes storage and transport easier by reducing bulk and making the product less susceptible to decay.
• It increases shelf life.
• It provides farmers with the opportunity to market their product to a wider array of customers.
• It reduces losses due to pathogen contamination and insect damage.
• It concentrates nutrients in the product by the removal of water, making it more marketable.
• It diversifies the uses of the product, improving perceptions about the commercial value of sweet potatoes.

Traditional sun-drying methods are highly dependent on weather conditions, labour intensive and generally result in a product of uneven size and quality. It is difficult to prevent product contamination and chemical changes due to enzymes that are activated during drying. In addition, drying is often not thorough enough, leading to losses through mould.

Hot air drying is ideal for sweet potatoes, making them more digestible and resulting in faster drying times. Drying rate depends chiefly on the air velocity, dry bulb temperature and the thickness of the sweet potato slices or cubes that are dried. Moisture content needs to be reduced to 10-15% or less. A high quality product will retain its nutritional value (e.g. beta-carotene content), not have darkened unduly through oxidation, have an even shape and will be free from dust or pathogens.

While solar dryers are sometimes used as they are a relatively energy efficient kind of mechanical dryer, drying times are long unless dehumidified air is used and the process is more difficult to control and the product more subject to contamination than with tunnel or cabinet dryers. Sweet potatoes are typically processed by blanching and chemically treatments before tunnel, cabinet or spray drying in large commercial enterprises to maintain product quality during drying and storage. Energy costs are greater, but temperatures, drying times and air velocity can be more closely controlled. Slices are often dried at 50-80 °C for 4 to 12 hours, although 150-160 °C for 10-22 minutes (depending on the treatment the product has undergone) have been found to result in the best quality product. Batch cabinet dryers can accomplish this task either on a small or an industrial scale.

Drum drying is one of the techniques used to do this, for producing flakes or powder from puree.

Drying of Onions

Onions must be completely dry and dormant to be ready for long storage or transportation across large distances. Bruising and infection occurs much more easily if the onions are not quite dormant, resulting in rapid decay.

In properly dried onions, the skin texture and colour should be even and several layers of it should be without obvious moisture. One should not be able to slide the neck of the onion around when pinching it and it should be dry almost to the bulb surface. “Neck rot” is a common problem that occurs in onions necks where the surface is broken and not completely dried. It is impossible to halt once it is underway.

Although onions can be dried with unheated air, heated air can dry the product more thoroughly and quickly. Between 48-72 hours of drying at 38 °C are enough. Accurate measurement of the air temperature at the surface of the onions is needed, as excessive heat will damage them.

Drying may be accomplished using fans directed towards pallet bins containing the onions, possibly with a heater to encourage faster drying. Partial recirculation of the air, with additional outside air for humidity control, can control the drying rate and keep energy costs down. Wet-bulb temperatures of around 29-32 °C result in a good compromise between quick drying and energy efficiency.

As in other drying applications, fan sizing is crucial to obtain the right balance between high airflow and energy conservation. The onions usually lose 5-8% of their weight in the drying process. A dehumidifier can help to reduce problems with damage from high temperatures and is often used during the final stage of drying. An incidental advantage of drying is that the tops and roots can be easily removed during the grading or packing stage because they become brittle.

Drying of Tomatoes

Dried tomatoes are a popular product with a concentrated flavour, a longer shelf life and lower storage and transportation costs than fresh tomatoes. The tomato variety, moisture content when fresh, ambient air temperature, velocity and humidity and the drying technique all affect the process of drying.

Traditional sun-drying takes a long time, during which product losses from decay and contamination with dust, insects and pathogens can occur. On the other hand, accelerated industrial drying at temperatures that are too high can harm the colour and flavour and cause case hardening which prevents the inside from drying out properly. Ideally, tomatoes should be dried at 45-55 °C. This results a high-quality product which retains its taste and nutritional value.

Proper airflow in the drying system helps to ensure that the process is even and consistent.