Introducing dryers

FPANov2Drying is an amazing food preservation technology which has been practiced since man became a farmer and possibly even before.

 Drying preserves food by reducing moisture content of the food which slows down and stops chemical and microbiological reactions that spoil the food and make it unpleasant and or dangerous to eat. Most of the world’s cereals are dried and stored, allowing their use throughout the year. Some are dried on the plant in the field while others are harvested moist and dried before storage. Storage is managed to ensure that the cereal remains dry. On the other hand, fruits and vegetables are mainly eaten fresh and the modern consumer tends not to have a large demand for dried products.
Drying is the removal of water by evaporation. It therefore requires the energy of evaporation of the water to be supplied by an external source of energy. Depending on the cost of energy, this can be costly and negatively effect the “green” nature of food products.
On the positive side, the drying process reduces transport costs and energy use as the water removed is not transported. When compared to fresh produce, even more can be saved if processing takes place before transport as waste material such as pips and skin is not transported.
Drying foods also reduces storage cost and energy use as the products can be stored almost indefinitely at ambient temperature. There is a clear opportunity for food processors to buy fruit and vegetables when prices on the market are low.
A previous issue of FoodProcessing Africa, described electrically-heated and powered cabinet dryers and a simple sun heated box produced by Dryers for Africa of South Africa (for this, search for “dryers” on www.foodprocessingafrica.com). This edition describes the Hohenheim, photovoltaic powered and solar heated tunnel dryer produced by Innotech of Germany.
There are many other drying technologies – such as simple sun drying, the Unido hybrid dryer, natural convection external solar collector dryers, forced circulation box dryers, greenhouse dryers, grain dryers and fluidised bed  dryers.
How to choose
The question that arises: how does the entrepreneur choose which technology is the right one for his/her factory? The answer to this is complex and depends on the customer, quality, production costs and the environment.
Generally sun and solar driers have lower drying costs compared to dryers heated with electricity, fossil fuels or
biomass – which can cost $0.50/kg for dried material.
Dryers relying on the sun must, however, be carefully located where sunshine is reliable and consistent.
In these days of carbon footprint and natural foods, sun drying (where the product is prepared and laid on trays in the open) results in products that meet these demands with very low capital cost and zero drying cost. There is increasing attention on the hygienic issues associated with drying in the open air. However, much of the world’s dried deciduous fruit is sun dried, with care being taken to address the potential dangers of drying in the open.
Introducing dryers
Drying is an amazing food preservation technology which has been practiced since man became a farmer and possibly even before.
If the dryer relies on the sun it can, of course, not dry during the night or when weather conditions are unsuitable. But more important than the poor utilisation of equipment, is the fact that the drying of the fruit or vegetable is less controlled which can lead to poor quality and even batch losses.
The more expensive dryers use expensive fuel sources and drying trays housed in closed chambers, produce higher quality products and are ideal for markets where the consumer is willing to pay premium prices. A factory using these dryers, especially if they are electricitypowered, must ensure a continuous supply of energy to avoid loss in product quality due to interrupted drying.
Solar dryers dry the fruit or vegetables in a chamber with trays and use a variety of techniques to improve drying rates. Some of the more complex dryers such as those using external solar collectors and heat sinks with natural convection air flow can increase drying rates, but tend to be limited in their size and capacity.
Dryers that use solar energy when available, but are able to maintain drying using fuel are the most suitable dryers from the product quality and drying cost viewpoint – but are more complex and expensive.
The Hohenheim solar tunnel dryer
Innotech, a German company, offers a viable solar powered tunnel dryer that continues to prove itself in drying businesses and projects in many countries around the world. As discussed above, drying is often promoted as a way to use crop surpluses and to add value to fruit and vegetables.
The biggest shortcoming in these attempts is that they are seldom focused on the market and are often based on a perception that it’s easy to build a solar dryer. To successfully build a drying business requires a high quality product that will be able to compete in the market and a large enough turnover to employ the necessary staff and generate a profit for the owner. Too often attempts are made to build a business on seasonal availability using a small and therefore cheap dryer.
The Hohenheim Solar Tunnel dryer relies solely on energy from the sun – the fuel cost of drying is zero. The sun heats air, powers a photo-voltaic fan and dries the product in a metre long by two metre wide tunnel with a small cross-sectional area that gives good air velocities. The tunnel costs about $9,000 and it dries between 600kg and 800kg of material per batch. The actual capacity depends on the material, its size and shape, and the time required to dry (which will be 1 1/2 to 3 days). Businesses with a production of up to 150t/year have been based on this dryer – at this production, raw material supply becomes a major issue. The novel construction of the dryer and sizing of the fan provide a faster drying rate and a built-in control system that links the air flow to the amount of heat available for drying so that overheating does not occur and cool moist air is not drawn into the dryer when it rains or during the night.
Contact Albert Esper, Innotech, Ingenieursgesellschaft mbH: Tel +49 (0) 7031 744741; fax +49 (0) 7031 744742;   info@innotech-ing.de; www.innotech-ing.de