Innovative Drying Technology for Small-scale Cassava Industries From sun drying to steam drying Top 20 Innovations that Benefit Smallholder Farmers
Summary For cassava producers, value addition is important both to increase the shelf-life of the crop and to make new products which fetch more income. Such value-added cassava products include fermented cassava flour (normally referred to as fufu flour), and the unfermented High Quality Cassava Flour (HQCF), which is gaining market popularity for its suitability as a wheat flour substitute. However, the need for an efficient drying technology is a major constraint in the flourmaking process. Traditionally, the production of cassava flour relies on sun-drying, which is very slow, inefficient and impractical for year-round drying in areas where rains fall for a large part of the year. In the sunny season, sun-drying is typically limited to one batch per day at most. In the wet or rainy season, it may take between 3 and 5 days to dry the same batch. This puts a severe limitation on the sun-drying method where premium is placed on volume of output and quality of products. High-end dryers such as Flash dryers require regular supply of electricity, making them expensive to acquire and maintain. Hence they are normally outside the range of the rural farmer. The innovation The objective was to provide smallholder cassava processors with a drying technology that does not depend on the sun or dry weather and is also: suitable for a rural area, uses locally available energy sources; fast enough to meet market demands of supply of high quality flour; hygienic, to prevent food contamination; genderfriendly and labour-saving; and good for business, in terms of assured daily output to a reliable market. The steam dryer was designed to help farmers respond to trade opportunities. In Sierra Leone the steam dryer is currently used by seven smallholder cassava processing centres in the north, east and south of the country.
How to use the technology The steam dryer consists of a boiler (or steam generator) with conducting rods welded inside the base that spread heat quickly through the water. In operation it can be connected to up to two double-compartment drying cabinets. The latter may be inside a building, whilst the boiler is situated outside, the two being connected by copper pipes, as shown in Fig. 1. The boiler can hold up to 250 litres of water. A three-sided insulating housing is built around the boiler to minimise heat loss during operation, thereby reducing firewood or charcoal consumption. For operational safety, a pressure release cap is fitted on the boiler. This rises to let out steam when the pressure is high, and drops back to its rest position as the pressure returns to normal. The boiler is filled to three-quarters capacity (about 180 litres) at the beginning of the working day and placed on its platform. Fuel wood or charcoal is added at the bottom and lit. With the fire going, it takes about 1 hour to produce steam at the pressure required for it to be fed into the copper coil in the drying cabinet. Whilst waiting for the steam to be produced, the cassava pulp is sieved and spread thinly on the drying trays and placed on the shelves in the cabinet. The cabinet doors are then closed. With steam circulating smoothly through the coils, it normally takes about 90 minutes for the cassava to dry. Fig.1 Arrangement of the steam drying system Outside Wall of building Copper pipes Inside
The drying process Fill boiler to three-quarters capacity before a fire is lit underneath it. Thinly spread pressed cassava pulp on aluminum sample trays and place on shelves in the drying cabinet. Copper coils take steam from the boiler and circulate it through the chambers, releasing heat which dries the cassava. Excess water is driven off by an air current through narrow horizontal vents at the top and bottom of the doors. Take the dried contents out of the cabinet and mill into flour. Hot distilled water is collected as a by-product of the drying system. Components of the drying system (a) The steam generator The steam generator consists of a drum made from 3 mm thick stainless steel plates. Metal rods inside the boiler aid the flow of heat through the water. Each steam generator can run two drying chambers, thus doubling daily output. The generator is normally located outside the building, with the drying chambers inside; the connecting tubes are passed through the wall. (b) The drying cabinet The heating elements are fabricated from rolls of copper coils (Fig. 2). Each roll is made of flexible copper tubes 15 m long, with an outside, having outside diameter of 7.9 mm. The coils are opened out to optimise the transfer of heat to the cassava pulp. The heating element of each shelf consists of two
rolls of copper coils connected in series. The sets of coils are linked together within the wall such that the steam is fed through one inlet and goes out through one outlet. Constructed from timber and plywood with aluminium lining the walls, the drying cabinet looks very much like a double-door domestic refrigerator, having shelves to hold the food trays during processing. A glass window in each door allows users to observe the dryness of the contents without having to open the cabinet for inspection. Moisture settles on the glass, which becomes clear when the contents are dry and ready for removal. A capillary attached to the outlet. The mixture of steam and water from the inside pipe is sent vertically through the capillary. Further condensation of the steam takes place, thus providing a column of water that acts as an impediment for exiting steam/water mixture. This added pressure provides the necessary delay of the steam in the cabinet, thus making the process more effective. Finally, hot water comes out of the capillary tube. A suitable thermometer can be installed to measure the inside temperature of the cabinet. The cabinet with the capillary is shown in Fig. 3. (c) The food trays For the level of cleanliness demanded in the food industry, it is important that these be of rust-free, heat conducting material such as aluminium. Thin (1.5 mm) sheets of aluminium are cut and edged to make the food tray, with each tray measuring 0.72 m wide, 1.10 m long, and 0.02 m deep. A typical drying cabinet has shelves for 22 trays, arranged in two columns of 11 in each partition of the cabinet. Each tray can hold up to 2.5 kg of grated cassava pulp. Fig.2 Heating copper coils (or elements) and drying trays within the cabinet Fig.3 The drying cabinet, closed and open to show the food trays. The delay capillary is shown on the outside.
Use and care of the steam drying system Mount the steam generator on its platform. The latter sits inside the three-sided concrete insulating chamber. Add fuel wood or charcoal. Fill the steam generator at the start of the working day to the three-quarter mark with water. A dip stick is normally provided for this purpose. Close the lid tightly, making sure that the pressure release cap rests vertically on its supporting tube. Make fire underneath the steam generator. With the fire lit, it takes about 1 hour for steam to be produced at the pressure necessary to go through to the drying cabinet. Always ensure the cabinet is clean at the beginning of each working day. Clean the cabinet of cassava and other debris at the end of each working day. Give the inside of the cabinet a fresh clean before use, if it has been left unused for a week or more. Use the food trays ONLY for the intended purpose i.e. drying cassava pulp in the cabinet. DO NOT place items in them for sun drying. Always clean the food trays after use and store in the cabinet, with the doors closed. Recommendations All materials used in the process must be hygienically clean and rust-free. Recommended metals should be stainless steel, aluminium, or copper. For regular supply of fuel wood, it is recommended that the processing centre plant fast growing trees to be harvested as required. Day round operation of the drying system requires a constant energy supply that can best be achieved with electrical power. For this purpose a second generation dryer is proposed that uses solar panels linked to batteries and inverters as an alternative.
System specifications Item Steam generator Drying cabinet Food trays Dimension/ output 0.38 m diameter 0.20 m tall Capacity 250 litres 1.25 m length 1.25 m height 1.50 m width 0.72 m width 1.10 m length 0.02 m deep Quantity per unit Materials used 1 3 mm stainless steel plates, with mild steel for conducting rods 1 Timber and plywood lined with aluminium 1 set of 22 trays Aluminium
For more information: Contact: Samuel Nonie Tel: + 232 78 394079 E-mail: enonie1@yahoo.com Contact: Braima James E-mail: bdamajames.sl@gmail.com Cover illustration: Geert Gratama www.cta.int @CTA 2016