Dryer

October 16, 2008 [CASSAVA DRYING]

MALAWI INDUSTRIAL RESEARCH AND TECHNOLOGY DEVELOPMENT CENTRE

STATUS OF CASSAVA DRYING IN MALAWI

A SURVEY REPORT

PREPARED BY:

R W Mkandawire & J L TauloMalawi Industrial Research and Technology Development Centre

P O Box 357Blantyre

Tel: 01 986 859 01 847 845Fax: 01 847 844

Cell: 08 329 037 05 334 164

e-mail: [email protected]/[email protected] /[email protected]

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mailto:/[email protected]

mailto:[email protected]/[email protected]


MALAWI INDUSTRIAL RESEARCH AND TECHNOLOGY DEVELOPMENT CENTRE

REPORT ON STATUS OF CASSAVA DRYING IN MALAWI

ACKNOWLEDGEMENTThe survey team is indebted to IITA who considered MIRTDC to undertake this work. Further thanks go to FAO who financed this work. Managers of all visited sites, Universal Farms, Thyoka Cassava Processors, Masimbe Starch Factory, Mafayo Kalima Investments and Masinda Starch Factory, and the people that were assigned to attend to the Survey team deserve our heartfelt thanks.

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SUMMARYCassava is a crop that has a lot of potential in the packaging, food and wood industries. In Malawi, the

crop has been used as a staple food for the lakeshore areas and as a drought relief crop in other areas.

Industrial use for the crop is a new drive and is yet to be established.

Drying has been identified as the biggest challenge in increasing industrial production for the crop. A

survey to identify areas requiring improvements in the drying technologies presently in use in Malawi

was initiated by IITA with funding from FAO. MIRTDC was contracted to undertake the survey. The

survey covered five processing plants where cassava flour and starch are produced. The visited sites are

Universal Farms at Njuli, Thyoka and Masimbe in Lilongwe, Mafayo Kalima at Nkhamenya and Masinda

in Nkhota kota.

The survey established that all plants visited produce cassava flour/starch at less than five tons a day; a

production rate that can best be described as micro. Solar drying is the dominant drying technology in

use in Malawi. It was, however, established that the existing solar driers were designed without

consideration for optimizing harnessing of insolation and drying air. It is, therefore recommended that

new designs be developed and tested that would consider the omissions. Another observation was on

the need for training of the personnel used in drying. The importance of spreading the flour/starch to a

specific quantity per square metre and regular tumbling to enhance even drying; and, for chips, to have

regular sizes was, in most cases, not considered.

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TABLE OF CONTENTS

ACKNOWLEDGEMENT.................................................................................................................................2

SUMMARY...................................................................................................................................................3

TABLE OF CONTENTS...................................................................................................................................4

ACCRONYMS and ABBREVIATIONS..............................................................................................................5

1.0 INTRODUCTION...............................................................................................................................6

2.0 CASSAVA DRYING ............................................................................................................................7

3.0 FINDINGS AND DISCUSSION OF RESULTS.........................................................................................9

4.0 CONCLUSIONS AND RECOMMENDATIONS....................................................................................13

References.................................................................................................................................................15

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ACCRONYMS and ABBREVIATIONS

FAO Food and Agriculture Organization

IITA International Institute of Tropical Agriculture

MIRTDC Malawi Industrial Research and Technology Development Centre

PVC Polyvinyl Chloride

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1.0 INTRODUCTIONCassava also called manioc or mandioc or juca or yucca and its scientific names of manihot esculenta or

manihot utilissima or manihot aipi is a crop that has very high industrial potential in packaging, wood

and food industries. In Malawi cassava has been grown mainly as a food crop, used as a staple food in

most lakeshore areas and as a drought relief crop in the uplands. The industrial use of it has been very

limited. Current efforts by IITA and other stakeholders has seen a widening interest in industrial use for

cassava. However, for the crop to be utilized in the industry there is need for processing it into flour

and/or starch. Drying is cited as the bottleneck in cassava processing.

IITA-Malawi has worked on a number of solar drying designs that it has disseminated at some places in

Malawi. In a bid to increase knowledge and develop an optimum dryer for Malawi, IITA-Malawi

approached MIRTDC for a joint effort in the development of such a dryer. This work was commenced to

help MIRTDC review the existing dryers so that it can meaningfully participate in the development of the

Malawi cassava dryer.

This work was commissioned with funding from FAO Malawi.

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2.0 CASSAVA DRYINGCassava drying aims at reducing its water content to less than 15%. The recommended water content

varies from the type of final product ranging from 9% to 15% (Wenlapatit, 2004, International Starch

Institute, 1999, and IITA, 2005). IITA (2005) cites four factors that influence drying of cassava (chips,

flour and starch). These are temperature, airflow, humidity and tumbling frequency.

Cassava drying is best done at temperatures between 40 and 60oC, at temperatures of about 60oC

gelatinization of cassava starch sets in (FAO, 2008). Wenlapatit (2004) recommends the same drying

temperatures, however adds that when the water content drops to 35-40%, the drying temperature can

be raised to 170-200oC without gelatinization setting in.

While drying, hot dry air needs to flow through the dryer so as to pickup moisture from the products

being dried. On a hot summer day humidity is low hence the dry atmospheric air can be used for

effective drying. However, the drying potential can be enhanced through pre-heating/drying of the

drying air. The preheating may be imperative in the wet season.

When drying, the cassava is placed on a non-perforated material, usually plastic sheet or concrete floor,

thus products at the base will not dry; hence frequent tumbling helps expose the products giving even

drying.

The IITA (2005) identify four methods of cassava drying, namely natural, artificial, rotary and flash

drying. They state that Natural drying, also called sun-drying, is where cassava mash is spread on a

plastic sheet and exposed to the sun for drying. They cite a number of drawbacks inherent in this

method of drying. Some of the drawbacks are susceptibility to damage and contamination and slow

drying rates. An improvement to the natural drying is the solar drying.

Artificial drying, the IITA (2005) state, is where a controllable source of energy is used for drying

operations. Further classification to artificial drying is brought about by the source of energy used in the

heating of the drying air. Such energy sources include electricity, biomass, solar, other renewable energy

sources and fossil fuels.

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The rotary dryer consists of an insulated drying chamber that is rotated by a diesel engine, or any other

rotary power. Heat to the dryer is supplied by either charcoal or gas. This type of dryer has been tried

and proven in Nigeria.

The flash drier is a well insulated dryer with a semi-automated feeder. The product being dried is placed

on stainless steel trays.

Solar energy or radiant solar utilizes energy from the sun. Insolation reaching the earth ranges from

6800kJ/m2/day for northern Europe to 23,000 kJ/m2/day for arid regions near the equator (Hinrichs and

Kleinbach 2002). For Malawi, average insolation is 21,100kJ/m2/day (7hr-day). This energy can be put to

various uses including drying. Insolation is short for incident solar radiation, hence the maximum

radiation can be enhanced if the receiving surface is incident to the sun rays. Thus insolation received at

a particular location on earth’s surface depends on latitude, the season, the time of day, and the degree

of cloudiness (Hinrichs and Kleinbach 2002). Whereas the season, time of the day and cloudiness are

variable, the latitude for a particular place remain constant. Insolation is optimum if the receiving

surface is tilted at an angle equal to the latitude of the site, where the surface faces North in the

Southern hemisphere and South in the Northern hemisphere. Malawi lies in between latitudes 10 and

20o.

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3.0 FINDINGS AND DISCUSSION OF RESULTSA team of Messrs R W Mkandawire, JL Taulo, (of MIRTDC) A Nthonyiwa and Graham (of IITA) set out on

a survey of cassava flour and starch drying plants in Malawi from the 7 th to 9th October 2008. The survey

covered Universal Farms at Njuli, Nthyoka Cassava Processing Plant and Masimbe Starch Factory in

Lilongwe, Mafayo Kalima Investments at Khamenya in Kasungu and Masinda Cassava Starch Factory in

Nkhotakota. All the visited plants can be classified as micro plants; Wenlapatit (2004) classifies starch

plants as large-sized if they produced more than 200 tons per day, medium 100-200 tons per day and

small producing less than 100 tons per day. All the visited plants talk of production of less than five tons

per day.

The plant at Universal Farms dries starch using a solar dryer and sun drying. The solar dryer is made

from clear plastics all round; the walls as well as the roof. Drying trays are made from plywood, the

edges reinforced by softwood. A clear plastic is laid on the plywood so that the drying starch does not

come into contact with the wood. The drier is poorly designed as it has no vents, hence denying the

dryer of drying air circulation. The dryer, acting as a greenhouse, is likely to raise temperatures beyond

60oC on a hot summer day, hence posing a gelatinization danger.

The dryer at Universal Farms is neither tilted nor facing North, hence is not designed for optimum

insolation utilization. Whereas this factor may not be critical in the hot summer months, it may be

important during the wet and winter months when insolation is low. Lack of insolation optimization was

observed in all other dryers except at Masimbe Starch Factory.

In sun drying, Universal Farms dries starch pulp on a PVC coated cloth (tarpaulin). The tarpaulin is not

porous, thus drying is achieved through moisture evaporation. Since the pulp is at 75% moisture

content, 700g of water need to be evaporated to dry one kilogram of pulp to 15% moisture content. The

latent heat of vaporization for water is 2260 kJ/kg, thus to evaporate 700g, 1580 kJ will be required.

Thus, if some water was allowed to drain before or during drying, the drying would have been

enhanced.

The spreading of the starch would affect drying. Spreading at all factories visited was done unevenly by

hand. This resulted into some areas on the drying racks being heaped too high, as high as 20mm.

International Starch Institute (1999) recommends spreading chips at 10-15kg/m2 and tumbling

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periodically, preferably every 30 minutes. If drying racks were designed to have shoulders not exceeding

five millimeters, supplied with a doctoring rod or blade, the problem of over-heaping may be addressed.

Figure 1. Drying starch, Universal Farms (October 2008)

The dryer at Thyoka is also made from clear plastic, but unlike the dryer at Universal Farms, this dryer

has vents to allow drying air. The problem with this dryer, however, is that the position of both the inlet

and outlet for the drying air are above the drying trays. Since hot air rises, the positioning reduces the

contact between drying air and the products being dried, hence affecting heat transfer and the drying

process. This design was repeated at Mafayo Kalima, the only difference was that the dryer at Mafayo

Kalima has a pipe vent and concrete floor. The concrete floor can act as a heat bank, thus extending the

drying effect through heat radiation after the sun has set. In addition, unlike the other processors,

Mafayo Kalima Investments dewaters its cassava pulp by pressing the pulp with a screw press before

placing it in the solar drier. The dewatering removes excess water before drying, hence hastening the

drying.

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The dryers at Thyoka, Mafayo Kalima and Masinda have smaller inlet vents than outlet vents, a situation

that would affect the raising of the temperature. It remains to be analysed whether this was due to an

attempt to keep temperatures within the drier to below 60oC. Ordinarily, solar driers would have

restricted outlet vents so that as much heat remains trapped in the drier as possible. This also enables

the drying air to absorb the moisture from the drying products before escaping.

Figure 2. Cassava Drier at Thyoka (October 2008)

Masimbe Starch Factory does not have a solar dryer, per se. The Factory practices sun drying on

specially constructed drying racks. In order to optimize insolation, the racks face North at an angle of

7.2o. The facing of the racks to the North is correct, however, the inclination angle is wrong as latitude

for Lilongwe is 13o59’ south.

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Figure 3. Starch Drying, Masimbe Factory.

The dryer at Masinda Cassava Starch Factory is of slightly different design to those at Thyoka and

Mafayo. Here the dryer has its inlet vents at the bottom of the dryer and the outlet vents at roof level.

The sides are of both clear plastic and cardboards painted matt black. Since the drying air passes

through the starch being dried, it is expected that drying is more effective in this dryer. The contribution

of the matt-black cardboards to drying is not quantified as a study comparing similar designs where the

difference is just the boards has not been done. Drying racks in this dryer are made of aluminum.

Aluminum, being a good conductor of heat, would pass more heat to the items being dried, however the

contact between the aluminum and starch is limited, most of the starch being spread on plastic covered

plywood.

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4.0 CONCLUSIONS AND RECOMMENDATIONSThe survey established that solar drying is the most common cassava drying process in Malawi, all the

processing plants are either sun drying or using simple solar dryers. Solar drying needs to be encouraged

as it is already accepted as a norm. The driers in use, however, can be improved if the sizing and

positioning of the inlets and outlets were done properly. In addition, insolation needs to be optimized by

properly inclining and positioning the solar collector. Better results, especially in the wet season and

winter, can be achieved by preheating the drying air. The preheating can be provided by solar or

biomass. Adequate biomass is generated at all plants using wet cassava, a raw material at all the

surveyed plants, except Universal Farms. Universal Farms uses dry cassava chips. The biomass can be

densified into briquettes, a practice already in place at Masimbe Starch Factory. The use of biomass

would help in a bid to establish zero-emission plants that are heralded for their environmental

friendliness.

The drying racks need to have a calculated size that would enable those drying acquire even spreading

and recommended loading per square metre. A depth of five millimeters or the one that can give the

recommended 10-15 kg/m2 with a doctoring rod would make adherence to specifications easy. The

operators also need to be taught on the importance of turning the drying cassava. Drying is even when

tumbling is done every 30 minutes or at reasonable intervals.

The processors need to be supplied with moisture metres in order not to overdry their flour or starch.

Thermometers, preferably digital thermometers are also necessary so that when temperatures in the

dryers go beyond 60oC, the operators should reduce the temperatures. The design of the dryer should

allow for temperature adjustments. The existing driers do not provide this important facility.

MIRTDC should be mandated to design and install a prototype dryer at a site selected by IITA (preferably

Nkhota-kota) to assess the kind of improvements recommendations made in this report can have on

cassava processing.

Investigations can be made on utilization of liquid wastes that seem to be an environmental hazard at

Masinda. Using the waste water for washing incoming cassava or fermenting it to produce ethanol are

some of the options that can be investigated. Production of biogas is another option.

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ReferencesFAO. (2008). Cassava Processing. [Online] Retrieved October 16, 2008, from www.fao.org

Hinrichs R. A. & Kleinbach M. (2002). Energy- Its Use and the Environment. Thomsun Learning. New York.

IITA (2005). Cassava Starch Production. Integrated Cassava Project. Nigeria

International Starch Institute (1999). Cassava. Science Park. Arrhus. Denmark

Wenlapotit S. (2004). Manufacturing Process Development in Thai Cassava Starch Industry. Cassava & Starch Technology Research Unit. Kasetsart University.

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