Final Moisture content, wet basis (percent)


Drying time

2 weeks 8-14 days

2 weeks

The quantities dried are given in tons per 50 m of drying rack, it should be noted that the drying times are reduced from about 4 to 5 weeks for untreated rack dried sultanas to 8 to 14 days for treated sultanas dried on a similar rack vir.der average conditions.

* Cold Dip: 10 Kilograms (Kg) of Potassium Carbonate in 400 litres (It) of water plus 6 to 8 It of oil. The oil is a mixture of ethyl ethers of fatty acids and free oleic acids. The solution is used at ambient temperature. The maximum dipping time is 30 min,

**Hot Dip: a) 1,8 Kg of Sodium hydroxide per 450 It of water at 93°C.

b) 1,4 Kg of Sodium hydroxide per 450 It of water at boiling point. The dipping time is 4 seconds.

Practical Operation;

Number of units used in the past: approximately 2 000 Number of units currently in use: approximately 2 000

Periods of Operation:

(a) On experimental basis: 1914-1915

(b) In field operations: 1915 - to date

Operating Conditions:

It is preferable to use these racks in climatic regions characterized by hot, dry weather and gentle winds. The racks may be used under many climatic conditions, and their performance will be contingent on the prevailing local weather. Occasionally, when deemed necessary, the whble rack can be enclosed in plastic,or other sheeting,and heated air pumped in at one end so that it acts as an artificial dehydrator. The racks should preferably be aligned on a North-South axis lengthwise, in order to effectively make use of the early morning and late afternoon solar radiation, it is preferable to locate the racks on a rise of land, clear of any obstructions to permit free air flow. If many racks are used in one location, they should be at least 9 to 12 meters apart. To compensate for slower air movement near the ground, the lower racks are often given a greater tier spacing, up to 36 cm, and the upper racks spacing can be reduced to 18 cm.

Economic Details;

Cost of materials: Not reported. Labour content high.

Annual operating Expenditure: Maintenance costs almost negligible.

Cost of drying related to the Amount of Materials dried; Not reported.

Estimated Life of the Dryer: 20 to 40 years

Comments on the Dryer;

The loading of the racks with fresh fruit tends to involve a good deal of strenuous effort. In fine drying weather, there are few problems but if rain falls and is not quick followed by drying winds, the fruit can go mouldy on the racks.

Further Information: Possingham, J.V.

Chief, CSiro Division of Horticulture Research gpo Box 350, Adelaide South Australia 5001 Australia


(1) Grape Drying in Australia, Dried Fruits Processing Committee, Australia 1973,(CSIRO, Melbourne - ISBN-0-643-00053-4), 23 references, Glossary of Terms




Status : experimental and operational Heating mode: natural only

Type: vertical-tray dryer Air circulation: natural air flow

This case study presents the main results obtained from testings made on a vertical wire mesh drying system specifically designed for the drying of cassava particles. This drying system utilizes more efficiently the available energy of the ambient air to evaporate the moisture content of cassava. The drying method gives a high quality cassava that is easy to store and handle.


The vertical dryer shown in Figure 1, consists of two wire mesh panels pinned on two wooden uprights which are set in the ground and act simultaneously as supports and end walls. Top and bottom openings make the dryer easy to load and unload. The distance between the wire screen walls has been made variable in the experimental prototype so as to allow the drying space to be charged with different loading densities of cassava per square meter of exposed surface. The unit is covered with a wooden roof to protect the product from rain and to allow the drying to continue overnight. This vertical dryer was developed from experiments on several similar prototypes of smaller size. Figure 2 shows a number of these small units under testing.

Dimensions; wire mesh panels: 1,5 m wide x 2 m high wooden uprights: 2,6 m high rafters

Fig. 2. Small vertical dryers under testing

Materials of Construction:

All the wood used in the construction was locally available. The two uprights were joined on each side by three angle iron bars set horizontally. The vertical wire mesh walls are reinforced with metal crossbars to ensure an even thickness through the drying section.

Location: Centro International de Agricultura Tropical C.I.a.T. Experimental Station Cali, Colombia

Latitude: 3,5° North Longitude: 76,5° West

Climatological Data;

No general data has been given, but during the series of experiments undertaken, the following data was reported:

Relative humidity 40 to 95%

Wind velocity 0 to 4,5 m/sec

Dry bulb temperature 20 to 35°C

Solar radiation 0 to 75 cal/cm2/hr

Practical Operation:

Nunber of units used in the past: one Nuntoer of units currently in use: one

Periods of Operation:

(a) On experimental basis: April 1973 - June 1973

(b) In field operations: (simulated farm conditions: same dates)

Drying Data:

Cassava pieces with approximate dimensions of 1 cm by 2 cm of section and length variable from 1,0 to 5,0 cm were made using a disc cutter mechanism. These sizes and shapes were determined to maximize the use of wind and natural convection in the drying process. The cut particles were dipped in a 50% alcohol solution for 2 minutes before drying :

Initial moisture content: 66.7 percent wet basis on average

Preferred final moisture content (critical) : 14 percent wet basis (Fig.-3) Drying time: 3 days - at optimum layer thickness for the climatic conditions prevailing at time of experiments

Cassava chips, unless they reach 50 percent moisture content (wet basis) in the first day of drying, will deteriorate rapidly in quality. With this condition in mind, charts have been developped which indicate the quantities of Cassava which can be dried given the specific set of climatological conditions occuring on a specific drying day. The charts are presented in Figure 4.

Operating Conditions:

Cassava chips being white, have a very high reflectivity and do not absorb much incoming solar radiation. This is the reason why this dryer has been designed mainly to improve the use of the air flow (see Fig.5 ).

Economic Details:

Cost of materials: frame: $7.00 U.S./sq.m. of dryer wire mesh and frame: $4.00 U.S./sq.m. of dryer total cost: about $20.00 U.S./sq.m. of dryer, including the roof and labour

Significant reductions in coats are expected by using a more simple unit and less expensive materials. No restriction in cost was attempted in the experimental unit.

Cost of Drying related to a Unit of Material Dried:

The first tentative figures are: U.S. $10.00 per dried ton of cassava of which $2.50 corresponds to labour cost.

Estimated Life of the Dryer: 10 years

Comments on the Dryer:

. construction and operation are simple.

. Vertical racks are more difficult to fill than inclined wire mesh trays: 20 to 30 minutes are rerm-ired to load the experimental unit and 10 to 20 minutes to unload the processed product.

. Drying in vertical dryers takes about one-half the time required for floor and wooden tray sun drying. See Figure 5.

. Overexposure of the product when drying is completed should be avoided due to possible insect infestation.

. As cassava is normally grown for periods of 10 to 24 months, the harvesting during the dry season, where the drying conditions are more favourable, is recommended.

If the relative humidity on a day is higher than 75%, supplemental heat should be used to assist the drying process. Solar dryers will be the ideal complement of the natural vertical drying racks for this purpose.

. A mathematical model developed so as to predict with good accuracy the drying performances of the vertical dryer under different climatic conditions.(Figure 4).

Principal Investigator(s): Roa, Gonzola

Department de Ingeniera Agricola Universidad del Valle Apartado Aereo 21-88 Cali, Colombia


(1) Natural Drying of Cassava, Roa, G., Cock, II.J., Proceedings of the

Third Symposium of Tropical Root Crops, 2-9 December 1973, Ibadan, Nigeria

(2) Natural Drying of Cassava, Roa, G.,Unpublished Ph.D Thesis, Agricultural

Engineering Department, Michigan State University, East Lansing, Michigan, U,S .A.

Critical Moisture Content for Storage Teneur en eau maximum requise pour 1'entreposage

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