The separation of the starch granules from the tuber in as pure a form as possible is essential in the manufacture of cassava flour. The granules are locked in cells together with all the other constituents of the protoplasm (proteins, soluble carbohydrates, fats and so on), which can only be removed by a purification process in the watery phase. Processing the starch can therefore be divided into the following stages:
1. Preparation and extraction. Crushing of the cells and separation of the granules from other insoluble matter (i.e.' adhering dirt and cell-wall material) including the preparatory operations of washing and peeling the roots, rasping them and straining the pulp with the addition of water.
2. Purification. Substitution of pure water for the aqueous solution surrounding the starch granules in the mash obtained in the first stage, as well as the operations of sedimentation and the washing of the starch in tanks and on flour tables, silting, centrifuging, etc.
3. Removal of water by centrifuging and drying.
4. Finishing. Grinding, bolting and other finishing operations.
This method of processing is essential in the preparation of any kind of starch. For cassava, however, because of the relatively small amount of secondary substances, the separation at each stage is performed with great ease. Whereas with maize and other cereals the grinding of the seed and the mechanical separation of the germ and the pericarp from the grain present special problems in stage 1, and the separation of protein and other constituents in stage 2 can only be accomplished with the aid of chemicals, these operations can be reduced to a minimum in cassava preparation. It is indeed possible to obtain a first-rate flour from the cassava root without special equipment by using only pure water. This makes the processing of cassava flour particularly suitable for rural industries.
Supply of cassava roots
Most starch factories buy cassava roots from growers in their neigh-borhood, directly or through agents. Some factories, however, own their cassava plantations. Modern processing plants usually contract various growers in the area to supply roots. In such situations the factory should furnish financial and technical assistance to the growers, and an agronomist should be assigned to help producers develop better production practices and to conduct control experiments for determining the proper varieties, fertilizers, and methods of insect and disease control for the area.
In many countries, prices are set on the basis of a certain starch content, with a discount or a premium for deviations from that level, which is determined according to the locality and the varieties. The starch content in the tubers is determined subjectively by the factory's representative or objectively by chemical analysis. Subjective evaluation is done by selecting a medium-size root and snapping it in two. If the tuber snaps with medium force, the crop is generally regarded as mature and the flesh will appear firm, white and dry. Such roots are considered to have the maximum starch content of 30 percent. Lowstarch flesh from immature tubers is usually slightly yellow and, although firm, has a translucent watery core. If considerable force is required to snap the tuber, it is considered to have become woody and the crop to have passed its prime.
Chemical analysis of the tubers is a truer method for the determination of starch content, but it requires a laboratory and qualified technicians.
In most cases' root weight is estimated at the farm by simple means which are not entirely accurate and consequently do not reflect the exact yield. It is therefore advisable to have special weighing bridges in the factories for recording the weights of the roots as well as of the final products.