The first step in the pre-separation process of starchy root or cassava tuber is to remove the adherent soil from roots by washing in order to prevent any problem later caused by the soil and sand. The process is followed by disintegration of cell structure to break down the size mechanically (i. e. milling) or thermally (i. e. boiling or steaming) or by combination of both processes. Slurry will be produced from the disintegration process which contains a mixture of pulp (cell walls), fruit juice and starch. This slurry can be cooked directly to gelatinized starch. When it is required, it can also be separated to produce flour by exploiting the difference in density using hydrocyclone and/or centrifuge separators as presented in Table 1.

For direct fermentation from starch to ethanol, there are two techniques normally employed in preparing starch medium which are non-cooking and low-temperature cooking fermentation. In non-cooking technique no heating is required however an aseptic chemical or method may be required to avoid contamination. Since it is uncooked, some aeration or agitation may also be required to avoid sedimentation of the starch particle. In low-cooking temperature fermentation, the medium is either semi or completely gelatinized first prior to inoculation of fermenting microorganism. Gelatinized starch forms a very viscous and complex fermentation media. It contains nutrients that required by microorganisms to grow and to produce different fermentation products. During fermentation, various physical, biochemical and physical reactions take place in the media. The nature and composition of the fermentation media will also affect the efficiency of the fermentation process. Many difficulties in designing and managing biological processes are due to the rheologically complicated behavior of fermentation media. Due to that, a pseudoplastic of a non­Newtonian behavior of starch solution is essential for cooked or gelatinized starch. This pseudoplastic property of gelatinized starch is important because it has suspending properties at low shear rates and its viscosity becomes sufficiently low when it is processed at higher rates of shear. Any fermentation medium which does not apply any viscosity reduction agent such as enzyme, its viscous nature combined with non-Newtonian flow will affect the mass heat transfer, dissolved oxygen homogeneity, mixing intensity, cell growth rate and eventually, the product accumulation state. Thus, it is imperative to minimize the viscosity to eliminate these problems. Starch slurry or flour concentration, temperature, agitation speed and cooking/gelatinization time are the major factors affecting media preparation. Optimization study of these conditions is useful prior to single-step fermentation of consortium or co-culture microorganisms. Table 2 gives the gelatinization temperature for different sources of starch. This information is helpful to prepare cooked or gelatinized starch for direct bioconversion at low temperature cooking.