During cassava harvest season, fresh roots are plenty available and the price is low. Therefore, it is common to use them to make slurry by grinding and then mix with cassava chip. Alternatively, cassava roots are used as a main raw material and then cassava chips are used to adjust the solid concentration. Similar to dried chips, there are two processes for preparing cassava fresh roots for bioethanol production, namely "With fiber" and "De­fiber" process.

In "With Fiber" process (Figure 4), the roots are transferred to the root hopper, in which soil and sand are effectively removed by root peelers. The roots are then washed and subjected to the chopper and rasper. The puree of milled roots is then slurried without fiber removal and used for liquefaction. This process requires less equipment and investment cost and is recommended for batch-type fermentation (Sriroth & Piyachomkwan, 2010b). However, with the presence of cell wall materials, ground fresh roots has developed semi-solid like characteristic and should be slurried with water to reduce viscous behavior. This causes dilution of solid loading in a fermentor, yielding a low ethanol concentration in final beer. A pretreatment of ground fresh roots with appropriate cell wall degrading enzymes has been introduced to handle that inferior flowability (Martinez-Gutierrez et al., 2006; Piyachomkwan et al., 2008), allowing potential use of fresh roots with Very High Gravity (VHG), i. e. high solid loading (> 30%) process and resulting in a higher ethanol concentration (upto 14.6% w/w or 18%v/v) in beer (Thomas et al., 1996). By almost doubling the ethanol concentration in the final beer, the VHG process can not only minimize the energy consumed during the downstream distillation process, but also improve the plant capacity. This concept can be applied to improve fermentation of other feedstock as well.

Similar to wet milling process of cassava chips, in "De-fiber" process (Figure 5), the starch slurry is prepared from fresh roots by modifying a typical cassava starch production

process. After desanding and washing, roots are subjected to the chopper and rasper. The pulp is removed and starch is extracted by a series of extractors. After depulping, the starch slurry is then concentrated by a separator and subjected to a jet cooker for liquefaction. This process requires a higher investment cost and also generates high starch losses in the pulp. However, defiber process is more controllable and can be readily applied to current well — established technology of ethanol production from other materials. It is also practical for

applying in high solid loading and continuous fermentation process (Sriroth & Piyachomkwan, 2010b).