3.5. VHG fermentation of ethanol by thermal and non-thermal SSF processes
The applicability of enzymatic pretreatment for viscosity reduction of fresh root mash in VHG ethanol fermentation was studied using the conventional thermal and alternative non-thermal(uncooked) SSF processes. Direct fermentation of fresh root mash with no enzymatic viscosity reduction led to low ethanol production, with a fermentation efficiency of 55–60% owing to in efficient mixing and mass transfer in the system. This condition was considered impractical as problems in flow-through processing in the fermentation facility were encountered. Fresh root mash was prepared at 32% (w/w) solid content and treated with 0.50% (w/w) of the enzyme preparation for 2 h, which resulted in the decrease of viscosity from the initial 10,785 ± 1184 to 289 ± 9 m Pa s. Monosaccharides (17 and 12 mg/g solid for glucose and xylose respectively)were detected after mash viscosity reduction by the enzyme preparation. Fig. 3 shows the change in viscosity, glucose, and ethanol contents during VHG fermentation from pretreated roots by the standard thermal SSF process. The viscosity of pretreated roots slightly increased during the liquefaction step, which was due to there lease of soluble starch in the system upon gelatinization. The viscosity then dropped during the saccharification step and thereafter,reaching a final viscosity of 216 m Pa s. The reduction in viscosity was mirrored by a reduction of solid content from 31.16% at the start of SSF to 7.19% after 96 h. Glucose increased during the first 24 h, then subsequently decreased, and was nearly completely consumed (0.34 g/L) at the end of fermentation. This was also reflected as the final total soluble solid of 12.4◦Brix. A rapid increase in ethanol concentration was observed during the first 48 h. The final concentration of ethanol in the mash reached up to 19.65% (v/v)or 15.62% (w/w), equivalent to 87.55% of the theoretical yield after 96 h of fermentation (Table 4).