3.4. Investigation of digestibility of concentrated SE materials
A low substrate concentration (1% w/v) SSF resulted in a good ethanol yield from SE duckweed (Fig. 3). However, the low substrate loading resulted in very low ethanol concentration (from a commercial perspective), being in the order of 0.25% (v/v). Since the ethanol concentration required for distillation is generally considered to have to exceed 4% for second generation biorefining, the substrate concentration of pretreated duckweed for SSF would have to be at least 20% (w/v) biomass since it is 35.2% glucose (Zhao et al., 2015). Thus, concentrating the substrate prior to fermentation is required. This was achieved using vacuum evaporation and would be expected to potentially remove some of the volatile inhibitors (formic and acetic acids). However, SSF of 5% and 20% (w/w DM) resulted in reduced ethanol yields of 47.7% and 18.8% (w/w of theoretical) respectively (Fig. 3, white bars). For 20% (w/v) substrate, the ethanol concentrations were the order of only a few percent. These results indicate that high substrate concentrations would severely limit the ethanol yield commercially. This would increase considerably the cost of processing, particularly downstream distillation.