3.5. Improvement of ethanol yield of SSF at 20% substrate concentration
Two other possible reasons for the decrease in ethanol yield at higher concentration were (a) an increase in the concentration of inhibitors in the fermenting broth and (b) the high viscosity of the material. The former could reduce yeast performance whilst the latter could reduce diffusion of enzymes and products of hydrolysis and fermentation resulting in locally high concentrations and possibly negative feedback inhibition. The level of inhibitors present in concentrated material was certainly of an order which would be expected to impact on fermentation by yeast (Fig. 5; Elliston et al., 2015). Previously, we demonstrated that yeast could metabolise the inhibitors and adapt to their presence (Field et al., 2015). Furthermore, a previous study on enzymatic saccharification (Elliston et al., 2014) indicated that increased agitation may be beneficial for enhancing the ethanol yield from SSF of concentrated cellulosic biomass. Therefore the effects of adding larger quantities of yeast, preconditioning yeast on SE liquor, and stirring the reaction mixture more vigorously (300 rpm) were assessed.