Initially, the sole addition of yeast at 4 times the base level to 1% and 20% substrate was investigated, keeping other conditions the same (Fig. 3, light grey bars). At 1% substrate concentration the yield of ethanol was improved from 78% to over 92%. At 20% substrate concentration the yield was more than doubled from the low level of circa 20% to over 45% theoretical yield. However this was still unacceptable. Therefore the intensity of stirring was increased, and the effect of adding higher titres of yeast evaluated (dark grey bars). Extra stirring had no significant effect. However, addition of yeast at up to 50× the normal level increased the final ethanol yield to 65% of the theoretical maximum. Finally, the effect of adding preconditioned yeast at different concentrations was evaluated (Fig. 5, hatched bars). The addition of extra “SE” yeast at 4× the normal titre increased ethanol yield to nearly 70% (theoretical maximum) or 13.5% (g/g DM). Addition of “SE” yeast at 50× had no additional benefit. The enhanced levels of ethanol caused by addition of larger titres of yeast also reduced the levels of unfermented glucose (results not shown).