It is known that high ethanol concentrations (10% v/v) were inhibitory to the industrial yeast strains and would reduce the yeast growth and cell density in the high solids mash. However, high slurry solids saccharification and fermentation are right direction for cost effective production of fuel ethanol. To overecome the challenge, vacuum feremtation and vacuum distillation can be applied to remove ethanol produced during SSF. As a result,the ethanol concentration was maintained as negligible during the entire fermentation process, which can lead to high ethanol productivity.Combining hydrolytic enzyme hydrolysis and SSF under a vacuum allows the integration of all four unit operations (liquefaction, saccharification, fermentation and distillation) into one single step, which eliminates both substrate (glucose) and product (ethanol) inhibition of the yeast and allow very high slurry solids (~40%) ssacharification and fermentation.
The higher slurry solids and removal of water during vacuum distillation will also result in higher percentage of stillage solids at the end of the fermentation, which can be sold directly as wet grains without any need for centrifugation and thin stillage evaporation to remove water, therefore to reduce operational cost to a great extent.
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