High Solid Loading Hydrolyzate-Tolerant Strains of Scheffersomyces (Pichia) Stipitis Exhibiting Reduced Diauxic Lag and Higher Ethanol Productivity

During the fermentation of lignocellulosic hydrolyzates to ethanol by native pentose-fermenting yeasts such as Scheffersomyces (Pichia) stipitis NRRL Y-7124, the switch from glucose to xylose uptake results in a diauxic lag unless process strategies to prevent this are applied.  Further investigation showed that ethanol concentrations around 50 g/L fully repressed enzyme induction although xylose transport into the cells was observed to be occurring.  Increasing degrees of repression were documented between 15 and 45 g/L ethanol.  Repitched cell populations grown on xylose resulted in faster fermentation rates, particularly on xylose but also on glucose, and eliminated diauxic lag and stalling during mixed sugar conversion, despite ethanol accumulations in the 60-70 g/L range.  The process strategy of priming cells on xylose was key to the successful utilization of high mixed sugar concentrations because specific enzymes for xylose utilization could be induced before ethanol concentration accumulated to an inhibitory level.  S. stipitis strains now adapted to perform in high solid loading (18%) AFEX-pretreated corn stover hydrolyzates show enhanced fermentation rates and ethanol yields, improved ethanol tolerance and enzyme inductions, and reduced diauxic lag.