Impact of pre-adaption of recombinant S. cerevisiae on co-fermentation of glucose and xylose from steam pretreated wheat straw

One of the major hurdles for economic fermentative conversion of lignocellulosic biomass to ethanol is the inhibitory compounds generated during thermo-chemical pretreatment of the biomass. The inhibitory action affects both cellular growth and fermentation behavior, and thus impairs viability, ethanol productivity and overall yield. The use of more tolerant fermenting microorganisms is an appealing alternative to detoxification. Genetic engineering and adaptation provides means to obtain tolerance. Pre-adaptation, the concept of cultivating the yeast under conditions resembling the subsequent fermentation, provides a simple method of minimizing inhibitory effects. Pre-adaptation of S. cerevisiae yields increased ability to in situ detoxify and tolerate inhibitors in the medium. Since the presence of inhibitors during the cultivation inhibits cellular growth, a lower biomass yield is to be expected from cultivation on hydrolysate compared to cultivation without influence of inhibitors. Implicitly this means a higher cost for cultivation of adequate amounts of yeast for a specific fermentation capacity. The additional cost must be off-set by improved performance of the yeast to yield viable process economics.

This study investigates the impact of pre-adaptation at different inhibitor concentrations on co-fermentation of pentoses and hexoses utilizing recombinant S. cerevisiae. The aim is to find a level of adaptation that maximizes xylose utilization and ethanol yield on total sugar supplied in both cultivation and fermentation and evaluate the state of the cultivated yeasts. The results are evaluated with regard to fermentative capacity, vitality, xylose utilization, ethanol yield and by-product formation.