7.1

From the Laboratory to the Real World: Strains and Media

Successful expression of XI in S. cerevisiae enabled further engineering for high-yield production of ethanol from D-xylose under anaerobic conditions. D-Xylose fermentation rates reported for S. cerevisiae strains based on the

Piromyces sp. E2 XI were, in principle, sufficiently high for industrial imple­mentation. However, the studies on these strains that have hitherto been cited in this review were all performed under “academic” conditions. These in­volved the use of defined synthetic media controlled at pH 5.0 and, perhaps most importantly, the absence of inhibitors that are characteristic for real-life plant biomass hydrolysates [31,37,49,54].

The S. cerevisiae strains expressing the Piromyces sp. E2 XI are based on the S. cerevisiae CEN. PK platform. Interestingly, preliminary tests showed that the parental strain CEN. PK113-7D demonstrated an almost similar performance in industrial-grade molasses compared with industrial bak­ers’ yeast strains. Moreover, deletion of the GRE3 gene (which encodes a non-specific aldose reductase, [66]) was not detrimental for performance in molasses-based industrial fermentations (W. de Laat, unpublished data). Therefore, trials to test the glucose/xylose fermenting strain S. cerevisiae RWB 218 [44] were initiated in both wheat straw and corn stover hydrolysates. Results from these fermentation trials will be briefly discussed below.

7.2