Many compounds that result from the pretreatment and heating of the lig — nocellulosic material, e. g., furfural and hydroxymethylfurfural, are severely inhibitory to most microorganisms [117,137]. Detoxification procedures for lignocellulose hydrolysates are under development [134]; however, the prac­tical large-scale use of detoxification is technically complex and adds cost to the fermentation process [138]. Similarly to inhibitors, low pH is required in the industrial context both for the function of cellulases and for avoiding bacterial infections, which is a frequently occurring problem in fermentation plants [139].

S. cerevisiae is the most robust microorganism among those with po­tential for efficient pentose fermentation, but differences between different S. cerevisiae strains are considerable. Laboratory yeast strains have been se­lected with regard to properties such as biomass yield and stability under well-defined conditions [140], whereas industrial isolates have been nat­urally selected for tolerance to industrial conditions. Although laboratory yeast strains are useful to evaluate metabolic engineering strategies and to compare cellular physiology, these strains do not possess the robust­ness that is required in the industrial context. Several investigations have shown that laboratory S. cerevisiae strains are generally less tolerant to lig — nocellulose hydrolysates than more robust industrial strains [7,141,142]. In addition, tolerance and robustness varies between different industrial strains [7]. Whereas some industrial strains require detoxification of the hydrolysate for efficient fermentation [6], others are able to ferment un­detoxified hydrolysates [12,13,143]. However, it is important to note that hydrolysates prepared with different methods and from different raw materi­als contain significantly different concentrations of inhibitors as well as of the fermentable sugars, as detailed elsewhere in this volume (pretreatment and hydrolysis). Therefore, the hydrolysate to be used has to be taken into account when selecting a strain for a fermentation process [144].

5.2