Wheat straw is an abundant lignocellulosic crop residue with potential as a feedstock for ethanol production, especially in Canada and Europe. Wheat straw hydrolysate was therefore selected as one of the fermentation feed­stocks for evaluating the fermentation characteristics of S. cerevisiae RWB 218 under industrially relevant conditions (W. de Laat, unpublished data). Wheat straw was pretreated using steam explosion (Sunopta, Canada). The pulp thus obtained was then hydrolysed enzymically at pH 5.0 with cellulases and hemicellulases, yielding a hydrolysate that contained 50 g L-1 glucose, 20 g L-1 D-xylose, 6 gL-1 arabinose and 6 gL-1 of disaccharides (cellobiose, melibiose, maltose and sucrose, indicated as DP-2 in Table 2). The hydrolysate, which

also contained 3 gL-1 acetic acid and 0.3 gL-1 of lactic acid, was supple­mented with 0.4 gL-1 of (NH4)2PO4 as a combined source of nitrogen and phosphate. Fermentations were run at 32 °C, with an initial pH of 4.8.

When batch cultures on the wheat straw hydrolysates were inoculated with 1.5gL-1 of S. cerevisiae RWB 218, most of the available sugars were con­verted within 55 h (Table 2). The yield of ethanol on the consumed sugars was very high and, towards the end of the fermentation, even approached the theoretical maximum yield of 0.51 gg-1. This very high apparent yield might partially be caused by the additional hydrolysis of some oligosaccharides or by the presence of other sugars that were not identified in the analyses. Xylitol formation was not observed.

Even when a much lower initial biomass concentration of 0.1 gL-1 was used, S. cerevisiae RWB 218 reached the same degree of conversion in 80 h. Addition of vitamins, trace elements and/or the anaerobic growth factors Tween-80 and ergosterol [2,3] did not result in a faster fermentation. This demonstrates the modest nutritional requirements of S. cerevisiae during fer­mentation of hydrolysates of lignocellulosic materials, which often contain very low levels of nutrients required for microbial growth.

7.3