The native xylose fermenting yeasts Pichia stipits CBS 6054, NRRL-Y7124 and Spathaspora passalidarum NN245 were compared for their fermentative abilities in minimal and rich media under two operating conditions. To effectively compare performance of different strains, it was necessary to use nutritional and operating conditions that maximied ethanol production rates and yields.  Sub-optimal conditions could not distinguish the best strains from those showing poorer performance.  Fermentations were carried out in shake flasks for 96-120 hours with an initial sugar concentration of 16% xylose. On rich medium, Y7124 and CBS 6054 produced about 4.3 and 2.7 g/L of xylitol, respectively at 25˚C, 150 rpm.  However, we observed about twice as much xylitol accumulation at 30˚C, 100 rpm. On minimal medium, Xylitol accumulations for Y7124, CBS 6054, NN245 were 3.6, 1.6, and 1.6 g/L at 25˚C, 150 rpm; and 8.2, 6.7 and 1.6 at 30˚C, 110 rpm.  Richer media and lower temperatures generally resulted in higher ethanol concentrations and yields (Table 1).

Table 1. Summary of ethanol production and yields under various conditions.

Strain	Medium	Agitation	Temp.	Ethanol
		rpm	C	Conc. (g/L)	Yield (g/g)
NRRL-Y7124	Rich	150	25	58.5	0.376
NRRL-Y7124	Rich	110	30	50.0	0.322
NRRL-Y7124	Minimal	150	25	49.1	0.361
NRRL-Y7124	Minimal	110	30	44.4	0.314
CBS 6054	Rich	150	25	55.4	0.354
CBS 6054	Rich	110	30	46.6	0.328
CBS 6054	Minimal	150	25	49.2	0.372
CBS 6054	Minimal	110	30	48.0	0.315
NN245	Minimal	150	25	52.1	0.339
NN245	Minimal	110	30	43.9	0.321