Tuesday, May 1, 2012
Napoleon Ballroom C-D, 3rd fl (Sheraton New Orleans)
Co-fermentation of glucose, xylose, and cellobiose is critical for bioconversion of lignocellulosics. Pretreatment and enzymatic saccharification result in a mixture of glucose, hemicellulosic sugars and oligosaccharides. Their sequential utilization extends fermentation time and can result in incomplete substrate consumption. Simultaneous co-fermentation of such sugar mixtures is problematic because most microbes use glucose preferentially. Surprisingly, we have found that the yeast Spathaspora passalidarum can co-ferment xylose and cellobiose in the presence of glucose. This could be highly advantageous in simultaneous saccharification and fermentation (SSF) processes. We examined the co-fermentation of xylose/cellobiose mixtures, or glucose/xylose/cellobiose mixtures (100 g/l total sugars in each fermentation), in duplicate bioreactors under oxygen limitation. In the absence of glucose, xylose and cellobiose were metabolized at essentially similar rates until xylose was depleted. In the presence of glucose, co-utilization of cellobiose and xylose were delayed, but all three sugars were co-utilized at similar rates and sugars were consumed by 68 h. The maximum ethanol production rate from xylose and cellobiose was 1.07 g/l·h and from all three sugars was 0.73 g/l·h. Ethanol yields during the phase of maximum production rate were 0.43 g/g and 0.42 g/g for xylose/cellobiose and glucose/xylose/cellobiose mixtures respectively. The capacity for co-fermentation of sugars makes S. passalidarum attractive for industrial bioconversion processes.