The yeast, Saccharomyces cerevisiae, is the popular microorganism used for biofuel ethanol fermentation. In the production of cellulose ethanol, the yeast is added to a sugar mixture obtained after the breakdown of lignocellulosic plant biomass (by pretreatment and saccharification). Sugar mixtures from pretreated and saccharified lignocellulosic biomass often contains glucose (a 6-carbon sugar, or a "hexose") , and xylose/arabinose (both 5-carbon sugars, or "pentoses"). Generally, the yeast only utilizes the glucose in the sugar mixture for ethanol production. The pentoses (xylose and arabinose) are often left unutilized. One of the challenges for cost-effective cellulose ethanol production is the development of a S. cereviseae strain which can convert not only the hexoses (glucose) into ethanol, but also the pentoses (xylose and arabinose). In some way, a "sugar omnivore" can be considered a good ethanol-fermenting yeast. Recently, scientists from the Delft University of Technology (Netherlands) reported the use of evolutionary engineering to develop a strain of S. cerevisiae that is capable of fermenting mixtures of glucose, xylose, and arabinose with a high ethanol yield (0.43 gram per gram of total sugar). The conversion was achieved without the formation of undesirable side products, such as xylitol and arabinitol. The strategy involved "a regimen consisting of repeated batch cultivation with repeated cycles of consecutive growth in three media with different compositions (glucose, xylose, and arabinose; xylose and arabinose; and only arabinose)". The regimen allowed rapid selection of an evolved strain (IMS0010), exhibiting improved specific rates of consumption of xylose and arabinose. Details of their work are published in the journal, Applied and Environmental Microbiology (URL above)..