Acid-pretreatment of lignocellulosic biomass for ethanol production usually destroys the tight lignin-cellulose matrix of the biomass, and results in the liberation of simple sugars which can then be yeast-fermented to ethanol. There are undesirable by-products, though, that can be produced by acid pretreatment, (depending on the reaction conditions used). These by-products can inhibit ethanol fermentation in yeasts. "Furan aldehydes" ("furfural" and "hydroxymethyl furfural (HMF)", from the degradation of xylose and glucose sugars, respectively) are major fermentation inhibition by-products from the acid pretreatment process. Scientists from Central Michigan University, John Hopkins University, and the Agricultural Research Service, Department of Agriculture (all in the United States), investigated the physiological/biochemical ethanol-inhibition effects of furfural (in the yeast, Saccharomyces cerevisiae). They found that "furfural was shown to cause cellular damage that is consistent with ROS (reactive oxygen species) accumulation in cells, which include damage to mitochondria and vacuole membranes, the actin cytoskeleton, and nuclear chromatin". Furfural concentrations of about 25 mM could allow repair response mechanisms for the (growth and fermentation) recovery of yeasts (after a long lag phase). Details of their study are published in the open access journal, Biotechnology for Biofuels (URL above)..