The typical inhibitory compounds from lignocellulosic biomass dilute acid pretreatment are sugar degraded products (furfural, HMF, formic acid, and levulinic acid etc), lignin degraded products (vanillin etc), extractives, and some metals.
The maximum concentration of each inhibitor that a microorganism can tolerate varies depends on the factors of types of microorganisms, its adaption to the medium, process, the numbers of inhibitors, and very importantly, their synergistic effect. For example, ethanol production was stimulated by acetic acid only (up to 10 g/l) in medium without furfural, or by furfural only (up to 2 g/l) in medium without acetic acid. However, the combination of both will negatively affect the cell growth, cell mass yield, and ethanol yield, i.e. the toxicity of biomass hemicelluloses hydrolyzates results from the aggregation of several toxic compounds (alcohols, aldehydes, and acid) rather than individual compounds.