Bioethanol production from lignocellulosic biomass is still struggling with many obstacles. One of them is lignocellulosic inhibitors. The aim of this review is to discuss the most known inhibitors. Additionally, the review addresses different detoxification methods to degrade or to remove inhibitors from lignocellulosic hydrolysates. Inhibitors are formed during the pretreatment of biomass. They derive from the structural polymers-cellulose, hemicellulose and lignin. The formation of inhibitors depends on the pretreatment conditions. Inhibitors can have a negative influence on both the enzymatic hydrolysis and fermentation of lignocellulosic hydrolysates. The inhibition mechanisms can be, for example, deactivation of enzymes or impairment of vital cell structures. The toxicity of each inhibitor depends on its chemical and physical properties. To decrease the negative effects of inhibitors, different detoxification methods have been researched. Those methods focus on the chemical modification of inhibitors into less toxic forms or on the separation of inhibitors from lignocellulosic hydrolysates. Each detoxification method has its limitations on the removal of certain inhibitors. To choose a suitable detoxification method, a deep molecular understanding of the inhibition mechanism and the inhibitor formation is necessary.

Nitrogen explosive decompression pretreatment was tested on barley straw, aspen wood, and pine wood in a two-step pretreatment approach. The two-step approach aims to avoid the uncontrolled degradation of hemicellulose during pretreatment and to recover high sugar yields from hemicellulose and cellulose. Additionally, the pretreated biomass was filtered after each pretreatment step with a fine metal sieve so that fermentation and hydrolysis inhibitors were removed. A total sugar yield of 43 g per 100 g of dry biomass was achieved from barley straw and aspen wood. For pine wood, the total sugar yield was significantly lower, with 12 g per 100 g of dry biomass. Possible reasons for this low sugar yield are the adsorption and deactivation of cellulases on softwood hemicellulose and on repolymerized lignin after pretreatment.