Formation of inhibitors during acid pretreatment of lignocellulose

Pretreatment of lignocellulosic biomass under acidic conditions, for example through steam explosion, is advantageous considering that it is efficient for a wide variety of feedstocks including not only agricultural residues but also more recalcitrant forms of biomass, such as softwood. It is also a pretreatment method that can be implemented in an industrial context already today. The hemicellulosic hydrolysate that results from acid pretreatment will contain by-products that inhibit microorganisms that are used for conversion of lignocellulose-derived sugars to the desired fermentation product. Groups of inhibitory compounds that are relatively well known include aliphatic carboxylic acids, furan aldehydes, and phenolic compounds. As inhibitory effects observed in studies of lignocellulosic hydrolysates are often difficult to explain considering only aliphatic carboxylic acids, furan aldehydes, and phenols, an effort was made to identify other groups of substances that contribute to inhibitory effects. Using mass spectrometry for analysis of hemicellulosic hydrolysates produced from different feedstocks under different pretreatment conditions and yeast as the fermenting microorganism, benzoquinones and small aliphatic aldehydes were identified as groups of inhibitory substances that deserve more attention in the future. Work in this area contributes to knowledge about substances that are important to monitor in pretreated biomass and paves the way for the development of efficient detoxification methods that target the most important inhibitors. By identification of critical inhibitory substances, it also offers guidance for engineering and selection of robust microbial catalysts.