Potential of a Bio-Based Additive in the Prevention of Lignin-Induced Recalcitrance under Severe Pretreatment Conditions

Second-generation biorefineries converting lignocellulose to renewable fuels and chemicals require pretreatment to open the cell wall structure and enable enzymes to access polysaccharides. However, lignin from the commonly used hydrothermal pretreatments can restrict enzyme action by covering the cellulose surface and non-productively binding enzymes. The resulting residual lignin typically contains a relatively high amount of carbohydrates, restricting its solubility and utilisation in applications. The aim of this work was to elucidate how pretreatment severity affects lignin inhibition in enzymatic hydrolysis and how lignin from softwood and herbaceous plants changes during pretreatment. Understanding these factors can aid in developing pretreatments with simultaneously improved hydrolysis yields and improved applicability of the hydrolysis lignin.

Spruce wood and wheat straw were hydrothermally pretreated at 180-220 °C with or without an acid catalyst. Increasing the pretreatment severity increased the inhibitory effect of lignin in Avicel hydrolysis. The inhibition arose from increased non-productive binding of enzymes to lignin. For both raw materials, the levels of aryl ether bonds decreased as pretreatment severity increased. Furthermore, for wheat straw lignin, a decrease in the amount of hydroxycinnamates and tricin was observed. Several publications have reported that introducing additives, such as 2-naphthol, to pretreatment can reduce lignin condensation resulting in improved hydrolysis yields. The flavonoid tricin, found in herbaceous plant, has an electron-rich aromatic ring structure similar to 2-naphthol. Softwood pine was hydrothermally pretreated at 200 °C with the additives tricin or 2-naphthol. Pretreatment with tricin resulted in higher sugar yields in the following enzymatic hydrolysis step when compared with the pretreatment with 2-naphthol. This may be due to modifications made to the lignin structure during pretreatment, which could be promising for both saccharification and lignin valorization pathways.