ENZYMATIC HYDROLYSIS OF PRETREATED WHEAT STRAW: IMPACT OF LIGNIN

Bioethanol produced from lignocellulosic biomass has been investigated for decades as a great alternative to fossil fuel. During the SHF process (Separate Hydrolysis and Fermentation), the cellulose contained in the biomass is hydrolysed into glucose in the presence of cellulolytic enzymes. The glucose is then converted into ethanol during a fermentation step, and the alcohol is recovered by distillation and incorporated into fossil fuel. Due to the cost of enzymes, the hydrolysis step is one of the main bottlenecks of the process. As a consequence, the action of enzymes has to be facilitated by first reducing the inherent recalcitrance of the substrate. This recalcitrance is partly linked to lignin, which is a very complex and heterogeneous polymeric material associated with cellulose and hemicellulose. It is known to hinder the enzymatic hydrolysis of biomass, since cellulases may adsorb on its surface in a non-productive way and lose access to a part of cellulose. However, the involved phenomena remain unclear and the impact of lignin on the enzymatic hydrolysis is still difficult to predict. The purpose of our work was to better understand the role of lignin in the recalcitrance of biomass during enzymatic hydrolysis. A pretreated wheat straw was partially delignified by acidified sodium chlorite at 70 degrees C and six different grades of delignification were obtained. The respective reactivity of these substrates was compared using an enzymatic cocktail from Trichoderma reesei. It appeared that the different grades of delignification did not have a significant impact on the hydrolysis of the studied straw in the experimental conditions tested. Inhibitive impact of lignin in terms of non-productive adsorption was then explored using soda lignin from wheat straw and kraft lignin from softwood. The addition of this particular type of lignin had a strong negative influence on the hydrolysis of a highly crystalline cellulose (Avicel), whereas it impacted only slightly the hydrolysis of delignified wheat straw. These findings comfort our previous results and complete other works carried on wood. They highlight the imperative necessity to characterize properly the physico-chemical properties of pretreated substrates in order to improve their enzymatic hydrolysis. Keywords : biomass, biofuel, enzymatic hydrolysis, lignin, straw