Ethanol production from lignocellulosic biomass

The reported study intends to describe the state of the art in the domain o f ethanol production from lignocellulosic biomass. It was sustained ana man aged by a specialized group of the French Agrice (Agriculture for Chemical and Energy Organization). Its first goal was to pinpoint the main technical and economical bottlenecks of the processes which are today under consider ation, and to identify which research and development efforts could be impl emented to overcome them (in the short or middle term). Lignocellulosic biomass is a complex substrate, and essentially made of cel lulose, hemicellulose and lignin. The processes which have been considered, attempted to recover a maximum amount of sugars from the hydrolysis of cel lulose and hemicellulose, and to ferment them into ethanol. The hydrolysis processes used in the past are essentially chemical processe s, but the acid recovery costs and the formation of toxic products make the m uncompetitive. They are now substituted by enzymatic processes, which are more specific and allow higher hydrolysis yields under less severe conditi ons. However, the cellulose that is the target of the enzymatic hydrolysis, is n ot directly accessible to the enzymes. It is the reason why a pretreatment step has to precede the enzymatic hydrolysis, in order to improve the enzym atic susceptibility of the cellulose, and to hydrolyse the hemicellulosic f raction. Different types of pretreatment have been studied, but three metho ds appear more efficient: dilute acid hydrolysis, steam explosion wit catal yst addition and thermohydrolysis. These pretreatments could result in high hydrolysis yields of the cellulose fraction (close to 100%), and in a maxi mum recovery of the sugars from the hemicellulosic fraction. Enzymatic hydrolysis has yet to be improved in order to reduce the cost of consumption of the enzymes. Research works will have to focus upon the enzy me specific activity, in order to achieve higher efficiencies such as those obtained wit amylases. The SSF (Saccharification and Simultaneous Fermenta tion) process improves the enzyme efficiency by reducing the feed-back inhi bition from the hydrolysis products. The screening of efficient fermentativ e microorganisms under high temperature conditions (45 degrees C) has thus to be further implemented. The last technological barrier of the process concerns the ethanolic fermen tation of the pentoses. Indeed, the pentoses, originating from the hemicell ulosic fraction, can represent up to 40% of total sugars in some lignocellu losic substrates. Nobody has yet identified a microorganism which is able t o ferment the pentoses into ethanol with performances similar to those of S accharomyces cerevisiae on glucose. But recent genetic improvements focused on the transformation of Saccharomyces cerevisiae and Zymomonas mobilis co uld result in good fermentative performances on pentoses.