Biological pretreatment of grassland biomass through ensiling

Grass biomass may become an important low cost lignocellulosic raw material for renewable fuels and chemicals in the future, being a perennial crop and allowing for several harvests (2-4 cuts) during a season contributing to a very high yield (  ̴17 tonne/ha). The low dry matter (DM) at harvest of typically 18-20% (w/w) makes conventional dry bale storage troublesome and expensive. Ensiling, the traditional storage method for animal feed, can alternatively facilitate biomass storage at much lower DM (20-50% (w/w)). Ensiling encompasses moist solid state anaerobic fermentation by lactic acid bacteria (LAB), where production of organic acids decreases pH and preserves the biomass. During storage parts of the structural carbohydrates gets hydrolysed due to the low pH, giving ensiling its function as pretreatment method. We have studied the pretreatment effect of ensiling systematically against important factors of biomass composition, DM at ensiling and use of biological inocula. The results showed that low DM ensiling (<25%) resulted in highest glucose yield and cellulose convertibility for all cuts of grass. The grass biomass composition is largely determined by the maturity; less mature grass resulted in higher cellulose convertibility both with and without ensiling, due to the lower lignin content. But most importantly ensiling improved the cellulose convertibility compared to dry storage. However ensiling cannot, without further optimisation, be an efficient stand-alone pretreatment. Approaches for optimisation and pretreatment combinations will therefore be presented.