Sequential acid-alkali pretreatment of rapeseed straw

Rapeseed straw is an agricultural residue increasingly produced in recent years due to growing interest in production and consumption of biodiesel. In 2012, 34.2 million hectares of rapeseed were cultivated worldwide (FAOSTAT, 2013). Rapeseed straw is a lignocellulosic material with a carbohydrate content up to 60% which is a raw material interesting for the production of second generation fuel ethanol by means of a biochemical process.

This research aimed to fractionate rapeseed straw evaluating the effectiveness of a new configuration of sequential acid-alkaline pretreatment. The strategy of two-step pretreatment of rapeseed straw allows the solubilization of hemicellulose fraction in a first step and the improvement of the enzyme accessibility to cellulose increasing the hydrolysis yields in a second step of delignification.

After a first acid treatment (130ºC, 2% H₂SO₄, 60 min) a prehydrolysate rich in sugars (52 g/L) was obtained recovering 63% of the hemicellulosic sugars contained in the feedstock. In a second oxidative alkaline step the influence of temperature (30, 60, 90ºC) and hydrogen peroxide concentration (1, 3, 5% w/v) on the solid material resulting from the first stage was studied. By testing the enzymatic hydrolysis the efficiency of alkaline pretreatment was evaluated and the best conditions for highest hydrolysis yields were found to be 90ºC, 5% H₂O₂, 60 minutes, representing 69% delignification and 96% enzymatic digestibility (corresponding to hydrolysis yield of 75% based on glucose content in the raw material).