Suitability of oxygen delignification to increase enzymatic digestibility of polysaccharides in sugar cane bagasse

Several pretreatments can be used to decrease the recalcitrance of lignocellulosic materials. Lignin removal and sulfonation during sulfite-based pretreatments have proved efficient to increase digestibility of varied materials, including hard and softwoods as well as grass monocotyledons. However, high sulfite or SO₂ loads point out for expensive pretreatment processes, making recycling of inorganics a requirement. In this context, we exploited the use of the oxygen delignification to selectively remove lignin from sugar cane as a simpler pretreatment process. Crude sugar cane bagasse or alkaline-sulfite pretreated sugar cane bagasse were treated with 6 bar O₂ under alkaline media. Chemical composition of pretreated materials and mass balance data indicated that O₂ alone or mild alkaline sulfite (5% Na₂SO₃) provided limited delignification of the crude material (18% and 26%, respectively). In contrast, a sequential process with mild alkaline sulfite followed by O₂ delignification increased lignin removal to 63%. Enzymatic glucan conversions from these materials, after 48h-hydrolysis, were: 15.0%, 25.5%, 46.8% and 71.5% for crude material, O₂-delignified, mild alkaline sulfite-pretreated and mild alkaline sulfite plus O₂ delignification, respectively. Oxygen delignification proved efficient to enhance glucan conversions to levels only attained with severe alkaline sulfite pretreatments, since 70-80% glucan conversions were obtained, without O₂-delignification, only when severe alkaline sulfite pretreatments (10% Na₂SO₃) were employed.