Physicochemical fractionation and enzymatic treatment of birch outer bark towards the evaluation of ‘oleochemicals’ and ‘active’ ingredients

Efficient conversion of lignocellulosic materials to ethanol and value-added biochemicals remains a challenging proposition from energy and environmental concerns. Apart from the polysaccharide part, the lignocellulosic feedstock is rich in natural waxes, such as triterpenes and insoluble polyesters (suberin), that serve as structural components of the outer barriers of plants. In order to improve the transformation of multiple biomass feedstocks, better understanding of the extraction and biodegradation of these components is needed. Moreover, degradation products, some of which are found almost exclusively in the suberized plant cell walls, could have considerable value as sources of oleochemicals. Silver birch (Betula pendula), the dominant pulpwood species in Northern European countries, contains ~3.4% outer bark, of which 44% consists of suberin and 40% of extractives. In this study, different pretreatment methods, including hydrothermal, steam explosion and organosolv treatment, were applied to birch outer bark in order to disrupt its structure and evaluate the potential of fractionation into its components. The extractive-free residual bark was characterized by IR and NMR methods and morphological alterations after pretreatment were detected with scanning electron microscopy. The solid fractions were subjected to sequential treatments with cellulases and esterases and the produced sugars, fatty acids and other organic acids, including phenolics and glucuronic acid, were characterized. The use of enzymes for the treatment of biomass offers the key advantage of selectivity of bonds that are to be disrupted and maintainance of different group functionalities (epoxy, hydroxyl and carboxylic) that may be destroyed with other harsh chemical treatments.