The biomass raw material consists of a large number of different chemical compounds – both small molecules and large (macro)molecules. The main macromolecules are cellulose, hemicellulose and lignin, but there can also be a considerable fraction of lipids. Cellulose and hemicellulose are carbohydrates (although variously modified in hemicellulose), while lignin is a complex macromolecule composed of short aromatic molecules. In order to use the biomass as a resource, it is usually desirable to separate out the main components before further processing. This is done during a stage called ‘pre-treatment’. After pre-treatment, the biomass can be fractionated; separated into different fractions of chemical compounds. Different raw materials give rise to various possible fractions of pretreated biomass, for example, lignin, hemicellulose, sugar, or vegetable oil. Additionally, the macromolecules are often more amenable to enzymatic treatment after this pre-treatment stage, as the surface structure has changed. This allows enzymes the possibility to modify the structure or hydrolyze the polymers into shorter fragments such as monomeric sugars.