This thesis is about the use of an agricultural residue as a feedstock for fermentable sugars to be used for second generation (2G) bioethanol. The main focus of this thesis work is upon the recalcitrance of different anatomical fractions of wheat straw. Biomass recalcitrance is a collective reflections of plant species, tissue or organ types, genetic traits and environment. Effects of cultivar type, anatomical distribution, chemical composition, fertilizer level and growth year have been observed during in vitro and in vivo trials. A similar approach is here taken to further investigate: 1). How recalcitrance or degradability varies between leaf and stem as found in different wheat cultivars. 2). Chemical, physical and structural differences between wheat straw leaf and stem which may be responsible for observed differences in their bio-degradability.

As a tool to assess the variation of lignocellulosic biomass’ degradability, a high throughput screening (HTS) platform was developed for combined thermochemical pretreatment and enzymatic degradation in Copenhagen laboratory during this thesis work. The platform integrates an automatized biomass grinding and dispensing system, a pressurized heating system, a plate incubator and a high performance liquid chromatography (HPLC) system. In comparison with the reported HTS platforms, the Copenhagen platform is featured by the fully automatic biomass sample preparation system, the bench-scale hydrothermal pretreatment setup, and precise sugar measurement solution, which together enables reproducible and reliable analysis of the biomass degradability in large sample set. The large variance of degradability reflects the different recalcitrant nature of the biomass samples.

The leaf-to-stem (L/S) ratio of wheat straw biomass was experimentally identified as an important parameter affecting the bio-degradability during enzymatic saccharification. The leaf fraction (leaf blade and sheath), which contributes up to 50 % of the whole wheat straw on a weight basis, is more degradable than the stem fraction via a biochemical process. Water was used to assess the different accessibility of the cell wall glucans, which was found to be a critical factor contributing to the different bio-degradability of wheat straw leaf and stem. The distribution patterns of intra- and inter- molecular hydrogen bonds within the glucans may be used as an indicator for the recalcitrance level of different biomasses.

The knowledge on wheat straw biomass gained from this thesis work may be translated to more focused plant breeding research for 2G ethanol production, and the Copenhagen HTS platform is an ideal tool for screening large biomass sample set. The success of lignocellulosic biomass based 2G bioethanol industrialization will need cooperative efforts from biologists, plant researchers and processing engineers. It is the hope that this thesis may contribute to this development process.