Definitions and calculations
The following definitions are used to quantitatively evaluate the four poplar wood samples for bioconversion to sugar and ethanol. The discussion of biomass recalcitrance has been qualitative in the literature [27]. We use a quantitative definition, Plant Biomass Recalcitrance (PBR), proposed in our previous study [13] to com- pare the recalcitrance of the four samples examined in this study