We applied HTT to the extractive-free sawdust (4 g) in a swing reactor equipped with temperature control both in the heating block and inside the reactor. A schematic overview of the AqSO process is shown in Fig. . Several adjustable parameters influence the outcome of the lignin extraction and its structure, most importantly the reactor temperature (T), residence time (t f ), and liquid to solid ratio L : S. Since the heating period significantly affects the extraction process, we work with the reaction severity instead of the residence time. The HTT reaction severity is quantified in terms of the P-factor, which is calculated according to