Once the desired severity was reached, we immediately transferred the reactor into cold water. We subsequently separated the HTT solids and hydrolysate by filtration using a glass crucible (pore size 3 µm) and exhaustively washed the solids with deionized water. We then extracted lignin from the washed HTT solids with 90% (v/v) aqueous acetone. We rotary-evaporated the solution (at 40 • C) to produce acetone-extracted lignin, which was finally vacuum-dried at 40 • C to constant weight to determine the yield.