*2.1. Raw Material*

The used corn stover was harvested in 2018 in Fulda, Hesse (Germany). The material was separated into leaves, stalks, corn cobs and further impurities and was air-dried. After conditioning by air-drying to a stable dry matter content of 90.7%, the material was chopped with a garden chipper into segments with a length between 6 and 8 cm. For steam refining experiments, only leaves and stalks were used.

For raw material analysis, the ash content was measured according to TAPPI standard T 211 om-16. Extractives were determined by Accelerated Solvent Extraction (ASE) of milled (≤1 mm) material with an ASE 350 (Thermo Scientific™ Dionex™, Waltham, MA, USA). Three extraction steps for 10 min at 10 MPa with solvents of different polarities (petrol ether (70 ◦C); acetone/water 9:1 (70 ◦C); water (90 ◦C)) were conducted.

Two-step acid hydrolysis was performed subsequently for the determination of monomeric carbohydrates. Two hundred milligrams of dry material were prehydrolyzed for 60 min with 2 mL of 72% H2SO4. The reaction was stopped by the addition of 6 mL deionized water and the sample was transferred with 50 mL deionized water into a 100 mL volumetric flask. The second step of hydrolysis was conducted for 40 min at 120 ◦C and 0.12 MPa overpressure [27].

Afterward, the samples were cooled to room temperature and then filtered through a sintered glass frit (G4). The undiluted filtrate was used for further analysis, described in detail in Section 2.6. The acid-insoluble residue was washed, dried at 105 ◦C and gravimetrically weighed [12,27].
