*2.1. Raw Material*

Hazelnuts were purchased in a local market (Ourense, Spain). HS were milled, sieved to obtain a particle size between 0.250–1 mm, homogenized, and stored.

### *2.2. Hydrothermal Pretreatment (Autohydrolysis) of Hazelnut Shells*

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HS samples were treated in a 600 mL stainless steel reactor (Parr Instrument Company, Moline, IL, USA). The raw material was mixed with distilled water at a liquid to solid ratio of 10 kg water/kg of dry HS, heated under nonisothermal conditions up to reach the target temperature (in the range 190–225 ◦C), and then cooled by circulating water through an internal loop. Figure 1 shows the heating and cooling profiles that provide the reaction time to reach the temperatures of the experiments performed in this study. At the end of the treatments, liquid and solid phases were separated by vacuum filtration. Solids were washed with distilled water, and air dried. The solids from hydrothermal pretreatment (autohydrolyzed solids, AS) and autohydrolysis liquors (AL) were characterized for moisture, solid yield, and composition, as described in Section 2.4. –

**Figure 1.** Heating and cooling profiles obtained for the autohydrolysis experiments performed at the assayed temperatures.