**4. Conclusions**

This study deals with the hydrothermal pretreatment (autohydrolysis) of HS, conceived as the first step of an overall utilization of this feedstock in the scope of biorefineries. Operating at the temperature considered as optimal (210 ◦C), most hemicelluloses were broken down into soluble fragments, appearing in liquid phase as oligosaccharides. Unwanted, nonsaccharide products present in the liquid phase from autohydrolysis treatments were removed by DD. The target products (refined OS present in the retentate) accounted for 90.87% of the NVC fraction. OS presented a wide DP distribution, and the backbones were made up of xylose structural units substituted with acetyl and uronic groups. Phenolics bound to OS accounted for 5% of the total NVC, and showed dose-dependent antioxidant activity. The phenolics identified included gallic, vanillic, and *p*-coumaric acids; vanillin; and catechin. These results confirm the potential of the target products obtained in this study for applications in a number of fields, including the food, cosmetic, and pharmaceutical industries.

The autohydrolysis stage also produced solids containing up to 93.13% of glucan and Klason lignin, which represented recovery yields of these components above 90% and 85%, respectively. These data confirm the interest of hydrothermal processing as a first stage of an integrated process for the global valorization of HS.

**Author Contributions:** S.R. and A.M. designed the research. S.R. performed experiments and analyzed the data. S.R. and J.C.P. prepared the original draft. All authors discussed the data. All authors have read and agreed to the published version of the manuscript.

**Funding:** S.R. contracts were supported by Ministerio de Economía y Competitividad (FJCI-2015-23765) and Ministerio de Ciencia, Innovación y Universidades (IJC2018-037665-I). This research was also funded by "La Caixa" and Triptolemos Foundation.

**Acknowledgments:** S.R. thanks Ministerio de Economía y Competitividad and Ministerio de Ciencia, Innovación y Universidades for her "Juan de la Cierva" contracts (FJCI-2015-23765 and IJC2018-037665-I). She also thanks to "La Caixa" and Triptolemos Foundation for the financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.
