**Sandra Rivas \* , Andrés Moure and Juan Carlos Parajó**

Department of Chemical Engineering, University of Vigo (Campus Ourense), 32004 As Lagoas, Spain; amoure@uvigo.es (A.M.); jcparajo@uvigo.es (J.C.P.)

**\*** Correspondence: sandrarivas@uvigo.es; Tel.: +34-988-387075

Received: 7 May 2020; Accepted: 25 May 2020; Published: 27 May 2020

**Abstract:** Hazelnut industries generate a large amount of byproducts. Among them, waste hazelnut shells (which account for about 50% of the nut weight), are potential raw materials to produce value added products. Hydrothermal pretreatment enables the solubilization of hemicelluloses, while cellulose and lignin remain in the solid phase almost unaltered, allowing their subsequent processing for an integral valorization of the feedstock. When the reaction was performed at the optimal temperature (210 ◦C), hemicelluloses were mainly converted into soluble substituted oligosaccharides (OS). Further membrane processing of the liquid phase from hydrothermal pretreatment enabled the refining of the OS, which accounted for up to 90.87 wt% of the nonvolatile solutes (NVC) in the refined solution, which also contained 5 g of natural bound phenolics/100 g NVC. The target products showed a dose-dependent antioxidant activity, conferred by the phenolic components. Substituted OS were made up of xylose backbones with a wide degree of polymerization distribution, and showed structures highly substituted by acetyl and uronic groups. The data included in this study provide the basis for assessing the large-scale manufacture of substituted oligosaccharides with bound phenolics as bioactive components of functional use in foods, cosmetics, or pharmaceuticals.

**Keywords:** biorefinery; hazelnut shells; hydrothermal pretreatment; hemicelluloses; oligosaccharides; antioxidant activity
