Hop stems, a new fibrous bioresource

Abstract

The valorization of agricultural wastes in accordance with the concept of biorefinery is currently a subject of great interest. The present work deals with hop stem, a co-product of hop cultivation that is currently not properly used. Hop stem was first decorticated to produce ≈ 20 % w/w of technical fibers and shives. Four different degumming processes were investigated: water retting, enzymatic hydrolysis, alkali treatment, and steam explosion at 200 °C during 4 min. The fibers produced were comprehensively characterized in terms of chemical composition (Klason lignin and simple sugars by ionic chromatography), mechanical strength, elongation, and morphology by scanning electron microscope (SEM). In contrast to the other processes tested, steam explosion (SE) resulted in efficient defibration due to a significant decrease in pectin (≈ - 70 %) and lignin content while maintaining good tensile properties (≈ 500 MPa). It was also shown that the hop single cell fiber has characteristics close to those of hemp with a length of 6 mm ± 5 mm, a diameter of 15 µm ± 3 µm and a microfibrillar angle of ∼ 7°. The hop shives were used for the production of an adhesive-free panel with good mechanical properties.