Extraction and Characterization of Oleoresin from Hop Residues with Supercritical CO₂ for Nanoemulsification and Application for the Control of Xanthomonas Citri

Abstract

The valorization of hop (Humulus lupulus L.) residues from the brewing industry was investigated through supercritical CO₂ extraction, followed by chemical characterization of the oleoresin and the formulation of nanoemulsions for the control of Xanthomonas citri, the causal agent of citrus canker. The oleoresin yield from previously processed hop residues was 1.87 ± 0.03 g per 100 g of dry biomass. GC–MS analysis identified 19 volatile compounds, with humulene (28.20%) and caryophyllene oxide (19.82%) as the predominant constituents. The oil also contained significant levels of palmitic acid (42.78%) and oleic acid (39.99%), and retained α-acids, including cohumulone (0.42%) and adhumulone + humulone (1.46%). Two emulsions were prepared: a water-based formulation (F1) and a chitosan-based formulation (F2). F2 exhibited a larger particle size (508.7 nm), high colloidal stability (+ 74.3 mV zeta potential), and bactericidal efficacy, achieving complete inhibition of X. citri at a concentration of 0.03125% (312.5 µg/mL). In contrast, both F1 and the crude oleoresin required concentrations of 2% to exhibit observable antimicrobial activity. Transmission electron microscopy and atomic force microscopy analyses confirmed spherical nanoparticles with uniform morphology and smooth surface for F2. These findings support the sustainable reuse of hop processing residues in the development of effective, low-toxicity biocontrol agents, with commercial potential for integrated citrus disease management.