Surface properties of cork in relation to reactive washing

Abstract

Reactive washing consists of treating cork stoppers with hydrogen peroxide under strong alkaline conditions at elevated temperature. This causes changes in the surface of the stoppers, affecting their wettability. This study aimed to relate the reactive washing with changes in the surface properties of cork stoppers and their affinity for hydroalcoholic solutions with different concentrations of ethanol. The surface composition was evaluated by Fourier Transform Infrared-Attenuated Total Reflectance spectroscopy, confocal Raman spectroscopy and the free surface energy by contact angle measurements with liquid probes using Owens-Wendt-Rabel-Kaelble method. The surface morphology and topology was assessed by scanning electron microscopy and 3D optical profilometry, respectively. Different wettability of the top and lateral of the cork stoppers is due to the difference in the exposure of phellogen cell wall layers. After reactive washing, the partial degradation of suberin and the increased exposure of hydrophilic biopolymers lead to an increase in the polar component of the free surface energy, thus promoting wettability with aqueous solutions. The changes in the surface observed imply an enhancement in the wettability of cork stoppers towards hydroalcoholic solutions mimicking alcoholic beverages, as revealed by modeling studies employing a wetting envelope.