Microscopic Characterization of Pinot Noir Grape Surfaces Following Coating with Chitosans of Different Molecular Weights to Reduce Volatile Phenol Uptake

Abstract

Wildfire smoke exposure introduces volatile phenols (VPs) into grape berries, where they permeate the cuticle, undergo glycosylation, and contribute to smoke taint in the wine. This study investigated the role of chitosan molecular weight (MW) in governing the performance of cellulose nanofiber (CNF)-chitosan coatings as barriers to volatile phenol uptake. Coatings containing chitosan of four MWs (190, 360, 440, and 780 kDa) were applied to post-harvest Pinot noir grapes prior to controlled VP exposure. High-MW chitosan (780 kDa) significantly reduced concentrations of guaiacol (48.0 to 27.0 ppb), 4-methylguaiacol (23.1 to 10.3 ppb), and cresols (89.2 to 69.2 ppb) compared with noncoated controls (p < 0.05). Microscopic analyses revealed that increasing the chitosan MW produced thicker, more cohesive, and less porous surface films with greater continuity across the grape surface. FTIR analysis indicated the masking of cutin-associated functional groups. These results demonstrate that chitosan MW governs the coating microstructure and barrier properties, providing mechanistic insight for designing surface coatings to mitigate smoke-related volatile phenol uptake in grapes.