Preharvest Cellulose–Chitosan Spray for Mitigating Smoke Impact in Wine Grapes: Enhanced Blocking <i>m</i> ‐Cresol, Validation in Model Table Grapes, and Vineyard Field Evaluation

Preharvest spray coatings composed of cellulose nanofibers (CNF) and chitosan (CH) were investigated to reduce volatile phenol (VP) absorption and phenol glycoside (PG) formation in wine grapes exposed to wildfire smoke. Coatings with CNF:CH ratios of 1:2 (LCH), 1:5 (MCH), and 1:11 (HCH) were cast into films and exposed to m-cresol vapor (0.1 or 0.05 g). HCH films showed the lowest m-cresol desorption (9.5 ± 1.5 µg/mg), while CNF-only films desorbed 17.7 ± 1.1 µg/mg, indicating CNF-enhanced reversible absorption. On table grapes exposed to a VP mixture (600 mg/L of seven compounds), LCH and HCH coatings increased VP and PG levels compared to controls, suggesting partial absorption and diffusion through coatings. Field trials with Pinot noir grapes included two smoke exposure timings, post-veraison and preharvest, and two smoke intensities (PM_2.5 = 5.0 and 13.9 mg/m³). In early-smoked grapes, cresol and phenol reached 12 and 8 ppb, respectively; coatings reduced cresol and phenol by 3-4 ppb. In late-smoked grapes, guaiacol reached 5-7 ppb, and 4-methylsyringol gentiobioside (4MS-Gb) peaked at 30 ppb. Coatings reduced guaiacol by about 1-2 ppb and 4MS-Gb by up to ∼8 ppb. Coatings had no significant effects on grape weight, diameter, °Brix (20.9-22.5), pH (3.23-3.39), titratable acidity (6.1-6.7 g/L), or total monomeric anthocyanins (320-346 mg/L). SEM imaging showed that LCH formed a continuous film, while HCH appeared patchy but adhered to cuticle waxes. CNF-based coatings showed potential for VP mitigation, though field variability limited conclusive outcomes. Further optimization and consistent field smoke exposure are needed to evaluate commercial feasibility. PRACTICAL APPLICATIONS: This study demonstrates the potential of cellulose nanofiber-chitosan spray-on coatings as a preharvest strategy to mitigate smoke impact in wine grapes. These biopolymer coatings may serve as a physical barrier to reduce volatile phenol absorption during wildfire smoke events. The work also provides insight into the importance of spray and smoke timing, offering practical guidance for vineyard interventions under smoke exposure risk.