Kaolin foliar spray induces positive modifications in volatile compounds and fruit quality of Touriga-Nacional red wine

Solar radiation and temperature play crucial roles in grapevine metabolic processes and are known to have a positive impact on grape berry composition; however, excessive exposure to these factors can be detrimental. Kaolin-based particle film technology has emerged as a valuable solution for mitigating the effects of heat and water stresses in vineyards. This work aimed to evaluate the effects of kaolin application on the phenolic composition, chromatic characteristics, oenological parameters, volatile compounds and sensory profile of red wine. The study was carried out in one growing season in the commercial vineyard "Quinta dos Aciprestes" in the Douro Superior sub-region. Twelve rows in triplicate of the Touriga-Nacional variety underwent foliar kaolin treatment (applied at the pre-veraison stage, at the manufacturer-recommended and -tested dosage of 5 % (w/v)), and another twelve rows comprised the non-treated control group. Kaolin increased the phenolic compound and tartaric acid concentrations (2.4 % and 20.8 % respectively), total acidity (2.4 %), the deep reddish colour of the berries, total and coloured anthocyanin (2.8 %), and total and polymeric pigments (3.6 %); meanwhile, a decrease was observed in pH (-1.4 %) and alcoholic degree (-4.8 %). No significant differences were observed in any sensory parameters between the wine from the kaolin-treated and control vines, but the tasters found the aroma of the former to be fruitier and more complex, with an agreeable acidic taste and persistence. It was possible to group the volatile compounds into two distinct groups based on the results of the Pearson’s correlation matrix. This grouping corresponds to the sensory descriptors common to each of the respective volatiles. Overall, the results further support the potential efficacy of utilising kaolin to alleviate summer-related stress in grapevines.