Beyond Sugar Accumulation: Dynamic Changes in Bioactive Phenolics During Ripening of Tropical Highland Muscat Grapes

Conventional harvest decisions based on soluble solids may overlook changes in grape bioactive composition, particularly in emerging viticultural regions. Because technological, phenolic, and aromatic ripening may follow different trajectories, a high sugar content does not necessarily indicate the highest concentration of relevant phytochemicals. This study investigated whether technological ripening parallels phenolic changes in Muscat Petit Grain (Vitis vinifera L.) grapes cultivated in a tropical high-altitude vineyard in Garanhuns, northeastern Brazil (841–1000 m a.s.l.). Berries were sampled weekly at nine stages from véraison (87 days after pruning) to 143 days after pruning, following a randomized complete block design with three vineyard blocks. Soluble solids, pH, instrumental color, berry firmness, and skin hardness were monitored. Phenolic compounds were characterized by liquid chromatography coupled to diode-array detection and quadrupole time-of-flight mass spectrometry (LC-DAD-QToF-MS). Eighteen phenolics from five chemical groups were identified. Soluble solids increased from 9.87 ± 0.06 to 23.87 ± 0.06 °Brix (+142%), whereas pH increased from 2.83 ± 0.06 to 4.03 ± 0.06. Berry firmness and skin hardness decreased by 61% (2.11 to 0.83 N) and 84% (0.99 to 0.16 N), respectively. The berries also became less green and more yellow: the color coordinate b increased from 9.34 to 15.07 (+61%), while lightness (L) decreased from 42.20 to 30.74 (−27%).  In contrast with the overall increase in sugars, individual phenolic compounds followed distinct temporal patterns. Gallic acid, catechin, and gallocatechin declined during ripening, whereas epicatechin, selected procyanidins, rutin, and quercetin derivatives reached their highest concentrations at intermediate stages before declining toward the final sampling. Gallic acid decreased from approximately 11 to 1 mg kg⁻¹ (around 90%), and catechin from approximately 25 to 3 mg kg⁻¹ (around 88%). Conversely, rutin increased from approximately 4 mg kg⁻¹ at S1 to 27 mg kg⁻¹ at S3–S4 before declining to about 3 mg kg⁻¹ at S9. Several phenolic maxima occurred during S3–S5, when soluble solids were 15.70–18.60 °Brix, substantially below the final value of 23.87 °Brix. Thus, the maximum soluble-solids content did not coincide with the peak accumulation of several phenolic constituents. These results reveal divergent technological and phenolic ripening trajectories and suggest that °Brix alone is insufficient to characterize harvest-stage composition in this growing region. However, phenolic maxima at intermediate stages do not alone establish the optimal harvest date for winemaking, which also depends on acidity and aromatic maturity. Integrating chemical profiling with conventional maturity indicators may enable harvest decisions better aligned with the desired bioactive composition of Muscat Petit Grain grapes.