The Touriga Nacional grape variety is renowned in Portuguese red wines for its intense color and aromatic complexity, largely attributed to its rich phenolic composition. Several factors influence the phenolic profile of wines, including edapho-climatic conditions, grape variety, and winemaking techniques such as fermentation, maceration, barrel aging, and maturation. In this study, the technique for winemaking was the only controlled variable, allowing for a specific evaluation of its impact on phenolic composition and antioxidant capacity. Ten single-varietal Touriga Nacional wine samples from the 2019 vintage, produced in the Cima Corgo sub-region of the Douro by different wineries, were analyzed. The phenolic composition was determined using colorimetric methods to quantify total phenols, ortho-diphenols, flavonoids, anthocyanins, and tannins. Antioxidant capacity was assessed through the 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), and Ferric Reducing Antioxidant Power (FRAP) assays. Since all wines shared the same grape variety, region, and harvest year, the fermentation technique was the main differentiating factor, enabling a direct comparison of its influence on phenolic extraction and antioxidant properties. Additionally, Reversed-Phase High-Performance Liquid Chromatography with Photodiode Array Detection coupled with Mass Spectrometry (RP-HPLC-DAD-ESI-MS/MS) was employed to identify and quantify individual phenolic compounds. This study highlights the key role of winemaking techniques in modulating the phenolic composition and antioxidant potential of Touriga Nacional wines.

The authentication of wines with Protected Designation of Origin (PDO) status is a key requirement for quality assurance, traceability, and consumer trust, particularly in traditional wine-producing regions such as the Douro Demarcated Region (Portugal). Among the certification criteria, the reliable identification of grape varieties remains technically challenging, especially when rapid and non-destructive analytical approaches are required. In this study, Fourier-transform infrared spectroscopy coupled with chemometric analysis was evaluated as a rapid screening approach for the differentiation of monovarietal Douro wines produced under standardized microvinification conditions. Twenty-one monovarietal wines were analyzed using mid-infrared spectra (1800–1000 cm⁻¹) and classification models were developed using Partial Least Squares Discriminant Analysis (PLS-DA). The PLS-DA models showed preliminary discriminatory capacity, with apparent error rates of 10.2% for calibration and 19.3% under leave-one-out cross-validation. The results indicate that FTIR-ATR spectroscopy combined with chemometrics captures chemically relevant spectral variability associated with grape varietal differences and shows potential as a rapid exploratory screening approach within PDO traceability frameworks. Although the study is based on a limited number of biological replicates from a single vintage and sub-region, the findings provide a methodological baseline for future multi-vintage and multi-region investigations aimed at consolidating FTIR-based approaches for varietal authentication of Douro wines.