Development and Validation of a Selective Method to Quantify Low-Molecular-Mass Flavan-3-ols in Grapes and Wines

Quantifying low-molecular-mass (LMM) flavanols in wines is relevant because these compounds, though typically minor, reflect flavanol structural composition (seed vs. skin contributions) and relate to cultivar and winemaking technique. Their determination is challenging because oligomeric and polymeric tannins interfere with standard spectrophotometric assays. This study introduces a coupled procedure that isolates and selectively quantifies LMM flavan-3-ols by combining the well-established methylcellulose precipitation assay (MCP) to remove oligomers and polymers with dimethylaminocinnamaldehyde (DMAC) determination of the MCP supernatant. The sequential workflow uses DMAC specificity and sensitivity and minimizes interference caused by higher-mass flavanols. Additionally, samples are quantified following dilution in the highly stable MCP supernatant medium. A Small Flavanol Index (SFI, %) is also introduced, expressing the LMM fraction relative to methylcellulose-precipitable tannins and providing a descriptor of flavanol composition. The method was validated in terms of linearity, limits of detection and quantification (LOQ in the supernatant, 1.58 mg L⁻¹), precision, and recovery. Applicability is demonstrated in Marselan and Tannat (Vitis vinifera), resolving compositional differences by cultivar, grape tissue (skins vs. seeds), and maceration technique. Compatible with microplate formats and simple instrumentation, this robust analysis enables tandem determination of LMM flavanols and condensed tannins and represents an analytically valuable tool for commercial wineries and research.