Where have all the polyphenols gone? Check your filter!

Wine represents a complex matrix of polyphenolic compounds. Polyphenols alter the sensory qualities of the wine, represent chemotaxonomic markers and are responsible for many of its health benefits. It is critical to accurately quantify these compounds, which are the subject of extensive current research. Quantification usually requires HPLC analysis, where the main step in sample preparation involves filtration. With the assumption that sample compounds remain on the syringe filter, this study investigated whether diluting the sample before filtration would enable a better recovery of polyphenols, yielding more accurate results. For this purpose, a red, rosé and white wine were diluted with 4 different solvents (water, methanol, ethanol and a 1:1 methanol-ethanol mixture) prior to filtration, and their polyphenolic composition was analyzed. Only 7 polyphenols present in low concentrations were not affected by dilution, while 38 compounds were all detected in higher concentrations when applying certain solvents, compared to undiluted wine. The methanol-ethanol mixture enabled the best flow of polyphenolics through the filter, while the water dilution showed that 30-40% of the sum of anthocyanins remained on the filter if a dilution step was not included in the sample preparation procedure. This study confirmed the need to incorporate a preliminary evaluation of the filtration step during sample preparation to improve the accuracy of subsequent HPLC analyses.