The presence of sulfonated oligomeric and polymeric procyanidins in red wines impacts the estimated mean degree of polymerisation of condensed tannins by phloroglucinolysis

The calculation of proanthocyanidin mean Degree of Polymerisation (mDP) is a relevant analytical tool for gaining insight into the characteristics of wine. Despite its limitations, it remains an effective method for studying the polymeric forms of proanthocyanidins. Recently, sulfonated monomeric and dimeric flavan-3-ols have been detected in wine, and their main mechanism of formation suggested the formation of even larger sulfonated analogues. This work aimed to explore if aged wines contain sulfonated polymeric proanthocyanidins and how these compounds might impact the calculation of mDP. For this purpose, an experiment based on 4 single cultivar red wines (Sangiovese, Nerello Mascalese, Sagrantino, and Nebbiolo), four levels of SO2 addition (0, 30, 50, and 100 ppm), and two storage conditions (Control and Room Temperature) was realised. The results showed that the epicatechin 4β-sulfonate released after phloroglucinolysis is 10 to 352 times higher than the free homologue, indicating the presence of oligomeric/polymeric sulfonated procyanidins in wine. Incorporating the quantified terminal unit of epicatechin 4β-sulfonate into mDP calculations resulted in a reduction of mDP values by 1 to 36 %. This overestimation of mDP values increases with wine ageing or storage at elevated temperatures.