Tannins are chemically diverse polyphenols contributing to important sensory attributes of food and beverages. In wine, their structure and quantity depend on several factors, such as the grape variety, climate, soil, viticultural and enological practices and the wine-aging process. Therefore, their accurate and cost-effective quantification is essential to optimize polyphenols profiles during wine making. Conventional tannin quantification methods are primarily colorimetric, some of which suffer from a high interference with anthocyanidins and therefore are unsuitable for many emerging disease resistant grape varieties. Even though precipitation-based methods remain a relatively good proxy for astringency they are difficult to scale to large number of samples. Fluorescence or chemiluminescence-based assays could provide a more practical alternative by offering high sensitivity while minimizing interference artefacts. We recently discovered a novel fluorescence quenching mechanism of synthetic rhodamine fluorophores with a high selectivity towards tannic acid (TA) and catechin-3-gallate (C3G). Specific chemical conjugates of silicon-rhodamine with alkyl linkers attached to bulky apolar moieties (SiR) had a detection limit near 500 pM and a linear range spanning 5–100 nM for TA. The technique was applied to quantify condensed tannins present in wine and was compared to a methylcellulose precipitation and acidic butanolysis. We validated the assay on 18 distinct red wine samples, which showed high linearity (R2 = 0.92) with methylcellulose precipitation. The variability between the individual measurement points was consistent for SiR-quenching and methylcellulose precipitation for all samples. Additionally, when tested with pure standards, the assay was two orders of magnitude more sensitive towards certain wine catechins than to malvidins, making it suitable for wines with high anthocyanin content like new resistant grape varieties. In conclusion, a novel assay was developed and validated that allows the sensitive and selective quantification of tannins abundant in food and beverages.
