Characterization of Enological Oak Tannin Extracts Using a Multianalytical Methods Approach
Kevin Pascotto, Hélène Halle, Aude Watrelot,
Aurélie Roland, Emmanuelle Meudec, Pascale Williams,
Stéphanie Carrillo, Bertrand Robillard, Nicolas Sommerer,
Céline Poncet-Legrand, and Véronique Cheynier*
*UMR1083 Sciences pour l’oenologie, 2, place Viala,
34060-Montpellier, France (veronique.cheynier@inrae.fr)
Oak tannin extracts are commonly used to improve wine properties.
The main polyphenols found in oak wood extracts are ellagitannins
that release ellagic acid upon hydrolysis and comprise numerous
structures. Oak tannin extracts also contain other compounds,
giving a complex mixture. Consequently, the official OIV method
based on gravimetric analysis of the tannin fraction adsorbed
onto polyvinylpolypyrrolidone is not sufficient to describe their
composition and highlight their chemical diversity. Eight
commercial oak tannins were characterized using a combination of
analytical approaches. Polyphenols were analyzed using the OIV
method, UV spectrophotometry, UPLC-UV-MS analysis before and
after acidic methanolysis, and HPLC-SEC-UV. Neutral sugars and
polyols were determined as alditol acetates by GC-FID analysis,
before and after hydrolysis. Finally, samples were compared using
a non-targeted metabolomic approach based on UHPLC-HRMS/MS.
Gravimetric analysis, absorbance values at 280 nm, and the
quantities of ellagic acid released by methanolysis revealed some
differences among samples, indicating variations in tannin
composition. This was confirmed by SEC analysis evidencing
differences in tannin size distribution, particularly in larger
polymer content. All samples contained significant quantities of
sugars, particularly xylose, mostly found in the linked form, and
quercitol, a polyol marker of oak origin. These compounds
contributed up to 25% of the whole extract composition. The
proportions of free and combined sugars and polyols also showed
large variations among tannins. Non-targeted UPLC-HRMS analysis
detected major ellagitannins such as vescalagin, castalagin, and
roburins A through E, but also a large number of derivatives and
other molecules such as lignans and quercotriterponosides, and
highlighted large differences among samples.
This work demonstrates the variability in composition of
commercial oak tannin extracts, likely to impact their
properties, and emphasizes the need for detailed multi-method
characterization, in the frame of quality control and selection
of tannins for specific applications.
Funding Support: IOC
