Influence of toasting oak wood on ellagitannin structures
Abstract
Ellagitannins (ETs) have been reported to be the main phenolic compounds found in oak wood. These compounds, belonging to the hydrolysable tannin class of polyphenols, are esters of hexahydroxydiphenic acid (HHDP) and a polyol, usually glucose or quinic acid. They own their name to their capacity to be hydrolysed and liberate ellagic acid and they have an impact on astringency and bitterness sensation, which is strongly dependant on their structure. The toasting phase is particularly crucial in barrels fabrication and influences wood composition. Burning the inner oak wood barrels surface with an open fire inducing severe changes in ellagitannins structures and compositions. Up to now thermal ellagitannin products or the reaction mechanisms underlying the ellagitannin degradation are not well searched, thus the goal of the present study is to research oak wood ellagitannin changes during toasting. For this purpose a purification protocol was established, 100 mg ellagitannins crude extract was fractionated on Toyopearl TSK HW-40 (F) gel from Tosoh Corp, ellagitannins were eluted in the acetone/water fraction. This fraction was fractionated for two times on a C-18 column. The final fraction containing only the eight principal ellagitannins was dry-heated in a lab oven for 60 min at 220 °C. After cooling, it was further fractionated on C-18 column and separated by means of preparative HPLC before being injected in UPLC/TOF-MS. Reduction process is occuring during toasting whereas oxydation can occur without heating; Thus vescalagin, is reduced into deoxyvescalagin whereas castalagin oxidation form is presented before and after toasting. Additionally to deoxyvescalagin, other ellagitannin derivatives which showed [M-H]-ion peak at m/z 1055.0631, 1041.0792, 1011.0756 and 971.0456 were produced by the toasting and identified for the first time. LC-MS/MS analyses gave strong evidence that decarboxylations as well as ellagic acid loss are the key steps in ellagitanin thermal degradation.
Issue: Macrowine 2016
Type: Poster
Authors
*Institut des sciences de la vigne et du vin