astringencyandellagitannins - J Agric Food Chem 2007 55...

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Identification and Sensory Evaluation of Dehydro- and Deoxy-ellagitannins Formed upon Toasting of Oak Wood ( Quercus alba L.) A RNE G LABASNIA ² AND T HOMAS H OFMANN * ,‡ Institut fu ¨r Lebensmittelchemie, Universita ¨t Mu ¨nster, Corrensstrasse 45, D-48149 Mu ¨nster, Germany, and Lehrstuhl fu ¨r Lebensmittelchemie und Molekulare Sensorik, Technische Universita ¨t Mu ¨nchen, Lise-Meitner-Strasse 34, D-85350 Freising-Weihenstephan, Germany Traditionally, spirits such as whiskey are matured in toasted wood barrels to improve the sensory quality of the final beverage. In order to gain first insight into the puzzling road map of thermal ellagitannin transformation chemistry and provide evidence for the changes in sensory active nonvolatiles in oak wood during toasting, the purified oak ellagitannins castalagin and vescalagin, their corresponding dimers roburin A and roburin D, and 33-carboxy-33-deoxyvescalagin were thermally treated in model experiments. Besides mouth-coating and golden-brown colored melanoidin- type polymers, individual major reaction products were produced as transient intermediates which were identified for the first time by means of LC-MS/MS and 1D/2D-NMR spectroscopy. Depending strongly on the stereochemistry, castalagin is oxidized to the previously unreported dehydrocastalagin, whereas its diastereomer vescalagin, differing only in the stereochemistry at carbon C-1, is most surprisingly converted into deoxyvescalagin. Comparative model experiments with 33-carboxy-33- deoxyvescalagin revealed castalagin, vescalagin, dehydrocastalagin, and deoxyvescalagin as typical reaction products, thus indicating decarboxylation as a key step in the thermal degradation of that ellagitannin. Similar to the ellagitannin monomers, LC-MS/MS analyses gave strong evidence that the corresponding dimer roburin A, containing the vescalagin configuration at C-1, was converted into the deoxyroburin A, whereas roburin D, exhibiting the castalagin configuration at C-1, was oxidized to give the dehydroroburin D. Human sensory experiments revealed that the ellagitannin derivatives imparted an astringent mouth-coating sensation with threshold concentrations ranging from 1.1 to 126.0 μ mol/L, depending strongly on their chemical structure. KEYWORDS: Ellagitannins; castalagin; vescalagin; roburin A; roburin D; taste; astringency; oak wood; whiskey INTRODUCTION Over centuries, spirits such as whiskey have been matured in toasted wood barrels for several years to give the beverage a final improvement in color, aroma, taste, and mouth fullness, the later of which is perceived as body, viscosity, or complexity in the oral cavity. For this maturation procedure American white oak ( Quercus alba L.) and European oak ( Quercus robur L.) have emerged as the wood of choice but, in particular, the toasting of the oak wood is commonly accepted as one of the most important technological steps in barrel manufacturing. During the toasting process, the inner surface of the oak wood
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astringencyandellagitannins - J Agric Food Chem 2007 55...

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