Pre-treatment of grape juices targeted to the separation of glycosidic aroma precursors by HILIC or RP-HPLC methods.

Wine aroma, a very important sensory parameter, is composed by a wide variety of compounds with different aromatic properties. A significant part of important flavour compounds is accumulated in grapes as non-volatile and flavourless glycoconjugates, which are known as glycosidic aroma precursors. The aglycone part of glycosides is often represented by monoterpenes, benzene derivatives, C13-norisoprenoids and long-chain aliphatic alcohols. The sugar moiety includes glucose or disaccharides. Flavorless glycoconjugates represent a high aroma potential as they can be hydrolyzed to release volatile flavorants which are involved in wine flavor. The component analysis of glycoconjugates has been mostly accomplished by preliminary enzymatic hydrolysis, to liberate the aglycones, and their subsequent identification by GC-MS. Intact glycosidic aroma precursors were previously fractionated by using the countercurrent chromatography (MLCCC). We have developed a fast and effective procedure to prepare grape juice samples suitable for either the preparative isolation or the analytical investigation. The key step revealed to be the treatment of crude extracts with lead (II) acetate which allowed to discard the phenolic compounds. No traces of aroma precursors were detected in the supernatant which could be subjected to hydrophilic interaction chromatography (HILIC) or to reversed phase (RP) HPLC for the separation of pure components. The glycosides were studied by 1D- and 2D-NMR spectroscopy which led to the identification of geranyl 6-O-b-L-arabinofuranosyl-b-D-glucopyranoside, glucosyl (E)-2,6-dimethyl-6-hydroxyocta-2,7-dienoate, glycosides of linalool oxides and others.