Acid-catalyzed hydrolysis of alcohols and their beta-D-glucopyranosides.

The hydrolysis, in model wine at pH 3, of the allylic, homoallylic, and propargylic glycosides, geranyl-beta-D-glucopyranoside, [3'-(1' '-cyclohexenyl)-1'-methyl-2'-propynyl]-beta-D-glucopyranoside, (3'RS, 9'SR)-(3'-hydroxy-5'-megastigmen-7-yn-9-yl)-beta-D-glucopyra noside, (3',5',5'-trimethyl-3'-cyclohexenyl)-beta-D-glucopyranoside, E-(7'-oxo-5',8'-megastigmadien-3'-yl)-beta-D-glucopyranoside (3-hydroxy-beta-damascone-beta-D-glucopyranoside), and their corresponding aglycons has been studied. In general, aglycons were more rapidly converted to transformation products than were the corresponding glucosides. Glycoconjugation of geraniol in grapes is a process that reduces the flavor impact of this compound in wine, not only because geraniol is an important flavor component of some wines but also because the rate of formation of other flavor compounds from geraniol during bottle-aging is reduced. However, when flavor compounds such as beta-damascenone are formed in competition with flavorless byproducts, such as 3-hydroxy-beta-damascone, by acid-catalyzed hydrolytic reactions of polyols, then glycoconjugation is a process that could enhance as well as suppress the formation of flavor, depending on the position of glycosylation. (3'RS, 9'SR)-(3'-Hydroxy-5'-megastigmen-7'-yn-9'-yl)-beta-D-glucopy ranoside hydrolyzed more slowly but gave a higher proportion of beta-damascenone in the products than did the aglycon at 50 degrees C. Reaction temperature also effected the relative proportion of the hydrolysis products. Accelerated studies do not parallel natural processes precisely but only approximate them.

Quantitative analysis of 4-ethylphenol and 4-ethylguaiacol in red wine.

2,3,5,6-[2H4]-4-Ethylphenol (d4-4-ethylphenol) was synthesised for use as an internal standard in a new, rapid and accurate analytical method, employing gas chromatography-mass spectrometry to determine the concentration of the important aroma compounds 4-ethylphenol and 4-ethylguaiacol in red wine. The concentrations of both compounds in wine stored in 44 American and 47 French new and used oak barrels from several suppliers were measured. Wine stored in shaved and refired oak barrels contained up to 85% less 4-ethylphenol and 4-ethylguaiacol than wine stored in normal barrels of the same age that were not shaved. The concentration of 4-ethylphenol found in 61 bottled commercial Australian red wines of various ages ranged from 2 microg/l in a Merlot up to 2660 microg/l in a Shiraz, with a mean concentration of 795 microg/l. 4-Ethylguaiacol was also detected in every red wine analysed, ranging in concentration from 1 microg/l (in a Pinot Noir) up to 437 microg/l (in a Merlot) with a mean concentration of 99 microg/l.

Determination of oak lactones in barrel-aged wines and in oak extracts by stable isotope dilution analysis.

The cis- and trans-isomers of 5-butyl-4-methyl-4,5-dihydro-2(3H)-furanone, the so-called oak lactones, are derived from oakwood, and the cis-isomer is an important contributor to wine flavour. Their deuterium-labelled forms, [2H4]cis-oak lactone and [2H4]trans-oak lactone, were synthesised from the unlabelled analogues, and were utilised in a new method employing gas chromatography-mass spectrometry to determine the concentration of these compounds in wine or extracts of oak shavings in a single analysis. The method can employ either liquid-liquid extraction or solid-phase microextraction, and is both rapid and accurate. There was some artefactual generation of cis-oak lactone during the analysis of model wine extracts of unheated oak shavings when diethyl ether extraction and injector block temperatures at or above 225 degrees C were employed.