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 determination of free and hydrolytically liberated beta-damascenone in red grapes and wines using a stable isotope dilution assay.

Quantitative analysis of free and hydrolytically liberated beta-damascenone in grapes and wines was developed, using a stable isotope dilution assay. Free beta-damascenone was isolated from grapes and wines by diethyl ether-hexane (1:1, v/v) extraction and the precursor(s) (glycosidic, polyols) of beta-damascenone using Sep-Pak Plus C18 RP cartridges. Hydrolytically liberated beta-damascenone was generated by acid hydrolysis from the precursor(s) extract. Red wines from Bordeaux (Merlot, Cabernet Sauvignon and Cabernet Franc, 1995 and 1996 vintage), Burgundy (Pinot Noir, 1995 and 1996 vintage) regions and Grenache wines from Chateauneuf du Pape and Côtes du Rhône (1995 vintage) were analysed to quantify free beta-damascenone. The wines made from Grenache and Cabernet Sauvignon (1996 vintage) grapes presented the highest mean amounts of free beta-damascenone, 5.4 and 5.5 micrograms l-1, respectively. Merlot, Cabernet Sauvignon and Cabernet Franc grapes of Bordeaux (1996 vintage) and their corresponding wines were analysed for quantification of free and hydrolytically liberated beta-damascenone. The levels of hydrolytically liberated beta-damascenone in grapes could predict closely the levels of free beta-damascenone in the corresponding wines after one year of ageing, i.e., almost half the levels found for the grape samples. The influence of enzyme and heat treatment of Merlot wine samples on their beta-damascenone levels was studied. Heat treatment doubled the levels of this compound, but enzyme treatment generated, in the corresponding wines, half the levels of beta-damascenone found in the non-enzyme treated wines.

Quantitative determination of beta-ionone in red wines and grapes of Bordeaux using a stable isotope dilution assay.

Quantitative analysis of beta-ionone has been developed, using a stable isotope dilution assay. This was applied to red wines from different cultivars and regions. The Burgundy Pinot noir wines exhibited the highest levels of beta-ionone. The variation in the levels of beta-ionone in grape samples and in their corresponding wines, of Merlot, Cabernet Sauvignon and Cabernet franc from Bordeaux regions was monitored at four different stages towards the end of maturation: the levels of beta-ionone were almost similar, exhibiting only a slight decrease during maturation. beta-Ionone occurred in all the grapes and wines samples analysed at levels higher than, or close to its odour threshold which was determined in a model wine solution and was found to be of 90 ng l-1. In a previously supplemented model wine solution with beta-ionone (250 ng l-1), the odour threshold was found to be of 980 ng l-1 (total levels).

Quantitative determination of 2-methoxy-3-isobutylpyrazine in red wines and grapes of Bordeaux using a stable isotope dilution assay.

Quantitative analysis of 2-methoxy-3-isobutylpyrazine (MIBP) in grapes and wines was developed, using a stable isotope dilution assay. This was applied to red grapes and wines from the Bordeaux region. The grapes and the wines of the 1995 and 1996 vintages came from the three most frequently used varieties of the region, Merlot, Cabernet Franc and Cabernet Sauvignon. The wines made from Cabernet Sauvignon grapes exhibited levels of MIBP (mean concentration, 12 ng l-1 for 1996 vintage and 13 ng l-1 for 1995 vintage) close to or higher than its odour threshold in wines (10 ng l-1) and slightly higher than the amounts found in the Merlot wines (mean concentration, 8 ng l-1 for 1996 vintage and 4 ng l-1 for 1995 vintage), especially those of the 1996 vintage. The variation in the levels of MIBP in grape samples and in their corresponding wines was monitored at four different stages towards the end of maturation. MIBP was present in all grapes and wines analysed, even in surmaturation. A linear trend was observed between grapes and wines of the three cultivars during maturation.

Glycoconjugated aroma compounds: occurrence, role and biotechnological transformation.

The present paper reviews the occurrence of glycosidically bound aroma compounds in the plant kingdom and discusses different hypotheses concerning their role in plants. Emphasis is on biotechnological methods for flavor release and flavor enhancement through enzymatic hydrolysis of glycoconjugated aroma substances.