Monoterpenic and norisoprenoidic glycoconjugates of Vitis vinifera L. cv. Melon B. as precursors of odorants in Muscadet wines
Introduction
Structural studies have allowed the identification in grapes of four groups of glycosides: β-d-glucopyranosides, 6-O-(α-l-rhamnopyranosyl)-β-d-glucopyranosides, 6-O-(α-l-arabinofuranosyl)-β-d-glucopyranosides [1], [2] and 6-O-(β-d-apiofuranosyl)-β-d-glucopyranosides [3]. These flavourless glycosides, as well as the cysteine derivatives discovered recently [4], make up in grape a reserve of aroma compounds that can be liberated in wine [5]. These glycosides can be conveniently extracted from grape using solid phases, like XAD-2 resin [6] or C18-RP adsorbent [7], [8]. However, due to the complexity of these glycosidic extracts, their direct quantitative analysis was restricted only to the most abundant benzyl, 2-phenylethyl and monoterpenyl glycosides [3], [9]. Therefore, the most used quantitative methods were indirect and consisted in the analysis of the volatiles liberated from the glycosidic extracts by enzymatic or acid hydrolysis [10]. The mild enzyme method was the most efficient technique to generate most aglycones unchanged. It proceeded for the diglycosides through a sequential mechanism, which required the occurrence in the enzymatic preparation used of β-d-glucopyranosidase, α-l-rhamnopyranosidase, α-l-arabinofuranosidase and β-d-apiofuranosidase [11]. However even this mild enzyme method cannot totally eliminate the formation of artifacts [12], [13]. Furthermore the aglycone fraction obtained can be spoiled by traces of polar non glycosidic compounds, extracted with glycosides and not totally eliminated through the processing of the glycosidic extracts. In spite of these limitations, which must be taken into account, this method allowed to cope with the noble part of these glycosides as regards aroma generation [10].
Among the aglycones identified in grape, monoterpenes and C13-norisoprenoids were the most involved in aroma generation, either directly from the glycosides or via polyhydroxylated derivatives [10]. The purpose of the present work was to make a qualitative and quantitative study of these isoprenoid aglycones in Melon B. grapes (grown mainly in the Nantes region in western France), not investigated up to now. Furthermore, the odorants generated from their glycosidic bound forms in the Muscadet wines made from these grapes were investigated.
Section snippets
Plant material
The Melon B. grape samples, grown in five different vineyards located in different areas of the Muscadet region (Nantes region, Western France) were harvested at technological maturity from the beginning of September to mid October, in 1998 and 1999 (encoded HAI, DRA, MON, LOR and LIM in Table 1). In 1999, Melon B. grapes from six complementary vineyards of the Muscadet region were added to the study (encoded A–F in Table 2). Grapes were frozen and stored at −20°C till analysis.
Chemicals
Theaspirane and
Glycoconjugates composition of Melon B. grapes
Glycoconjugates from Melon B. grapes were extracted using reversed-phase C18 cartridges [7], [8]. The enzymatic preparation used for the liberation of aglycones moieties from these glycosidic extracts contains the glycosidase activities required to hydrolyze the grape glycosides [5]: β-d-apiosidase (564.6 nkat/g), α-l-rhamnosidase (236.7 nkat/g), α-l-arabinofuranosidase (13 500 nkat/g) and β-d-glucopyranosidase (4380 nkat/g). As this enzymatic preparation was used in low amount, it was not
Acknowledgements
The authors thank F. Charrier (ITV Nantes, Vertou, France), for the technical help during the vintages.
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