Glycosidic / glycosidically flavour precursors, grapes

Figure 4 shows in generalized form the structural features of those glycosidically conjugated flavour precursors that have been identified in grapes to date. 

Countercurrent chromatography (CC) is a support-free, liquid-liquid partition system in which two immiscible liquids act as mobile and stationary phases. Absence of a solid adsorbent eliminates the possibility of analyte decomposition on an acitve surface, assures recovery of the sample material, and means that the capacity of a CC column is much greater than, e.g. a HPLC column of the same dimensions. CC has been used in glycoside research becasue it is ideally suited to separate such highly polar, water-soluble, and labile constituents without the need for their derivatization. The techniques of CC that have recently been applied in fruit flavour precursor studies, including those of grapes and wines, have been reviewed (Winterhalter 1993). 

Giinata, Z., Dugelay, 1., Sapis, J.C., Baumes, R., Bayonove, C. (1993). Role of the enzymes in the use of the flavour potential from grape glycosides in winemaking. In P. Schreier P. Winterhalter (Eds.), Progress in flavor precursors studies (pp. 219-234). Carol Stream Ils. AUured Publishing. 

Williams, P.J., Sefton, M. and Francis, I.L. (1992) Glycosidic precursors of varietal grape and wine flavour, in R. Teranishi, G.R. Takeoka, and M. Giintert (Eds), Flavor precursors - Thermal and enzymatic conversions, ACS Symp. Series 490, Am. Chem. Soc., Washington DC. 

In direct contrast to the monoterpenes and norisoprenoid grape flavour components, no evidence has yet been found for the existence of bound or precursor forms of methoxypyrazines in grapes and, in particular, there is no evidence of glycosidically bound forms. 

This chapter has identified two major challenges in the analysis of flavour components of grapes. One is that components may be present at trace or ultra-trace levels, and this places special demands on detection, characterization and quantitative analysis. The second is that components also exist in precursor forms which, in the case of the glycosides, necessitate techniques suited to analysis of relatively involatile compounds with wide structural diversity. Mass spectrometry, coupled... 

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