glycosyl-glucose assay or G-G assay, an experimental measure of grape and wine composition, developed in the mid 1990s, complementing, and additional to those already available (see grape composition and wine quality and grape quality assessment). The basis of the G-G assay is the recognition of the role of glycosides as flavour precursors of varietal wine flavour. The assay as applied to a grape, juice, or wine sample involves (a) isolation of the glycosides, (b) their complete hydrolysis to yield glucose, and (c) quantification of the glucose. The results are expressed as the amount of glycosides in micromoles, either per litre or per berry as appropriate. The G-G assay has been applied to both light- and dark-skinned grape varieties, at early stages of berry development through to harvest, to musts and juices during fermentation, and to wines before, during and after ageing. The assay is particularly important for white varieties, where there are few alternative ways of analysing secondary metabolites. Accordingly, the assay allows the viticulturist and oenologist to relate the glycoside component of grape composition, obtained before harvest, to the glycoside concentration of a wine, irrespective of its style, years after its vinification. It has revealed the decrease in G-G in wines with ageing as glycoside hydrolysis progresses, the range in glycoside concentration in juices of the same varieties grown in different regions, and the different rates of increase in glycoside concentration in fruit grown under different conditions. In the late 1990s, the assay was advocated as a valuable new tool to viticulturists wishing to investigate the influence of vine-growing practices on grape composition. As such it holds promise of an objective measure of fruit composition pertinent to wine quality. However, the assay is relatively complex and time consuming, requiring specialized skill. For this reason there has been little uptake by commercial wineries, and new methods utilizing infrared spectroscopy that offer rapid analysis of G-G in grapes are more popular.
