Characterization of Sugars and Organic Acids in Commercial Varieties of Table Grapes

Flavor composition has been defined as a complex attribute of fruit quality, in which the mix of sugars, acids and volatiles play a primary role. In table grapes (Vitis vinifera L.), sweetness and sourness are the most important flavor attributes for fresh consumption. However, most of the studies available have been performed on wine grapes, which are grown, cultured and processed differently to table grapes. Therefore, the objective of this work was to characterize the changes in sugars and organic acids during the development of 'Thompson Seedless', 'Red Globe' and 'Crimson Seedless' grown under the same agroclimatic conditions. Each variety was sampled weekly from 2 wk before véraison until commercial harvest. Sugars and organic acids were quantified by high performance liquid chromatography (HPLC) equipped with an evaporative light scattering detector (ELSD) and ultra violet detector, respectively. The ranges of acid and sugars concentrations found in grapes were as follows: tartaric acid, 1.28-7.45 g L^sup -1^; malic acid, 0.38-29.92 g L^sup -1^, citric acid traces-1.03 g L^sup -1^; fructose, 0.15-8.74 g (sugar) 100 g(grape)^sup -1^; glucose, 0.19-8.71 g (sugar) 100 g(grape)^sup -1^ and sucrose 0.02-0.91 g (sugar) 100 g(grape)^sup -1^. Among sugars, glucose was the most abundant one in early stages and then it decreased until the harvest period, when the amount of fructose and glucose converged to an average of 47% for each sugar. Despite organic acids reaching steady levels 3-4 wk before commercial harvest, there were important differences in the organic acid profiles among varieties, with 'Thompson Seedless' showing the lowest tartaric/malic acid ratio of 1.19. These differences are an important aspect in terms of overall flavor. Total soluble solids (TSS) showed the largest increase between 7 and 4 wk before commercial harvest for all three varieties studied (Figure 1). 'Red Globe' and 'Crimson Seedless' showed the largest change in TSS (close to 13% TSS) and a continuous rise throughout the sampling period. However, 'Thompson Seedless' reached a plateau from 4 wk prior to harvest to the end of the sampling period. A reduction in titratable acidity (TA) close to 4% was observed during berry development in the three varieties. 'Red Globe' was the variety showing the largest decline (Δ = -4.0% of acidity), while 'Crimson Seedless' was the variety with the smallest drop in TA (Δ = -2.6% of acidity). From 4 wk before harvest (H-4), the TA levels remai