Tannin Development in Grape Seeds

Based on previous findings that the color darkening of grape seeds from green to brown will take place during the ripening process, and the change coincides with the decreasing rate of flavan-3-ol monomer biosynthesis, quantification of the color change can possibly be a simple way to predict the change of tannins, or the tannin ripeness of grape berries. Thus, a procedure was developed to measure the color and results expressed in Lab values (L is the lightness, a is green when negative and red when positive, b is blue when negative and yellow when positive). Malbec and Cabernet Sauvignon in Davis and Oakville were selected for this study in 2002, and the physical and chemical changes during the ripening period (from June 25 to October 17) were monitored. Both berry weight, Brix increased and titratable acid decreased in a rapid manner after veraison started (in this study veraison period was from July 22 to August 10), and stayed fairly constant after commercial harvest on the week of September 16. Color of the seeds especially the L values also changed rapidly after veraison started, but did not change appreciably during the picking window later in the season. Monomers (mainly catechin, epicatechin and epicatechin gallate) increased at beginning stage and decreased at the end of veraison. The change in procyanidin did not show a significant and consistent trend. One key accomplishment in this study is that we have shown it is possible to quantify seed color in the Hunter L-a-b system by using inexpensive computer hardware, just a scanner and PC instead of an expensive investment in specialized colorimetric equipment. The data is then analyzed using desktop software Corel Photo-paint 10, which is relatively easy to use and moderately priced. The characteristic change of the seed color can be correlated to the polyphenolics change during the ripening process. A journal manuscript of all the results is in the final stages of drafting.