Abstract
Since Japan generally has a more humid and warmer climate during the grape-growing season, and hence a higher fungal disease pressure, than the major good-quality wine-grape cultivation regions of the world, grapes grown in Japan generally contain lower amounts of total phenols, especially anthocyanins. In order to make red wines rich in polyphenols but with a smooth flavour lacking astringency or bitterness, it is necessary to investigate in detail the behavior of polyphenols during fermentation and storage. For many years, we have been conducting such studies with the aim of establishing how to produce good-quality wines favored by consumers from low-polyphenol Japanese grapes. When grapes are crushed, polyphenoloxidase (PPO) oxidizes colorless diphenols to produce the corresponding quinones. o-Quinones formed enzymatically oxidize red pigments such as monomeric anthocyanins and oligomeric and polymeric pigments, thus decolorizing them. PPO can not directly oxidize these pigments. Fermentation is started by adding yeast to must, and polyphenols are extracted from seeds and skins. The maximum intensity of red coloration (red pigments) in the must is obtained within several days after the addition of yeast, but the concentration of colorless polyphenols (catechin, procyanidins, flavonols, etc.) continues to increase until the removal of the pomace (seeds and skins). In sensory evaluations, wines fermented on skins for more a few days after reaching the maximum red color intensity received the highest scores. We have produced red wines from Cabernet Sauvignon grapes annually for 22 years and stored them in an underground cellar. Color analysis of these wines showed that the total phenol content did not change appreciably for 10 years after the start of storage, whereas the total pigment content gradually decreased. Strong relationships were found in the 22 wines between antioxidant activity and total phenols, flavonoids, or red pigments. Resveratrol is a polyphenol and stilbene specific to grapes infected by the gray mold Botrytis cinerea. It is noted as an antioxidative substance that is thought to prevent LDL oxidation and/or blood platelet aggregation, and may have anti-cancer properties. Little resveratrol was found in grape juices, but its concentration in musts (wines) increased during fermentation on skins. Because grape varieties with high resveratrol productivity were found to be resistant to various fungal pathogens, and a wild grape originating from Tibet produced the a large amount of resveratrol, we have cloned the stilbene synthase gene of this wild grape for the purpose of comparing it with the corresponding genes of grapes with low resveratrol productivity.
