Grape berry physiology
A grape berry dissected. The more seeds, the bigger the berry and the longer it takes to reach maturity.
From a physiology point of view, the sole purpose of the grape is to produce seed and get it out in the environment for germination, says Washington State University viticulturist Dr. Markus Keller. Grapes are mature when the seeds are able to germinate, not when the berries reach a particular Brix or sugar level.
Keller, in highlighting the physiological aspects of grape ripening during a recent grower-winemaker seminar, said that seeds play a key role in grape ripening. Seeds tell the berry when to start accumulating sugars, change color, and they also control the rate of ripening. “Color is the advertising to birds ‘to come and pick me.’ Sugars get the birds to eat the fruit so they will later disperse the seeds for germination.”
Other physiological aspects he suggested growers and winemakers keep in mind as grapes mature include:
Berry composition—Berries are 70 to 80 percent water. It is the skin that limits how much berries can expand, not the irrigation applied late in the season to the grapes. Research conducted by Keller has debunked the myth that late-season irrigation plumps up berry size and dilutes sugars and flavors, a practice that winemakers in the past urged growers to avoid.
Sugar—The maximum sugar level that berries can accumulate is 23 to 25° Brix. Any level above 25° is due to water loss or dehydration by the berry. An increase of two degrees Brix means grapes have lost 10 percent in weight or yield. Fruit dimpling, which indicates dehydration, begins to show when berries have lost 10 percent in weight.
Flavors and aromas—Most flavors and aromas are bound tightly to the glucose inside the berry to prevent volatilization and can’t be smelled when winemakers chew on the berries to gauge ripeness. “You can taste the increases in sugar, you can taste acidity, the change in pH, astringency, and decline in methoxypyrazine (compounds associated with green, vegetal flavors), but you can’t taste or smell aromatic flavors.”
Determining ripeness—Grapes should be sampled frequently once berries change color at veraison. Growers and winemakers should keep a database of the sampling numbers, collecting data from sentinel vines in the vineyard, and refer to such records from year to year to gain knowledge on the vineyard history. And, be prepared for season-to-season variation in berry development due to changing temperatures from each growing season.