To everything, there is a season…


by Mary Jacobson, MD and Lynn Gretkowski, MD

Just as Vermonters are thinking about the changing colors of falling leaves, Californians, including the WineDoctors, have been thinking about the ripening and harvesting of wine grapes. Two very analogous processes that have common botanical roots are occurring 2,500 miles apart and remind us of our unique yet shared seasonal voyages.

All through the summer, grapes have transformed from green to purple with softening tannins, lowering acid levels, and concentrating sugars. Grape ripening indeed happens in autumn as maples in Vermont go from green to red. Sugar levels in grapes are concentrating with the longer nights of autumn.

In a two-step process, berries are formed during berry set, and approximately 60 days later, veraison, or color development, of the grapes begins to occur. Color of red grapes emerges as anthocyanins and xanthophylls increase while carotenoids increase in white grapes and chlorophyll levels decrease in both.  Acids and aroma compounds accumulate during berry development and sucrose (eventually becoming the fermentable sugars glucose and fructose) transport into the berries from the leaves.

Berry development and ripening processes are thought to occur in temperature-dependent viticultural areas. The temperature at which plant pulp freezes is 28 degrees Fahrenheit. This determines the season length. This interval occurs between the spring and the fall (depending on the growing area) when average temperatures equal 28 degrees Fahrenheit.

Dr. Albert Winkler at UC Davis devised another method of seasonality for grape growing using the degree-day concept. The alcohol in wine is dependent on the levels of sugars that are in the harvested berries. Higher levels of alcohol found in California and new world wines are related to high sugar levels present in the berries at harvest, thought to be related to an increase in the number of ‘degree days’ or days after berry set during which the average temperature is above 50 degrees Fahrenheit on average. As with leaves of deciduous trees, the level of color in the berries, and sugar content, can be a reflection of the year’s weather.

Leaves of deciduous trees like oaks and maples contain chlorophyll (just like grape leaves), where photosynthesis occurs where light that’s absorbed is converted to energy for the tree and vine. Carbon dioxide is converted into oxygen and carbohydrates, energy that is used for the plant to grow and grapes to develop. Warm temperatures and sunlight are required to keep the process going.

Leaves may contain carotene and anthocyanins, which impart yellow and red colors, respectively. Carotene and chlorophyll are in the cell membranes of leaves; anthocyanins are in the sap.  Anthocyanins are dependent upon the level of sugars interacting with protein in the sap of these trees.

At the onset of cool days, the abscission layer forms between the branch and the leaf, leaf chlorophyll levels drop, sugar levels increase, leaves change color. Cold temperatures also drop chlorophyll’s green and produce color.

Analogously, grapes soften and lose chlorophyll while sugars accumulate and pigmented changes develop.  Different varietals ripen at different rates based on other factors like climate and pruning techniques in the vineyard performed to increase fruit exposure to light.

All of the above are great reasons to think about wine and cheese as you stroll along a leaf-cluttered path during this most colorful and fruitful of seasons.

Mary Jacobson, MD and Lynn Gretkowski, MD are the WineDoctors ( They are committed to promoting an understanding of the impact of wine on health by providing transparent, accurate information as it relates to moderate wine consumption. They also produce a line of handcrafted, boutique wines.

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