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Achieving high sugar in wine grapes by letting fruit hang for extended time comes at an economic cost to growers, says a Washington State University viticulturist.

The maximum level of sugar that wine grapes can accumulate is about 24 to 25° Brix, Dr. Markus Keller, WSU viticulture professor, said to a room full of winemakers attending the Washington Association of Wine Grape Growers annual meeting. Higher sugar levels are the result of berry dehydration.

"If you want a higher sugar level, say 29 or 30° Brix, you have to wait for the water to evaporate and the berry to dehydrate," Keller said. "You cannot import more sugar into the berry after you reach the maximum sugar stage.

"Shrinkage is what increases the sugar, and you can lose ten percent of the berry weight to get from 25 to 27° Brix, and another ten percent to get from 27 to 29° Brix. That’s a huge loss to the grower."

Berries at harvest are 70 to 80 percent water, he said. "Almost everything that’s made in the grape has to be imported."

No physiological reason

With the potential yield loss to growers, why do winemakers continue to want high sugars in the grapes?

Keller believes that there isn’t a valid physiological reason. Berries are physiologically mature after veraison as seeds are ready to germinate. Acids like malate decline early in the ripening stage, although terpenes, which are involved in the aromas of grapes, do accumulate later. Another issue from riper, more mature grapes is the higher proline and arginine amino acid levels, which can result in yeast fermentation problems.

The reason for extending hang time given by some winemakers, he said, is that extended hang time gives them opportunity to ripen grapes to a particular style of winemaking.

But Keller warns that if hang time is being used to eliminate vegetal flavors in the grapes, "chances are those flavors will linger for almost forever."

Instead of using hang time to try to correct grape deficiencies, he recommends that growers and winemakers work together to grow balanced vines with open, productive canopies, under ideal microclimates, that can ­produce high yields of quality fruit.