Grapes

 


Few fruits contain 20 percent sugar when ripe, so grapes are unique in their high sugar content. Most grapes contain about equal amounts of glucose and fructose. However, Chardonnay contains more fructose than glucose. Zinfandel contains more glucose. Grapes also contain small quantities of sucrose and several other sugars.

Grapes develop in four distinct stages. A green stage - The tiny, green berries grow rapidly by cell division. Acids accumulate, but little sugar develops. A rest stage - Cell division slows considerably. The growth rate of the berries decreases, and the green berries rest. A veraison stage - The berries change color and soften. Berry size increases by expanding cell volumes. Water, sugars, aromas and flavors accumulate, but the acid level decreases. Vines produce sugar from water and carbon dioxide in the air using a process called photosynthesis. Photosynthesis requires green leafs and sunshine. Vines with yellow leafs (or without leafs) cannot produce sugar. A dehydration stage - Berries soften and become less firm. The skins may be slightly wrinkled. Sugar production stops. But, Brix may continue to increase slowly because the berries lose water. Most winemakers prefer to pick white grapes late in the veraison stage (20 - 23.5 Brix) but often prefer red grapes picked early in the dehydration stage (23.5 - 25 Brix).

Tartaric and malic are the two major acids in grapes, and they account for more than 90 percent of the total acids present. After veraison, tartaric acid (in grams per berry) remains about the same. But the tartaric acid, measured in grams of acid per liter of juice, decreases because of the increasing water content. On the other hand, vines respire malic acid during hot weather, so malic acid decreases after veraison both in grams per berry (by respiration loss) and grams per liter (by dilution). Grapes also contain about 5 percent citric acid and smaller quantities of other organic acids. For the same Brix, grapes grown in cool climates have higher acid content than the same variety grown in warm climates.

Grape clusters hanging under the canopy in the shade are close to emperature equilibrium with the air, and the internal temperature of these berries will be within a degree or two of ambient. The temperature of clusters hanging directly in the sun will be many degrees hotter than clusters in the shade. Grapes have a high specific heat, so they retain temperature for long times. Cool grapes in forty pound "lug" boxes will remain cool for several hours when stored in the shade. A truck load of cool grapes will remain cool for more than 24 hours even in hot weather.

On average, grapes contain about 5 percent stems, 12 percent skins, 3 percent seeds and 80 percent flesh (by weight). Stems contain 0.5 to 4 percent phenolic compounds, and stems account for about 20 percent of the total tannins in harvested grapes. The skins of red varieties contain about 60 percent of the total phenolic materials, but most of these phenolics are pigments. The skins of white varieties only contain about 10 percent of the total phenolic compounds. Skins contain much of the potassium in the berries, and pH increases when must is cold-soaked because of potassium leaching out of the skins. Seeds contain about 40 percent of the tannins in harvested grapes, and seeds are the major source of tannin in red wines. Seeds also contain grape-seed oil, and the oil can impart a bitter taste to wine. The flesh consists of large cells filled with juice, and they provide the free-run juice when the grapes crushed. Flesh only contains about 5 percent of the total phenolics and little potassium. Grape varieties with large berries, thin skins and small seeds produce 160 to 180 gallons of wine per ton. Other varieties like Cabernet Sauvignon have small berries, thick skins and large seeds. These varieties often produce 140 to 160 gallons of wine per ton.