Did I leave you fermenting for too long? Actually, the yeast Saccharomyces, needs time to transform the glucose in the grape juice into ethyl alcohol. White wines will ferment up to 13 days whereas red wines ferment in as little as 3-7 days. All this action needs to take place in the absence of oxygen so fermentation tanks are fitted with one-way valves that allow the carbon dioxide to escape while preventing oxygen from entering.
For each molecule of glucose fermented, two molecules of alcohol and carbon dioxide are produced. That means, if the grapes are harvested with 2% sugar or Brix, the wine should yield 1% alcohol. Most grapes are harvested at 20-26 degrees Brix to produce wines with the desired 10-13% alcohol level.
During the initial days of fermentation, the degrees Brix in the tanks does not drop as the yeast cells are metabolizing the trapped oxygen in the grape juice. In the second stage the fermentation goes into full swing as the density of the liquid drops which is reflected in increased alcohol and decreased glucose. The juice also warms up to near 60 degrees. Finally, the fermentation slows as the glucose is depleted, the juice cools to near 50 degrees and the yeast is fully metabolized.
Some times a fermentation can get "stuck" due to high temperatures or a lack of a specific nutrient to feed the yeast. One of the most memorable "stuck" fermentation happened in 1975 when Bob Trinchero was trying to produce a dry, full-bodied, oak-aged white wine from Zinfandel grapes. The fermentation stopped before all the glucose was metabolized and Sutter Home White Zinfandel was born. It just goes to show you that good and profitable things can come from adverse situations.
Before we go any further, step outside and enjoy spring! Or watch a movie while enjoying your favorite chilled wine. For something fun, try pairing a simple fizzy Italian Prosecco with a bowl of buttered popcorn or even fried chicken. The bubbles will cut the fat and create a memorable pairing!