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The must, whether from red grapes or pressed white grapes, is ultimately sent to the fermentation tanks. The fermentation tanks are airtight, made of stainless steel and can hold 1,500 or 3,000 gallons (5,678 or 11,356 liters). The tanks are cooled with glycol to maintain a temperature in the 40-F range (4-C range). The winemaker adds sugar and yeast to start the process of fermentation. The type of yeast and the amount of sugar added depends on the type of grape.

Fermentation tanks

When the yeast first hits the must, concentrations of glucose sugar (C6H12O6) are very high, so it is through diffusion that glucose enters the yeast. In fact, it keeps entering the yeast as long as there is glucose in the solution. As each glucose molecule enters the yeast, it is broken down in a 10-step process called glycolysis. The product of glycolysis is two three-carbon sugars, called pyruvates, and some ATP (adenosine triphosphate). ATP supplies energy to the yeast and allows it to multiply. The two pyruvates are then converted by the yeast into carbon dioxide (CO2) and ethanol (CH3CH2OH), which is the alcohol in wine. The overall reaction is:

C6H12O6 => 2(CH3CH2OH) + 2(CO2)

The fermentation process takes about two to four weeks. During this time, the winemaker samples the fermenting must and measures the pH or acid levels to determine that the fermentation process is proceeding as it should.

Photo courtesy Chatham Hill Winery
Wine is stored in oak barrels or stainless steel storage tanks.

Sulfur-containing compounds called sulfites are naturally found on grapes to retard the growth of bacteria and mildew. Most winemakers add sulfites to the wine to help stabilize it as it ages. However, some people are allergic to sulfites -- wines that are labelled as sulfite-free have had the sulfites chemically removed.

Once the fermentation process is completed, red wines are sent to the press to separate the skins from the wine. The red wines are then filtered to remove the yeast. White wines are allowed to settle and are filtered to remove the yeast. Once the yeasts are removed, the wines are stored in either stainless steel storage tanks or oak barrels (oak gives many wines a characteristic flavor) depending on the type of wine. In some red wines, a second type of fermentation, called malolactic fermentation, is undertaken while in storage. In malolactic fermentation, the winemaker adds a bacteria to the wine that breaks down malic acid, a byproduct of aerobic (oxygen-requiring) metabolism, into lactic acid, a byproduct of anaerobic (no oxygen) metabolism. Lactic acid is a milder acid than malic acid. The aging process can be anywhere from three months to three years.