Wine Fermentation Process
Fermentation is the conversion of sugar, by the interaction of yeasts into alcohol with a by-product of carbon dioxide (CO2) gas. Because they get their flavor from grape skin, most red wines are fermented with natural yeasts that grow on the grape skins. Winemakers can use cultured yeasts produced in a laboratory to ensure that fermentation goes smoothly and thoroughly. The crushed grapes ferment or soak with their skins, often at varying temperatures, until the fermentation is complete and the yeasts have converted the sugar to alcohol.
During fermentation, heat is produced. In general, warmer fermentation extracts more flavors, but the winemaker's goal is to find the perfect balance. Too much heat results in juice that tastes “cooked” and too little heat makes a fruity simple wine. Stainless steel vats wrapped with refrigeration jackets can control fermentation temperatures precisely. Some winemakers keep the temperature below fermentation point for a day or two to allow maceration. This boosts the extraction of color and flavor from the skins. Red wines can spend 5 to 30 days fermenting on their skins, depending on the winemaker's desired style.
The wine is usually “worked” or manipulated to maximize or minimize color and flavor extraction during fermentation. One technique is “pumping over”. Wine is pumped from the bottom of the tank to the top with hoses, leading to more evenly distributed contact with the juice and skins of the grapes. Another technique is called punching the cap. The grape cap, formed when crushed grape skins riding carbon dioxide bubbles float to the top of the vat, is punched down into the juice, either by foot (in the old days) or by plunger, which mixes the grape skins and must back into the juice. Large wineries use huge rotary tank fermenters that turn like a cement mixer, continually mixing the grape skins and must back into the juice. Either way, the grape skin cap is periodically broken up to encourage flavor extraction, allow more air in and encourage the yeasts to complete fermentation.